Does a drainage layer matter? Two jars, three months
Every guide insists on a drainage layer: a couple of centimetres of gravel or expanded clay at the bottom of the jar, a piece of mesh or fine net over it, then charcoal, then substrate. The argument is that excess water collects in the gravel instead of waterlogging the roots. The counter-argument I have read is that in a sealed jar there is no excess water unless you put it there, so the layer just steals height. I set up two jars on the 17th of November to find out which was true for me.
Both were one-litre pickle jars with the same mix of coir, fine bark and a little horticultural sand, the same two pinches of crushed charcoal, the same cushion moss and a small cutting of a creeping fig. Jar one had 2 centimetres of pea gravel under a circle of mesh cut from an old onion bag. Jar two had the substrate laid straight onto the glass. Each got 30 millilitres of rainwater, measured with a syringe.
At twelve weeks. The gravel jar has a visible water table: about 5 millimetres standing in the gravel most mornings, dropping to nothing by evening in a warm room. The substrate above it looks evenly damp. The no-gravel jar has a darker, wetter band of substrate at the very bottom, about a centimetre deep, with a few black specks that I think are the beginnings of anaerobic rot, and a faint sour smell when I opened it to check. The moss in both jars is equally green. The fig cutting in the gravel jar has rooted and put out two new leaves; the one in the no-gravel jar has rooted and put out one.
So the layer does something, mostly for the bottom centimetre of substrate, and it probably matters more in a tall jar with more soil than in a shallow one. I have also noticed that the gravel jar is easier to read: I can see at a glance whether there is standing water and judge whether I overdid it. That alone is worth 2 centimetres of height. New jars get gravel. The no-gravel jar has been given a pinch more charcoal through a funnel and left to see whether the sour patch resolves on its own.