One ingredient in your recipe for seed-starting mix is screened compost. Shouldn’t the compost be sterilized somehow to prevent fungus? Everything I’ve read tells me that damping off will occur with young seedlings if the starting medium contains fungus or harmful bacteria.
The seed-starting blend we use when we grow seedlings for the Organic Gardening Test Garden is about 50 percent compost. We do not sterilize the compost because to do so would kill all the beneficial soil microorganisms as well as the few organisms that are potentially disease-causing.
That means we have to use some extra common sense and diligence to prevent damping off. We avoid adding any obviously diseased plant debris to the compost piles. We take care to grow our seedlings where they get plenty of sunlight and air circulation, and we space them to avoid crowding in the flats. We rarely have problems with damping off, and when we do, it’s usually our fault from overwatering.
Damping off is a fungal disease of very young seedlings. If seedlings suddenly topple over (and appear pinched at the base of their stems), it’s probably damping off. If this happens, immediately transplant the survivors to fresh potting mix. If damping off is a chronic problem for you, try using a seed-starting mix that has more sphagnum peat moss in it, and cover the seeds with pure milled sphagnum instead of more seed-starting mix. You can use sphagnum from a bale of peat moss, which is typically ground fairly fine. It’s easiest to dust it over the seeds dry and then lightly mist the surface to moisten it.
When you think about it, soil outdoors is just as full of microorganisms as our unsterilized seed-starting mix—yet seedlings in nature manage to prosper and grow. It’s only when we have given them less-than-ideal growing conditions that damping off becomes a problem. —Doug Hall