https://swfrec.ifas.ufl.edu/docs/pdf/ve ... ns_cs1.pdf

So, the above-linked PDF is a science article that talks about what happens to plants when they have smaller containers. Something of interest for us, however, is how it talks about getting better post-transplant results (including production) from seedlings started in larger cells (as opposed to smaller cells).

I'm guessing growing conditions matter a lot (both pre-transplant and post-transplant).

I tend to use large cells, as they're less risky (small ones dry out faster, and I like a comfort window). I should do some experiments with various container-sizes next year. It's a little too late, this year.

I generally start in 1 3/4 x 3 inch deep containers. If I am running low on starting mix I use 1 x 1 inch containers. I don't see a difference in the starts. The smaller ones require more work because I have to up pot sooner. I have no idea how it affects the starts after they are planted. With a peat heavy starting mix the small containers hold a lot of water.

Recently I direct sowed about six seeds in a 15 gal container. As the seeds started sprouting I yanked all but one. I noticed the root was just a tap root about an inch long. I have never noticed this root behavior in any of my conventional starts.

Well, that makes me glad I used larger pots this year.

I don't think the size of cell matters as long as plants are potted up when the time comes, and not left to languish in tiny cells. I've experimented a bit with different cell sizes and transplant times for tomatoes, they all seem to work well iff transplanted when ready. Peppers seem to do best in smaller cells at first, because they're much slower to develop, at least in my temperatures, so I use the nine-cells (72/1020). Other crops get different cell sizes depending on how long I expect to keep them in the tray and how large I want the starts to be. Sixpacks (48/1020) work well for brassicas and lettuce. Anything larger seeded like zucchini for example gets a 4 pack (32/1020). This year I had the farm tomatoes in some bigger, slightly deeper sixpacks (36/1020) and I was amazed how big they grew and how long they held at that size - we were about a week and a half past the optimal moment to transplant them, but they didn't suffer too much. By contrast my tomatoes in the smaller sixpacks needed potting up a week earlier than the date the others were 'ready', and went into beer cups. At the end of the day, all of the plants look just as large, hale and hearty as one another regardless of the start they got.

I've heard about the tomato taproot and the necessity to start in large containers for that. But this root type is only important if you have deep soil and a long season, or if you're dry farming. For me it would not be a goal to develop a taproot, since this kind of root is less happy in a container. And even if I were growing in the ground, our short cool season means that shallow spreading roots are more advantageous for the plant.

I agree with Bower that the size of the starting cell is not as important as the transfer cup. I am transferring this year into both 18 oz Solo type cups and 32 oz yogurt cups. The difference in health of the plants is obvious. They prefer the large yogurt cups but do fine in the 18 oz. And I started all mine in damp paper towels, then 7 oz yogurt cups before finally to the larger cups.

I don't think the size of cell matters as long as plants are potted up when the time comes, and not left to languish in tiny cells. This is a quote from Bower and I agree.

This article deals with a wide variety of vegetables and flowers, mostly in a commercial setting rather than home gardeners starting seeds for home use. It even states that excessive time spent in a small pot has consequences. Home growers who pot up at the proper times and then put the seedlings in the garden or large container will not have the problems of commercial seedling sellers. This article is very general for all sorts of plants meant for the commercial growing audience, not the home gardener.

My way is not broke so it will not get fixed: small six packs with a dozen or more seeds; then individual 1.5"X1.5"X3" deep pots for each seedling, and then to the garden at 10'-12" plant. No problem with root restriction or lack of production. A commercial grower will absolutely worry about production a whole lot more than I will.

Transfer- I try to use plastic dishpans with soil.
Indoors as is,for outdoor use I switch over to ones I melt holes in the bottom for drainage.
It makes a big difference using lots of soil,especially if you start seeds late.

I start a LOT of tomatoes and peppers - 450 varieties of tomatoes this year. I start the seeds in used coffee K-cups, which fit perfectly in 50-cell trays. That way I can easily move slow or non- starters without disturbing them, which is a space boon for me. But I put as many as fifty seeds in a K-cup (Sungolds, by far my biggest seller at market) and transplant into 2" soil blocks when most have tiny true leaves showing. They take right off after transplant, and stay in the soil blocks until they are sold or planted in my garden, usually without any additional fertilization. I have planted out 12"-15" tall plants, living happily in the soil block. Again, this is a huge space saver for me, with no apparent detriment to the plants.