You could have started them in the 20 gallon pots, and the plants wouldn't have known the difference. But the small pots are vastly more convenient for YOU. If a plant is small, it doesn't "know" what pot size it is in. If having them in 20 gallon pots is now convenient for you (probably in that you aren't moving them around or trying to keep them under lights), go for it.

It's not like clothing sizes, where you want to get a size that fits.

I think Dan knows that I will disagree with him on this since I and others have posted about it so often.

Two big disadvantages - one you skip the triggering of the fibrous feeder roots development when you start in the growing container. The plant develops its tap root but only slowly and to a much less degree, the feeder roots it needs.

And two it stresses the young seedling unnecessarily with the inconsistent soil moisture levels in the container. Water perches (ponds) relative to the depth and size of the pot. It reaches a level where it resists draining regardless of the soil type and the capillary action of the soil to wick water up to the roots is cancelled out by the effects of gravity. The bigger the pot the lower the water table. Perched water in a big pot is unavailable to the plant's roots so it alternates between being exposed to far too much wet soil for it to cope with to far too dry soil as the water all drains to the bottom where its roots can't reach.

A bit more work for the grower perhaps but there are distinct advantages for the plant to keeping the container size somewhat relative to the size of the plant's root development.

Dave

I grow tomatoes in containers. I normally transfer them into a larger, interim container before placing them in the final (and largest) container. I think the reasons cited by Dave make sense. It never made sense for me to put the tomatoes into the largest and final container from a small pot. Others may disagree.

Nowadays I use grow bags. What I do is I fold/tuck the grow bag to make it smaller and fill it with soil and then the transplanted tomato plant. As the tomato plant gets larger, I untuck/unfold the bag to increase the size and add more soil and compost. This is my lazy way but it works. Also, I'm in the city, so using grow bags means less storage space in the off-season.

Dave makes good points, especially about perched water. But that just means that with larger containers, you have to be a bit more careful about keeping the soil moist. Not sure about the point about roots. Maybe it's just for tomatoes, but the same argument about skipping feeder root development can't apply to any vegetable that you direct-seed.

Hi Mike

Both previous answers have good points, but neither argument is absolute. This concept of "Perched Water" for example, is more applicable to self watering pots or pots with flooded bases, like nursery flats. Soil will wick up water to its height vs. gravity due to capillary action not due to the size of the pot, but rather the height of the soil and its composition (wicking ability and capacity) and the evaporation rate.

So if you start in Solo cups with a fill height of 4" you are probably marginally better off than starting in trade gallon 6.5" diameter "nursery" pots filled to the typical 5" in those (Classic 800S nursery pots are the ones I have). Further these nursery pots have typical large perforations going 7/8" up the side from the bottom, making them at worst just as good as a Solo cup bottom perforated.

I have taken plants from beginning to end in those nursery pots when playing catch-up with a last minute addition, reliably, and as long as you keep an eye on them, those pots are really convenient. Solo cups are a bit cheaper and easier to bring indoors and put under a lamp so I have different reasons for preferring them.

Dave and Dan know their tomatoes. When commenting, we really have no idea what your conditions are so Dave's is the less risky option, and Dan's is the simpler option with the TLC.

If you are in a 4" pot going to the container, to be practical, and have roots reaching or coming out of the bottom, let's say it is 3 1/4" tall. Don't overfill your container and bury the seedling up to its ears in there and that can keep things healthy and simple. As it grows larger you can be adding compost or other amendments and fertilizer and do a wonderful job rather than looking for a rule book, really there are so many successful approaches to transplanting.

Cheers

PC

I also think that interim potting up is not necessary. We all transplant from those 4" pots ( or solo cups ) directly into the garden all the time.

Sey

Thank you all. We have about 150 tomato plants between 12 and 24 inches tall. Hope to give a lot away, probably keep 30 or 40. would love to pot some up and put them out. But have winds in the 40 mile an hour range again today. we have some in 1 gallon pots already. We will probably just pot up a half dozen or so, to put back in the greenhouse as it gets colder. All of those points you kind folks have made make sense.

Thank You

Mike

One thing do the same rules follow for peppers. The growth rate being so different. The earliest tomatoes, and peppers I planted at the same time are no where close to the same size.

thank you Mike

As long as a plant has at least 2 sets of moderately well developed true leaves on it, I feel fine about planting out into the garden or into large containers. For me, the issue is more about whether the outdoor conditions are correct than about what size the plant is. And a tomato, as long as it's not a very young seedling, is usually going to do better with more soil to grow into than in a pot, provided conditions outside are good for it, i.e., no reason to rush a young plant out into the ground if temps are going to be too cool for growth, but if temps are warm enough, a plant will really take off in the ground/large container outdoors in a way that it won't in a pot indoors.

Pest conditions matter, too, though. If there is something in the garden that likes to eat tomatoes, a larger plant will withstand attacks better. In my garden, I have almost no tomato pests in the early part of the season, so that figures into the calculations for me.

Developing a network of fibrous feeder roots may be important when the final destination for the plant is in-ground. In containers it seems less so to me, as watering and feeding are something that you have to do with more frequency and regularity in containers anyway, and in the limited available space, the plant's roots are going to colonize available space regardless. But that's just my limited observations--Dave has much more experience growing in general and in containers specifically.

Like Sey points out... we plant 4 inch potted tomato plants into the garden all the time. Heck, I sow tomato seeds into a six inch pot and they sty in these pots for 4 weeks and then directly into the garden.

For plants that have fast growing roots s tomatoes do, planting a small plant into a large pot is not a big deal. Perched water tables, roots resting in too wet of media for too long is just not a big deal for fast growing plants.