Houseplants
Yes, I have lumped pretty much all houseplants in together as I treat them all the same whether they are epiphytic or terrestrial. I never use ‘houseplant compost’ straight out of the bag (in fact I don’t use it at all anymore, and I’ll be explaining why later). I have experimented with many houseplant composts, and I have to say that I don’t particularly get on with any of them (no surprise there, I hear you say). Most composts, straight out of the bag, I find much too heavy and water retentive for most houseplants. The main reason for this is that most tropical plants that we grow in our homes are adapted to what, for want of a better word, we might call ‘woodsy’ soil. Most of our houseplants are well adapted to low light situations, and those conditions are typically found in forests where the trees provide shade from the sun, and our beloved houseplants (I really dislike that word) are understorey plants (or occasionally epiphytes as is usually the case with orchids). They grow mostly in the detritus of the forest floor, so leaf litter, decomposing wood, etc.
I have found a few houseplant composts that have an amount (sometimes a large amount) of loam. I don’t disagree with this in principle but it tends to be unsuitable for many houseplants as it doesn’t replicate the conditions the plants naturally grow in. Loamy compost is heavy, often water retentive (the sand particles knit together making the mix very dense). for plants that naturally grow in loamy soil (such as our native plants), this is no problem, but tropical understorey plants need much more air around their roots, as well as moisture, for them to truly thrive.
Again, many of the problems associated with so called houseplant composts comes down to how they are treated before purchase. They typically arrive at the garden centre on a pallet along with the outdoor plant composts and are also often kept outside with them. As I have mentioned before, I firmly believe that no compost should be kept outside to become saturated by the rain and houseplant compost is no exception. I don’t see that it would be impertinent to ask before purchase where the compost has come from and how it has been stored. We surely all want the best for our plants; how are we to provide this if we are being sold products that have not been treated properly?
The advantage to using a proprietary mix is that it is sold in small bags (typically 10 liters) which makes them easy to carry. Also, very few people have enough houseplants (naturally I exclude myself from this generalisation) to warrant buying large quantities.
There are even more specialist composts on the market (such as for Streptocarpus, Citrus, Cacti, Bonsai, etc) but I mostly have not experimented with them as I don’t grow these plants (and many of them are terrestrials or trees and are better adapted to loamy soils, so the above doesn’t apply to them), so I won’t discuss them here.
From my experience, the single most important factor in growing healthy houseplants is aerated mix. This is not the same as placing crocks at the bottom of a pot, which has no effect other than removing space in the pot that could otherwise be taken up with growing medium and roots. In a poorly aerated mix, the roots can only really function in those areas of the pot that receive sufficient air; i.e. the sides, the top and possibly the bottom. This severely restricts the amount of root volume the plant will have and its general health. When you add to this the excessive water and compaction present in many pots, it is no wonder many houseplants do not thrive. A properly aerated mix allows the roots they entire volume of the pot to expand into, allowing the plant to reach its true potential.
So how do we achieve this? It’s easy. As I’ve stated earlier, I use the same mix for all of my houseplants. One third, or slightly more, of my potting mix (notice I’ve stopped using the word ‘compost’ because it is very misleading) is bog standard multi purpose compost from the garden centre (doesn’t seem to matter at all which brand, so you can go for a peat free option if you wished to). I like to keep a bag indoors so I can mix my potting medium fresh whenever I need any. This way it is always warm and fairly dry so it doesn’t compost down and doesn’t attract flies. Another third of my potting mix is fine bark chippings (Melcourt’s orchid propagating bark). If you can’t get hold of this, you can use a garden centre orchid compost, it will do roughly the same job. Now this is where it gets a little tricky because the final third of my potting mix is chipped scrap plastic. I have an ample supply of this as it is a waste product from my workplace, and would otherwise end up in landfill. I like to use this as it is nice and light and seems to provide good aeration. I’m well aware that this isn’t a product that can be easily obtained elsewhere, but don’t panic, you can replace this with coarse grit. I used to use grit, but it seemed senseless to pay for a product when I could replace it with something I can get for nothing.
Having said that tropical plants need a ‘woodsy’ potting mix, it would be a mistake to provide them with a 100% organic mix. If I used an altogether organic mix, I would achieve good results in the short term, but as the mix started to decompose it would compact and the aeration would diminish, eventually choking the roots. With the best will in the world, no potting mix is going to last forever, but we can take easy steps to keep it in good condition for as long as possible. Decomposed mix is only a problem if it is compacted and aeration is lost. By having what might be considered a relatively large percentage of inorganic material, we keep that aeration. Remember that two thirds of this mix is quite large particles, as another third is bark chips. This will keep the the mix aerated for quite some time. Remember also that this mix will not stay so wet as many so decomposition will be much slower. If the mix remains fresh for longer, this means that our plants will need repotting much less frequently (apart from when they outgrow their containers).
I usually include a handful of slow release fertilizer to my potting mix too, just to give the plants a headstart, but it isn’t essential. I’ll tackle feeding in another article.
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