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In Situ Plants Blog

Bags of Dirt: How to Choose a Growing Medium for Interior Plants

I had promised back in one of the parts of Watering 102 (see parts one, two and three of that massive tome here) to write a little about growing media, and how to select or make one yourself in order to grow great plants. There are several aspects to consider that will help you make the best choice as far as what to use, and a lot depends on the type of plants you grow, how heavily/frequently you water, and environmental factors such as heat and humidity.

We'll get this out of the way first, though: don't ever call it dirt. Dirt is the stuff that ends up under our fingernails, and what we sweep off the floor. It's something of a derogatory term, I feel. It's actually one of the first things we got drilled into us at school, and it stuck. We are all better off calling it soil, though even this is a bit of a misnomer, as what we typically use for indoor applications is a soilless growing media containing mostly organic material and no true mineral soil, such as you'd find in your backyard. 'Soilless growing medium' is a bit of a mouthful, though, so soil will suit our needs here.

Perhaps it's best to start with a rundown of some of the commonly used elements that make up a typical soilless mix. Here we go...

High-quality peat moss: note the large, fibrous-looking pieces. 
This is Lambert`s AFM soilless mix. Image © Lambert;
retrieved from their website.

Low-quality peat moss: a photo of the dark, small-particled stuff
that is sold at the consumer level. Image © unknown; retrieved
from LA Readers blog.

First and foremost is peat moss. This material is partially decomposed sphagnum moss which comes in varying grades and qualities, and is the major component of most mixes, from the dusty garbage they sell you at the garden centre to the tried-and-true stuff used to produce countless acres worth of ornamentals of all sorts. When of an appreciable quality, it is best described as 'fluffy', and if you compare qualities side by side, the difference is striking. Quality peat is light in colour and feel and has varying larger particle sizes, which allows for good aeration. It also has a good water-holding capacity without becoming mud when very moist. It is relatively long-lived, though it does degrade over time. The other stuff is dense, with mostly small particle sizes, and is basically a more decomposed version of the nice stuff (being dredged from deeper in the peat bogs from which the material is drawn [the sustainability of which practice may come up in a future post]). It does become mud when you try and water it. It's basically garbage, because it becomes a real challenge to ensure that a plant's roots are able to absorb enough oxygen to keep from dying.

It's important to make sure if you're going to be purchasing straight peat moss for use in your own recipes that it is buffered: peat moss on its own is highly acidic (and is used as an amendment for acid-loving plants such as plants in the Ericaceae family such as blueberries), and bufffering will raise the pH from 4 or so up to about neutral (7), which is where most species will be best suited.

There is an alternate material to peat moss made from the husks of coconuts, called coir. It also comes in differing types, from roughly chopped husks to finely ground fibres. It is longer-lasting than peat, and also is less hydrophobic, meaning it will more readily absorb water when it is dry, which is handy. One serious issue is with the source of this material: as coir is processed it is often washed in seawater, and unless it is thoroughly cleaned can impart some serious saltiness to one's plants, with the obvious devastating effect that you might expect. It is important to only buy this stuff from a reputable source (hydroponics stores usually stock it), or to process it yourself in order to ensure it is fit for use.

Perlite is usually the other component in soilless mixes. This is an expanded volcanic mineral which imparts greater aeration and water-holding capacity to the mix. It is not fertilizer, nor is it styrofoam, or anything else I've heard it being referred to as. It comes in varying sizes, with larger pieces being used for things like bark-based orchid media. It is admittedly ugly and unnatural looking, and has the bad habit of floating to the top of the container when watered heavily.

On that note, bark and sand are two more materials which can be found in some mixes, also in order to create more room for air in the growing medium. Sand is a bit outdated, but it's cheap, and can be found in the dirt that's marketed for cacti and succulents. Bark is more useful, but does break down eventually. Vermiculite is another mineral that is sometimes also used for enhanced water retention and aeration porosity. Other materials in specialty mixes are long-figer sphagnum moss, which is the undecomposed version of peat moss (you've probably seen orchids grown in this), and which greatly increases the water-holding capacity of a mix, tree fern fibre (a durable material derived from tropical tree ferns which creates a really nice open mix for things like epiphytes) and charcoal, which has little value past increaasing aeration porosity and its longevity.

So with that out of the way, basically what we're looking for is a medium that provides the right mix of water-holding capacity to air-holding capacity for the type of plants we wish to grow, and for the amount of water we are applying. A decent basic soilless mix such as Berger's BM6 contains about 75% peat to 25% perlite, and I've grown an extensive array of plants in this without any amendment. It can hold a lot of water if you let it, but it's also a very good mix for keeping lightly moist, as it does drain quite freely. Again, if you let it go bone dry it will be difficult to rewet due to the hydrophobic nature of the peat, so bear that in mind. A lot of growers of gesneriads advise amending a mix like this to more of a 1:1 peat to perlite ratio, and while there's definitely no harm to it, I've been able to grow most gesneriads I've worked with in just the basic mix by simply avoiding overwatering.

