So that's what we planned to do with this year's seedlings. However, as soon as our first batch of seedlings (parsley) started to poke through the seed-starting mixture in our little seed-starting tray, we discovered the difficulties with this approach. For one thing, what would we use for the larger containers? Paper cups? Would those fit in our juice-carton containers? Would we have to scrap our entire seed-starting setup and switch to commercial seedling trays? And even once we found suitable containers, how well would the seedlings survive the transplanting process? It's always seemed to me that plants thrive best when they can grow where they first take root (hence my frustration that the weeds in my garden, including vegetables that seed themselves from last year's crop, always seem to outcompete the carefully-nurtured seedlings). Each time they're transplanted, some of them don't make it—so why would we adopt a system that requires each seedling to be transplanted twice, once from starter to seedling pot and once more out into the garden?
While mulling over this problem, I came across an article on the Weekend Gardener web magazine (unaffiliated, so far as I know, with the book by the same name that I've described elsewhere on this blog as my gardening bible). The author, Hilary Rinaldi, says she doesn't bother with the "traditional routine" of seed starting, which involves starting seeds in one tray and moving them to larger containers. Instead, she starts them directly in the larger containers where they will live until they're set out in the garden, thus allowing the root ball to stay intact and producing "healthier, sturdier plants." The key, she says, is to use a potting mix that's "fluffy and light," but also rich in organic matter. It should contain a combination of soil, vermiculite or perlite, and peat moss. She adds that it's also important to premoisten the potting mix before it goes into the containers. (We've observed ourselves that it's very difficult to water seedlings in dry potting mix; because it's so loose, the water sort of flows under the soil rather than sinking into it, creating a puddle with dry soil floating on top. We've taken to watering our seedlings with a spray mister.)
This sounded like an appealing idea, but we puzzled over where to find a potting mixture that would meet her requirements. Neither bagged potting soil nor seed-starting mix seemed to fit the bill; the potting soil wasn't loose enough, and the seed-starting mix was lacking in nutrients. We could have tried making our own by buying compost, vermiculite, and peat moss separately and mixing them together, but some of these components are hard to find in stores, and even if we did, it would be a big and messy job. Then Brian got the idea that maybe the thing to do was to take the bagged soil and seed-starting mix we already had and simply combine them in a one-to-one ratio. The potting mix contains about half peat moss and half vermiculite, and the soil is just soil, so the two products together would give us the mix of materials that Rinaldi was recommending. And since we already had the ingredients, it would be a lot easier and cheaper than putting something together from scratch.
So this is our new plan: each time we start seedlings from now on, we'll start by mixing equal parts potting soil and seed-starting mix, moistening it, and loading it into the seed-starting tubes. Then the seeds will go straight into the tubes, and there they'll stay until it's time to put them in the ground. We'll test this mixture with both indoor seedlings grown under our grow light and seedlings that are winter sown (grown outdoors in miniature greenhouses made from plastic jugs) and see how it performs under different conditions.
Of course, it was too late to try this with the parsley seedlings we'd already started, so Brian compromised; he filled up a new set of tubes about two-thirds full with plain potting soil, then carefully removed the parsley seedlings, along with about a third of the seed-starting mix they were in, and popped that in right on top of the potting soil in the new tubes. So they'll now have seed-starting mix on top and more nourishing soil underneath, which we hope will allow them to draw up the nutrients they need from the soil as their roots spread. I'll keep you posted on how it goes with them and with other seeds grown according to our new method.