If you grow the seedlings in a warm environment, 6-8 week old seedlings will be half a foot high. They'd be grossly pot-bound in a 2.5" pot, and even somewhat pot-bound in a 4" pot. Now, pot-bound tomatoes that are planted out generally do OK eventually, but they are stunted. I've found that those tomatoes in the tiny six-pack seedling planters you see in the stores manage to look reasonably healthy, but if you plant them alongside a seedling that was given more room, they don't grow as fast. I guess if you know that, you'd be less inclined to buy grossly pot-bound tomato seedlings.

Is there some reason to wait 6-8 weeks? Lately, I put my seedlings in 3" diameter paper cups, where it is warm, and plant out at 4 weeks, when they are about 3-4 inches tall.

I start mine in cell packs and pot up to 4" square pots after 2 weeks or so. I agree that six weeks would be the maximum age that I'd shoot for. I prefer mine to be somewhere around 35 days give or take.

Thank you! If my plant sale spans 2 weeks, should I shoot for them being 4 weeks at the start of the sale, then 6 weeks at the tail-end?

You need to think about plant size, not plant age. In a 4" pot, you're shooting for 4" tall plants. If it's warm where you grow them, it'll just take a few weeks to get there, and 4-6 weeks of growth would be hideously pot-bound. If it's a lot cooler, it could take two months.

4 weeks of growth would not be close to root bound in a 4" pot if grown in cool ~65 temperatures. At 6 weeks they might start getting root bound but not enough to stunt them. I would shoot for 5" plants at 4 weeks and they should be 8-9" by week 6.

I grow my seedlings at 80F. 8-9 inch plants don't belong in 4 inch containers. I sure wouldn't buy them.

I wouldn't sell them to you either :-p

But the truth is that a 8-9" tomato in a 4" container is fine.

I plan 5 weeks start to finish. I start them in 72 cell trays. I pot them up into 3" pots (1801 inserts) 2 weeks before I want to sell them - otherwise they get too big. They are perfect selling size at that point (7-8" tall), and due to the compost based soil we use, will hold well for a week or two for customers. It's amazing how the growth slows once they are removed from the greenhouse...

That's exactly right. What no one here seems to appreciate is that time-to-maturity for tomato seedlings is a steep function of temperature. You can't talk about time-to-size without talking about temperature.

An 8-9" tomato in a 4"pot won't grow as fast when planted out as a seedling treated better. It's pre-stunted.

An 8-9" tomato in a 4"pot won't grow as fast when planted out as a seedling treated better. It's pre-stunted.

Not true.

Most of my transplants are typically that size. I pull off lower leaves and plant deep. They continue growing with no slowdown.

True for me. I usually plant out when they are 4" high, but some get away from me, and I don't plant out until they are 8" high. The difference in growth after plantout is obvious.

If you don't let them get that big, you don't need to pull off leaves. But I guess you can include that in the instructions when they are sold. "These are too big. Pull off lower leaves and plant deeply!"