If you are heavy-handed with the watering can you may benefit from amending your mix, but there is such a thing as too light a mix as well. With increased aeration comes reduced water-holding capacity, and less than adequate watering can quickly lead to dessication. High temperatures and low humidity can also contribute to a more rapid drying of the media, and in some cases it may even be beneficial to have less aeration porosity in your mix: there are some unforgiving plants which are goners after they dry out even once (you'll notice that the growing media of most ferns you see available, for example, do not even contain perlite).

How you use the mix will play a large role in how it performs. One of the most important things that needs your attention is that the media loses its functionality when it is crammed into the pot by hands more used to garden soils out of doors. While it does seem to hold a plant in place very well, this basically reduces all of those nice large spaces between soil particles that would ordinarily hold air, reducing them to a size that is only capable of holding water. That's bad news for plant roots, and a bit of a slight to the folks who spend all that time developing an optimizing their growing media for the perfect aeration porosity. Slightly overfilling the container with soil and them watering the plant in gently but thoroughly will do wonders for securing the plant as the soil settles into itself, and the integrity of the mix will be preserved.

When watering these mixes, particularly when the container has been freshly planted, it is best to use a watering can or hose with a breaker in order to keep from disturbing the soil surface: in time, the soil's microbiology will colonize the media and produce exudates which hold everything together nicely and make it easier to throw water on without having to worry too much about taking the extra time (good news for professionals).

Remember, too, that if you're using any organic material at all, it will eventually decompose, and the soil will need to be changed. I won't go into repotting here, but if you seem to have problems that could be explained by overwatering even though you've been doing everything the same way for a long while, decomposition of the media may be the issue.

A note on terrariums, particularly ones which actually have half a chance of success and aren't just a few succulents thrown into a glass bubble because someone saw it on Pinterest (yes, I have a serious hate on for the current terrarium craze; so much so that I'll likely rant aout it in another post): I want to draw your attention to the fabled 'ABG mix', one of a few substrate recipes whipped up by Atlanta Botanical Gardens for plants in their collection. This stuff has really caught on in the vivarium hobby (basically a planted terrarium that houses creatures like lizards or frogs), as it has perfect drainage and very long life, which are two things that can plague a planted glass enclosure. The mix commonly used is as follows:

1 part peat moss (or coir)
1 part milled sphagnum moss (ground or chopped long fiber sphagnum moss)
1 part fine horticultural charcoal
2 parts tree fern fibre
2 parts fine bark

It's pretty tough to dispute the effect of a perched water

table when it's captured in photos like this. 

Image © unknown, retried from The Garden Professors Blog

You can have many years of enjoyment with this mix in the warm, humid conditions of a terrarium without it breaking down, and another bonus is that it looks way more natural than a peat/perlite container medium. If you play around with the ratios you can also create a great mix for houseplants that also resists decay and can allow you to go for quite some time without needing to repot due to substrate decomposition.

OK, one last thing about container media that has to do with drainage. There is an adage that many of you have no doubt heard about putting pot shards, gravel, or other material in the bottom of the container before filling with soil to allow for better drainage. This is a useless and counter-productive practice, and here's why: because of the way water interacts with itself and with soil particles (cohesion and adhesion, respectively), it will not move readily from an area with small spaces between particles to one with large space between particles. The top of a layer of gravel or whatever is pretty much the same to water in soil as the bottom of the pot, so basically all you're doing is eliminating valuable real estate for plant roots, and potentially creating conditions in which too much water is held in the soil because it's not draining properly. Use the same medium throughout the container and you will generally have better results.

The photos to the left illustrate perfectly the effect of having a larger-particled substrate over a finer-particled one: the water distributes itself throughout the top layer and does not move into the layer below until fully saturated. The myth of drainage is one of those many things in horticulture that has just been repeated so often everyone thinks it must be true (and have a look at the comments on the page I grabbed the image from to see the type of stubbornness to change that is basically rampant in the scene, professional and amateur alike. This was another of the first things that we learned at school, and we did a whole whack of labs that really drove the point home for us.

So there it is, long-winded as ever. The takeaway points, I guess, are: don't buy shitty quality soil, don't cram it into the container, don't bother trying to improve drainage with other materials, and don't put succulent plants in terrariums.

The Myths of Phosphorus and its Overuse in Indoor Horticulture

I can't dispute that phosphorus is an essential element in plant growth: it is present in every cell, and is directly involved in many processes in the plant, including, energy generation, respiration, nitrogen fixation, and most importantly photosynthesis. A plant can't do much without it. There are a lot of old ideas, though, that are repeated in the horticultural industry, despite the fact that they have been obviously disproven. In the interest of attempting to dispel the dogma and disseminate some valuable insight about how plants actually make use of the phosphorus that we provide them (or, more importantly, how they often don't), what follows are a few points that hopefully will help professionals and amateurs alike choose the best fertilizer for the needs of their plants.

