Potting soil/drainage inquiry

Asked February 21, 2020, 12:55 PM EST

Hi there, I am currently enrolled in the Master Gardeners training, and have just learned that adding course material at the bottom of pots makes the bottom of soil more saturated, and that it decreases rather than increases drainage. I am wondering if this is the case only if the course material is concentrated at the bottom of the pot, or if it is also problematic if it is spread throughout the pot. I'm thinking here about perlite in potting soil- does this help with drainage? Would a layer of perlite at the bottom of pots be better than other material to increase drainage? I'm planning on building a glass terrarium, and want to make sure I don't create root rot, but in a terrarium with no drainage holes at all, I am stumped about how to go about increasing drainage without using gravel/pebbles at the bottom. Thanks!

Washington County Oregon

1 Response

We always want to improve drainage but adding any rock or gravel or tiles below a soil in a pot above the drainage hole actually compromises the drainage and does not improve drainage. When a customer is overly worried about soil coming out of the drain hole I simply cover that hole with a chunk of shade cloth or panty hose. Even that is unnecessary. Soil just doesn't come out of that hole after the first watering. Otherwise, and I have given lectures/seminars for Master Gardener classes as to the WHY. Adding material with larger pore spaces than the soil actually causes a perched water table...upside down but the principles are the same. That is amazingly deleterious to drainage. The small pores of the soil have to become saturated before the water even begins to enter the larger pore spaces of gravel, rocks, packing peanuts or tile. Not at all a healthy situation for potted plants. This has been one of the biggest myths for plants in pots that has been very tough to change. Even among Master Gardeners. I don't like even panty hose. I also insist that the bottom of pots be raised off the surface of the saucer or patio or whatever that pot is sitting upon. THAT truly enhances drainage. The ONLY time plants should be planted in pots or terrariums without drain holes are the pots of bonsai and succulents. Bonsai needs to be watered daily (very different than all other plants) and succulents only need the surface of the soil where their shallow roots reside sprayed/moistened or the water below roots will cause rot and anaerobic conditions. And by golly, I am so tired trying to get people to understand garden soil mixed with whatever is not meant ever ever ever for use in pots ,potted plants! I am slowly getting very experienced and educated gardeners to see the light. Good Luck with training and you can always check there for local experts on this matter of terariums.