Gardeners' Holidays 2022: Seed-Starting Day, with a new twist

Our gardening schedule for this year was a bit disrupted by a snowstorm that hit near the end of January. Last weekend, our yard was full of snow, so we had no chance to dig out the dirt we'd need for seed starting. So it wasn't until today, with the snow mostly melted that Brian finally managed to extract some dirt for this purpose. And under normal circumstances, his next move would be to bake it in the oven to sterilize it before adding it to the seed starting tubes with a layer of commercial seed starting mix on top.

Except...we're not actually planning to do that this year.

See, this whole seed-starting method was kind of cobbled together in the first place. We'd had disappointing results using plain seed starting mix, which doesn't provide nutrients for the growing seedlings, and we didn't want the hassle (and the risk of transplant shock) of transplanting them into real garden soil after starting them. So after some research, I dug up a fact sheet from Purdue that suggests two methods that sounded feasible: either start seeds in "a mixture of about one-third loam garden soil and two- thirds vermiculite...sterilized before seeds are planted in it," or "partially fill a flat or pot with sterilized soil mix, and then top it with a layer of vermiculite or milled sphagnum moss." We combined these two approaches, using baked garden soil and sterile potting mix on top, and this gave us pretty good results.

But I'm beginning to wonder whether the baking step is really useful. I came across an article on a gardening blog that claims baking soil in the oven, as Purdue recommends, doesn't truly sterilize it; there are still some microbes remaining that will reproduce fast, and the ones that return fastest are the fungi that cause damping-off disease. So it can actually do more harm than good.

Now, we haven't actually had any problems with damping-off while using this system. And normally I would say, if it ain't broke, don't fix it. But baking the dirt for seed starting really is a smelly nuisance, and if it's not doing our seedlings any good, then I'd be happy to skip it and simplify the process. On the other hand, I don't want to risk all our seedlings on an untested method.

I discussed this with Brian, and he suggested an experiment. He routinely starts twice the number of seedlings we expect to need for each crop, just to make sure we have plenty of healthy ones, so he proposed dividing those seedlings into two batches. Both batches would start in a layer of sterile seed-starting mix, but one batch would have that mix sitting on top of a layer of plain, unbaked garden soil — microorganisms and all. And the other would have a layer of commercial potting soil on the bottom.

This method, we think, would be pretty close to the start-and-transplant method originally suggested to us by the landscaper we hired in 2012. The seeds would sprout in the potting mix, which is (reasonably) sterile and lightweight. Then, as their roots developed, they'd move down into either potting soil (still fairly light and fluffy, but less rich in nutrients) or garden soil (richer but heavier, and containing a mix of microorganisms that could be both helpful and harmful). And as they grew, we could compare the two batches to see which type of soil produced healthier plants.

Unfortunately, to conduct this experiment we need to acquire some potting soil first. We don't have any at the moment, and we didn't come up with this idea until after we'd come home from our weekend shopping. Which means today isn't really Seed-Starting Day; it's just Coming-Up-With-A-New-Idea-For-Seed-Starting Day.

I'll keep you posted as we put this new method into practice, and by the time Planting Day rolls around in March, I should be able to give you the lowdown on how the Battle of the Potting Media turned out. Stay tuned!