Note that I put the word 'interior' in the title, hoping to imply that the plants I'm discussing are growing in a soilless growing mix indoors, and not in the ground: that's a whole other can of worms, and the concepts here will be greatly simplified without having to worry about it. We're working with basically a blank slate here, nutrient-wise. It's also easier for us here in Toronto because the water is fairly low in minerals save the bloody bicarbonates, which I'll likely talk about at a later time. 

An old-timey picture of a rock phosphate mine, because this post, like many
others, has gotten pretty long and I need to break it up a little. That and actual
rock phosphate isn't really much to look at. Public domain image;
retrieved from altervista.

The phosphorus used in fertilizers for the most part is derived from rock phosphate, which is becoming more scarce with time (here's an article that discusses a few aspects of that whole thing as it pertains to global agriculture: Phosphate: A Critical Resource Misused and Now Running Low). There are other sources, notably from organic sources such as bonemeal and bat or bird guano, which can be combined with a growing media and which makes itself available to plants gradually and thus acts as a kind of organic slow-release fertilizer for plants.

As mentioned above, phosphorus is one of the macronutrients that are required for healthy plant growth (as opposed to the micronutrients, which, while also essential, are required by the plant in much smaller quantities). In most fertilizers, the three primary macronutrients are listed on the packaging as a ratio in the order of nitrogen, phosphorus and potassium, such 20-10-20 and 15-30-15. These may or many not have the additional macronutrients calcium, magnesium, and sulphur, as well as the fairly long list of micronutrients. In most fertilizers that you can buy 'over the counter', so to speak, the amount of phosporous will typically be equal to or greater than nitrogen or potassium. There are several reasons why this is ridiculous.

Plants do not use most of the phosphorus that is contained in these typical fertilizers. These high-phosphorus recipes have their origins in field crop production, where phosphorus behaves quite differently than in our soilless media, and where yields can be significantly affected by the availability of this nutrient. Producers of fertilizers for domestic use typically market high-phosphorus fertilizers as producers of better root growth and better blooming, when in fact the extra fertilizer is of no use whatsoever. They make quite a killing at it, too, I'm sure. It's pretty wasteful, though, as it happens, and it seems that, at least in some cases (see below), lower phosphorus can produce better flowering and better quality crops in general.

Additionally, the fact that the extra phosphorus doesn't leach away and remains present in the soil can have some negative effects: too much can inhibit the uptake of other negatively-charged elements such as iron and manganese. It also readily precipitates with other elements, forming insoluble compounds which are unavailable to plants, such as calcium and magnesium phosphates, particularly at higher pH levels.

Figures 1 and 2 from the bedding plant study linked below, showing
increased flower production and less wilting in plants grown with less
available phosphorus.

But so how much phosphorus do plants actually use? Not very much! This paper provides a good look at the use of phosphorus by azalea plants, and indicates that the addition of phosphorus above a certain (low) threshold made no significant difference to the growth of the plants in the study: Nitrogen and Phosphorus Uptake Efficiency and Partitioning of Container-grown Azalea During Spring Growth. And here's another paper that shows that lower phosphorus levels can actually produce better quality crops, with more flowers that held for longer time, as well as increased drought tolerance: Improving Bedding Plant Quality and Stress Resistance with Low Phosphorus.

So from these and other studies, we can determine that phosphorus seems not to promote better blooming, yet somehow these fertilizers can seem to be effective, hence their continued use. What is it about them that makes them work? The answer is in the number to the left: nitrogen. Many of the 'bloom booster'-type fertilizers have either reduced nitrogen or increased phosphorus/potassium levels, and a reduced nitrogen to potassium ratio is one of the ways to shift plants more into reproductive mode, wherein they obviously produce more flowers at the expense of foliar growth. Balancing plants between vegetative and reproductive growth is the art of the commercial grower, and it is a fine art indeed, and worth examining to the amateur grower. 

Recommendations for commercial crop production of most tropical plant species (which, to be fair, are mostly grown for their foliage) is for the use of a fertilizer with a 3-1-2 ratio, though even this seems a bit high: compare it, for example, to the MSU orchid fertilizer, which is 13-3-15, and which is a fantastic recipe, in our opinion. This article from the American Orchid Society discusses some of the reasoning behind the ratio they use, and how it works to create beautifully balanced plants that grow and bloom at an optimum: Without High Phosphorus A New Fertilizer Proves Itself with Orchids. It's important to note that this fertilizer was not developed specifically for orchids, but is rather a well-suited mix for plants of any type, being built on sound principle and good science. For those in the business, or those who really go through the stuff, it would be fairly easy to reverse-engineer it from the guaranteed analysis on the label if you have access to the raw materials. (A hint if you don't want 200 lbs of elemental fertilizer sitting around- hydroponics stores often sell smaller bags of potassium nitrate, etc., though for an inflated cost.) If anyone is interested in doing this and doesn't know how, say so in the comments and we'll see if I can't help you sort through it. 

Remember, the job of the fertilizer company is,
first and foremost, to sell fertilizer. Properly
managing plant nutrition is the responsibility of
the grower.

I find it a little funny that even though the recommendations for production are to use a 3-1-2 ratio, I see professionals further down the line such as interior landscapers or garden centres using something like 20-20-20 (or even 10-52-10 for new transplants). Phosphorus is about four times more expensive than it was ten years ago, and I don't presume the cost will be going down any time soon. Given the plants aren't using it, why throw your money into the dirt, so to speak? Even local orchid societies are applying high-phosphorus fertilizers to their collections, despite vendors at their meetings carrying the MSU feed! Old habits die hard, I suppose.

I'm not recommending that anyone rush out, buy a reverse osmosis filter and start mixing up batches of MSU feed to start doing their houseplants (well, I sort of am, though it is expensive and maybe almost as wasteful as the extra phosphorus I'm bitching about here, due to the waste water that RO filters produce- your plants would love you for it, though, particularly those that struggle with the ever-present nasties like flouride [like, say, every Dracaena], and with high total dissolved solids in general [like some orchids and most carnivorous plants]). But it is worth considering when choosing or mixing your own fertilizers. There are a lot of ones that you can find on the market that are in and around the 3-1-2 ratio (particularly if you're not looking at the major brands, which have got a good thing going with their 'root-' and 'bloom-boosters'), and you can give yourself a pat on the back for having used less of an increasingly scarce fertilizer. You may also save yourself some nutritional problems down the road. And if your plants aren't blooming as well as you'd like, there are other avenues to experiment with besides lowering your nitrogen (though it will probably help), like lowering your night temperature a little (tricky out of season, unfortunately), giving the plant more light, or reducing somewhat the amount of water you give it, all of which are known to promote reproductive growth. Some plants won't even set buds until some triggers are hit, like shortened day length or extended drought. Another reason to know what you're growing, I guess. 

On Biophilia and the Value of Interior Landscaping

`...that the naturalist`s journey will go on forever. That it is possible to spend a lifetime in a magellanic voyage around the trunk of a single tree. That as the exploration is pressed, it will engage more of the things close to the human heart and spirit. And if this much is true, it seems possible that the naturalist`s vision is only a specialized product of a biophilic instinct shared by all, that it can be elaborated to benefit more and more people. Humanity is exalted not because we are so far above other living creatures, but because knowing them well elevates the very concept of life.’

E.O. Wilson, from Biophilia                

You may have noticed that we reference and tag a word fairly often here at In Situ: biophilia. Biophilia as a concept was developed by the great Edward O. Wilson, biologist, ecologist and prize-winning author, who used it to describe humans’ innate need for affiliation with other living things. From the plants we have kept in our homes since at least the beginning of recorded history, to the out of work domestic animals we still keep around (there aren’t many professional mousers or herders among them these days, at least from an urban perspective- I’m sure there’s still work in the country), to the way we design our cities and parks, human beings have always surrounded ourselves with other organisms. Here’s another (admittedly long) quote by Wilson, who can put it all much more elegantly than I ever could:

‘I have suggested that the urge to affiliate with other forms of life is to some degree innate, hence deserves to be called biophilia. The evidence for the proposition is not strong in a formal scientific sense: the subject has not been studied enough in the scientific manner of hypothesis, deduction, and experimentation to let us be certain about it one way or the other. The biophilic tendency is nevertheless so clearly evinced in daily life and widely distributed as to deserve serious attention. It unfolds in the predictable fantasies and responses of individuals from early childhood onward. It cascades into repetitive patterns of culture across most or all societies, a consistency often noted in the literature of anthropology. These processes appear to be part of the programs of the brain. They are marked by the quickness and decisiveness with which we learn particular things about certain kinds of plants and animals. They are too consistent to be dismissed as the result of purely historical events working on a mental blank slate.’

E.O. Wilson, from Biophilia                

So while at the time the book Biophilia was published (1984), there had been no empirical study on the presence of an innate biophilic instinct shared by all of humanity. Since Wilson’s introduction of the hypothesis, many studies have been done that highlight the importance of proximity to nature and other living things to our mental health (see this literature review for a good discussion on many of the studies that have been done: Biophilia: Does Visual Contact with Nature Impact on Health and Well-Being?). Beyond all this, and at the risk of relying on intuition when so many of our human eccentricities are indeed counterintuitive, it just feels correct. At least to me (biased as I might be). Any client I’ve ever spoken with has always pleased with having plants around, and they often comment on how much better a place feels, which seems like mission accomplished and hypothesis confirmed to me.

But how did all this come about? Rooted in our history the habits may be, but the fact that humanity is itself rooted in the natural world is what has caused our deep-seated need to be surrounded by nature. We are the product of a particular habitat, and to this day we still find space in our urban centres for an approximation of it. Think of practically any city park you can imagine. Open grassy areas, with scattered copses of trees; sometimes a pond, fountain or the like. Maybe something somewhat reminiscent of this?

The cradle of humankind: the plains of Africa. Image © Gossipguy; retrieved from Wikimedia

Our species came to be in just such a habitat, and we still seek these same landscapes for comfort, relaxation and meditation. We select our homes in similar ways: perched atop a hill, overlooking water, with a few trees (not too many) here and there describes some of the most sought-after property available (and indeed will often fetch a hefty price).

Turn now to the indoors, where people have been keeping plants for at least as long as we've been recording history. All ancient civilizations have depictions of potted plants indoors in the images they created, and sometimes went through great lengths to cultivate plants difficult to grow outside of their native environment (the Romans were building greenhouses even before glass was invented). It is safe to assume that many of these were functional from a medicinal or culinary standpoint, though the Chinese have cultivated ornamental plants indoors for at least three thousand years. Plant mania swept homes and offices in the 1970s, to an extent that has not yet been rivaled (though what those early pioneers of the interior landscape industry would have made of vertical gardening technology!). The interior landscaping industry was born in this era, and has persisted since. 

Said industry has often toted the benefits of keeping plants indoors, primarily from a health and employee productivity standpoint (I went through some of the science that these claims are based on here), and some companies have begun to reference the biophilia concept as another selling feature. They are certainly right to do so, but I doubt that many who belong to these organizations have actually read and understood the ideas behind the concept, and are genuinely interested in fostering the sense of interest and wonder in the natural elements we surround ourselves with.

What In Situ is trying to do (and what we would like to encourage the rest of the industry to try to do) is to create more of those moments when nature really takes us in, where time falls away and we are free to explore with our senses the structure of a leaf, say, or the contrasting textures or colours of different plants growing together, to go on Wilson’s ‘magellanic voyage around the trunk of a single tree’. We wish to recreate the forest edge, viewed from our comfortable place amongst the figurative grasslands of our urban interiors, which draws us nearer, showing us glimmers of the mysteries held deeper within the forest. We want to replicate indoors the richness and splendour that has captivated us as a species forever, has inspired countless works of art, and that still, in the lives we live primarily apart from it, holds a special place in our imagination.

A photo of the Atlantic Forest of Brazil, a view of which prompted Darwin's
words to the right. Image source unknown; retrieved from Projeto Entre Serras.

‘Among the scenes which are deeply impressed on my mind, none exceed in sublimity the primeval forests undefaced by the hand of man; whether those of Brazil, where the powers of Life are predominant, or those of Tierra del Fuego, where Death and decay prevail. Both are temples filled with the varied productions of the God of Nature: -- no one can stand in these solitudes unmoved, and not feel that there is more in man than the mere breath of his body.’

Charles Darwin, from Voyage of the Beagle          

By using new and interesting species, and using familiar species in interesting ways, we hope to satisfy the biophilic instinct by completing indoor environments with the engaging natural elements that have been a part of our species’ evolution since time immemorial. When used this way, plants can create a kind of biotic warmth that tempers the sterility of many modern interiors. I don’t advocate turning every indoor space into a jungle (...): rather, the contrast between our manmade constructions and these natural elements are what highlights their presence and what really makes them come to life. The studies I referred to above seem to indicate that having these elements in sight from any area of an indoor space is the optimal placement, and this is the model that many in the industry use when designing their interior landscapes. It`s sound to me, but I feel it’s only effective if the elements are actually visually captivating: this is why we try whenever possible to use plants that people are not usually familiar with, and that have very unique textures or colours, or some other interesting facet to their biology that creates real interest. We seek to foster a true biophilia, through which we can draw inspiration, comfort and knowledge, secure in the surrounds of our earthly cohabitants. 

Some Notes on the Natural Habitat of Tillandsia spp., and Inferences Therefrom on their Care in the Interior Environment

Tillandsia recurvata growing on power lines, Tamaulipas, Mexico.

Image © 0+000; retrieved from Wikimedia Commons

While I don’t wish to discourage anyone from keeping Tillandsia at home, I think that it is important for people to develop a greater understanding of these fascinating plants in order for them to have greater success in their cultivation; they are not as easy of care as most retailers would have you believe, and some species, despite being common in cultivation, are unsuitable (or at best, very challenging) to grow indoors.

The genus Tillandsia is made up of more than 600 species, which is about a fifth of all species in the Bromeliaceae, which includes other favourites such as pineapple. They are distributed, as are nearly all bromeliads, across North, Central and South America (there is one species of Pitcairnea which made it Africa somehow). There is a large variety of form across the genus, as different species have adapted to widely different habitats, from montane rainforests that are some of the wettest places on earth, to arid coastal deserts that receive a scant 3mm of rain annually, at best, and knowledge of where a species originates is of great use when determining how to care for it. 

Cultivators of Tillandsia often lump species into two general categories: grey- and green-leaved varieties. This is a pretty good place to start, in terms of their care; the grey-leaved species are typically from more exposed, sunny habitats, and are evolved to make use of the sometimes very little water they receive in these environments, while green-leaved varieties are typically from more shaded, moister habitats. There are exceptions, of course, and so I reiterate that knowing where a species is from will be give you the best chance at success. 

Closeup of a Tillandsia sp., showing trichomes. 

Image © Josef Špaček; retrieved from Botany.cz

The grey colour is due to the leaves’ trichomes, which can perform quite a few functions in plants, but which in this case are designed to capture airborne moisture such as fog, as well as reflect sunlight (up to 45%, apparently). Species will have more or less of these depending on how water-deprived or exposed its habitat is. 

The ones more often in cultivation are typically the grey-leaved varieties (though Tillandsia cyanea is definitely another big player, even if it’s typically sold potted along with all the force-flowered Guzmania and Vriesea bromeliads), and I’ll mention a few things about their different habitats below.

Dry Tropical Forest

Many species inhabit dry tropical forest which sees precipitation for only part of the year. Examples of species include T. brachycaulos, T. caput-medusae, and T. xerographica. As epiphytes, these plants are found at different heights on host trees, where there is good air movement (which allows the plants to dry after rainfall) and varying amounts of sunlight depending on where in the tree a particular species is found (and indeed, different species seem to have different preferences for what part of the tree they occupy). These species, because of their requirement for good air movement, are not particularly suitable for most terrariums (despite what you’ve heard), though they do much better in higher humidity than is typically found indoors. A larger terrarium with air movement via a computer fan or the like (see here for some ideas on how to do this, with the understanding that your own endeavors with electricity in moist environments are obviously your own problem) would probably be best, and indeed that’s where I’m having the most success with plants from this type of habitat. Being hung outside under a tree through the summer months (at least here in Toronto) would probably treat them just fine as well.

A note on the seasonality of these habitats: these forests are marked by distinct dry seasons with little to no rain, during which time host trees may drop all their leaves, presumably exposing any harboured epiphytes to more direct sunlight. There doesn’t seem to be much written on coercing these plants to bloom, but in my experience an increase in light can often do the trick, and may well be the trigger for flowering that the plants use in nature.

Tillandsia landbeckii, growing where little else will.

Image © Eduardo Vergara; image retrieved from Flickr.

Xeric (dry) habitats

There are also many Tillandsia species in cultivation which are from disturbingly arid habitats. These are the stiff, very grey-leaved species that are adapted to take what they can get, water-wise: T. tectorum, T. albida and T. edithae are a few examples of species. I say disturbingly arid because these can be some of the driest habitats on earth: the Atacama coastal desert of western South America, for example, has a few weather stations that have NEVER RECEIVED RAIN. Tillandsia species from the Atacama have evolved to take advantage of the coastal fog that rolls in from the ocean, allowing the plants to capture airborne moisture. Some xeric species inhabit high elevations in the Andes, and take advantage of the clouds that float through their habitats.

These species are much more tolerant of dry air, direct sunlight and restricted moisture, but the converse to this is that they need a lot of sun and also cannot be kept too moist. South-facing windows (or, optimally, a solarium or greenhouse) are probably best in the case of these plants, and they are definitely not suited for most terrariums.

A note here on watering Tillandsia: these plants use CAM (Crassulacean acid metabolism) photosynthesis, which is really neat and interesting and worth talking about at length, but which for our purposes here means that they open their stomata to absorb carbon dioxide at night rather than during the day as do most other plants. Wetting the plants before nightfall can inhibit this gas exchange and in effect suffocate the plants. The xeric plants, at least, receive moisture in their native habitat before dawn, allowing them to absorb carbon dioxide and then liquid water before the demands of the desert day begin. I don’t advocate waking up at 4:00 a.m. for any reason (though maniacal plant care is, in my opinion, a better reason than most), but watering your Tillandsia in the morning is probably best practice. And water them well: aside from the really effective water-catchers like T. tectorum, the oft-recommended misting with a spray bottle isn’t going to do much for the plant: give it a good soaking with a watering can or a quick dunk in a bucket.

Tillandsia biflora in habitat. 

Image © Jean-Francois Brousseau; retrieved from Flickr

Moist Habitats

Though not as common as the ones you find piled haphazardly in a basket at your local florist or garden centre (can you tell that gets on my nerves?), these are my personal favourites. Even T. cyanea, which is pretty boring as far as these go, foliage-wise, is an easy to grow species that does well in a pot. These species are, as far as I know, strictly epiphytic, and inhabit some of the wettest habitats on earth. The aforementioned T. cyanea, T. flabellata and T. biflora are three that can be found with a little digging (some more than others).

Conditions should be moist and humid, but with very good air movement, and lighting can be lower than for species from the previous two habitats I mentioned, so if you’ve been losing plants due to a combination of low light and excessive moisture, you might want to try one of these. T. cyanea is a tough plant, but T. biflora, in my experience, is not, and best results will be had in a greenhouse or large terrarium with good air circulation.

There are obviously gradients to these habitats I’ve mentioned here, and research into what species you have will give you the best idea of how to care for it. A little experimentation never hurts either (OK, sometimes it does), and you may find that some species are quite forgiving in cultivation. The lesson here is that, at least nine times out of ten, your local purveyor of Tillandsia hasn’t the slightest idea of how to keep them alive over the long term (you can even buy them in furniture stores now, apparently: I saw a bunch of mounted ones piled crudely on top of each other in a West Elm store here in town), so you should do your homework if you’d like to have success with this interesting group of plants.

Ferns from Scratch, Part Two

Sporophyte fronds of what I`m presuming is Adiantum

peruvianum, doing their thing in one of In Situ's vertical

gardens. Image ©  In Situ Plants.

Just a short one here, but I’m pleased to report that the little gametophytes I wrote about back in June have started to produce their first fronds, AKA sporophytes (being the part of the plant that eventually produces the spore which gifted us with the gametophytes in the first place). Again, I can only presume that these are Adiantum peruvianum, as this is the only fern species in this garden, but there are a lot of other ferns here at the lab (to say nothing of the effectiveness of travel by spore; these ferns really could be from anywhere), and so I’m still not 100%. (And there’s actually a terrarium in another room of the building that appears to be growing something similar, so the plot could still thicken here.) 

They’re pretty cute, though, either way. As soon as we start to see more mature foliage on these plants I’ll update again with a more conclusive ID. The plants pictured here are way up at the top of the wall, and should produce a nice (albeit unplanned) cascading effect once they get going (again, presuming they’re even A. peruvianum).

Plants at Work- The Science Behind How Plants Improve Life Indoors

We, as an industry, nearly always tote the benefits of interior plants, and I’m here to tell you that it’s not just bullshit: there are measurable effects in the way people think, behave and feel when they are in an environment that contains plants versus one that does not, and plants actually are able to clean the air we breathe.

Instead of doing what everyone else does, which is usually just to concisely (we all know that’s my strong suit, ha ha) list the same key points, I’ve done the legwork and actually rustled up a few of the papers from which said points were drawn from, and will point you to them so that you can read for yourself the results of some of the various studies that have been conducted over time.

By all means be skeptical, and don’t take our word for it: we’re very few of us scientists in this industry, but there has been real scientific work done which really confirms what we’ve been saying all along: that plants indoors have a direct effect on things like employee productivity, reduction of airborne pollutants, and combating stress and fatigue.

Cleaning the Air

So for starters, do plants actually clean the air? It would be a boring blog post indeed if I said no, so here are a few papers which highlight some of the work that different plants (and their associated colonies of soil-dwelling microorganisms) do to remove harmful chemicals from the interior atmosphere. What I’m not going to do is tote the old NASA study that gets thrown around so often: you can look that up for yourself, but Dr. Wolverton (and others) have continued to do good research into this phenomenon since the first study was published in the 80’s. 

Plants and Soil Microorganisms: Removal of Formaldehyde, Xylene, and Ammonia from the Indoor Environment

This study found that quite a few plant species, notably Boston ferns, chrysanthemums, and dwarf date palms, were able to remove appreciable amounts of these chemicals from the air inside sealed chambers. Based on prior research into indoor air pollutants by the EPA, the

Figure 1 from the study linked above, showing formaldehyde
concentrations being removed by a Boston fern.

authors calculated that an average-sized office constructed of typical building materials would contain 3916 µg (micrograms) of formaldehyde (to use the most sinister example in the paper). A single Boston ferns was shown to remove 1863 µg of this formaldehyde- per hour! The mums and palms were not far behind, and there was a decent list of other plants which were also quite effective at removing formaldehyde from the air.

The other part of the study looked at the microorganisms which colonize the rhizosphere (the area immediately surrounding plants’ roots), and the role they play in the removal of these chemicals. They found that unsterilized soil was able to remove formaldehyde from the air while sterilized soil was not, and that soil containing a plant was more effective still. They found that different types of bacteria had an effect on how much formaldehyde was removed, and the data indicated that different plants harbour different types of soil bacteria. Check out the paper for yourself: I’ve linked to it above.

Purification Ability of Interior Plant for Removing of Indoor-Air Polluting Chemicals Using a Tin Oxide Gas Sensor (sic)

This study performed similar experiments to the one above (you can read it yourself for the full details), with a slightly different method. Their results were similar: plants and their associated bacterial communities removed airborne pollutants quite effectively from the atmosphere. One point of note is that temperature and light had a large effect on the experimental results, suggesting that plants are more effectively cleaning the air when they are actively growing (see the portion in the discussion on uptake of gases through stomata if you like), which really bolsters the case for optimizing plant health in the interior landscaping in order to maximize this beneficial effect.

Improving Employee Productivity

This is a claim that is often used because it seems to infer a real economic benefit to the client. I’m inclined to agree with the science, and I can see that this certainly makes interior landscaping more marketable, but it almost feels like a bribe: surely plants can be desirable of their own merit, and surely the effect they have on people should not be measured in terms of productivity but of general mental and physical health? Do clients actually purchase plants to get more out of their staff? At any rate, the effect has been measured in the following papers (and I’m sure there are more); let’s call it here just an added bonus to the addition of plants to the workplace.

Interior Plants May Improve Worker Productivity and Reduce Stress in a Windowless Environment

This is one of the commonly cited ones, in which the authors noted a 12% increase in productivity (measured as reaction time to a computer task). I’m not sure that this is really a rock-solid study, and I wish I could have found another paper which replicated the experiment, but it’s here, for what it’s worth. One more interesting point in the study is the result on the blood pressure of the participants, which measured significantly lower during and after completing a computer-based productivity task in participants in a room with plants versus that of those in a room without.

Effect of Ornamental Foliage Plants on Visual Fatigue Caused by Visual Display Terminal Operation

I’m hoping that someone’s Japanese is better than mine and they might comment on this paper, but based on the English abstract and the figures in the results, these researchers found that viewing plants while performing tasks on a visual display terminal (presumably a computer screen of some sort) resulted in reduced visual fatigue when measured as critical flicker fusion frequency (a somewhat complicated phenomenon that you can look up on your own). I can’t comment much on this one, as I can’t even read it, but the numbers are there.

Plants Enhance Productivity in Case of Creative Work

Figure 2 from the productivity paper above, showing the number of correct
associations by students who reported a high level of physical exhaustion.

This is more of a press release than an actual paper, I think, but it highlights the results of an experiment carried out by researchers in The Netherlands, which found that, while no improvements to productivity tasks were noted, there was a marked improvement in performance of creative tasks. These improvements were even more dramatic with test subjects with self-reported stress or exhaustion (the study used students as their guinea pigs).

General Health and Wellbeing

This is probably the most important one for me, because it has much to do with the concept of biophilia, which I will be addressing soon (likely at great length), and which is tied very closely to In Situ’s raison d'être. We believe that humans have an innate subconscious need for proximity to natural elements, and keeping plants indoors proves to be a noteworthy way of satisfying this in our modern urban settings.

General mental health seems a difficult thing to quantify, but the works below are able to convey a few measured benefits to having plants around us while spending, as we typically do, the majority of our time indoors.

Stress Recovery during Exposure to Natural and Urban Environments

Figure 2 from the study above, showing changes in pulse transit time
while watching first a gory video and then one of several others. 

This almost creepy lab study measured several parameters (heart rate, muscle tension, etc.) during and after showing the poor participants videos of people getting into violent industrial accidents, followed by a video of either a fast-moving stream, a wooded area, or varying degrees of busy vehicle or foot traffic. The results clearly showed that the wooded scene was very effective in recovery from the stress indicated in the physical tests.

The self-report from the participants also indicates that the nature scene was the most positively affective by far, and best able to reduce anger, aggression and sadness.

Greening the Great Indoors for Human Health and Wellbeing

This extensive study looked at various aspects of how keeping plants indoors relates to human well-being, from mental and physical standpoints. In section 5, the authors had their subjects complete Profile of Mood States questionnaires (apparently a widely accepted method for measuring different psychological states) before and after the placement of varying numbers of plants in their workspaces for a period of three months. The questionnaires covered such feelings as tension/anxiety, fatigue, and confusion.

The data shows that plants did in fact affect these parameters, and that the control group with no plants scored even worse on the questionnaire than it initially had done, while the subjects with plants saw their scores improve markedly.

The Role of Nature in the Context of the Workplace

This is also mostly a literature review, and includes quite a few statistics from other authors’ papers (which is why I’ve included it here), but the author points to two of her own studies, and I’d like to summarize here the gist of the second one: in a survey rating employee satisfaction, the availability of a view out of doors was considered far more valuable and restorative if it contained natural elements, and became even more so the more natural elements could be seen. Further to this, respondents with clear outside views to natural elements reported feeling more positive about their work in general. From the above:

“These results point to the range of impacts that a view of nature can affect. Those with a view of nature felt less frustrated and more patient, found their job more challenging, expressed greater enthusiasm for it, and reported higher life satisfaction as well as overall health.”

Pretty interesting stuff, I think. It will be interesting to stay on top of the science and see what further studies come from this quarter. If anyone has any further information on this they want to share (for or against, of course, though I bet you’d be hard pressed to find a study against plants in buildings), be sure to include it in the comments.