From what I can tell, collards have a shallow fibrous root system. They may or may not have the same taproot structure as Swiss Chard, which is in the same family.

The Root Development of Vegetable Crops is a comprehensive survey of many vegetable root systems. From it I chose Swiss Chard, but maybe there's something closer to collards in the list.

Collards are in the cabbage family, along with broccoli, cauliflower, brussels sprouts and such.

Swiss chard is in the beet family, which would explain the tap root.

Maybe that helps.

Jim

I say, that if it can grow in Mel's square foot gardening with only 6-8 inches of soil, than it can grow in a pot.

Jimster, I suspect they all have long taproots in that big family of plants. Take a look at cabbage at that link. But I'm not sure how important it really is to have enough soil-depth for a taproot since there are so many other roots that are shallow.

I grew Kale in the what was stamped 1600 and was 11.5" in dia at the top by 10.25" high by 9.75(+ or - a touch). I planted 3 plants in potting mix with SRF mixed in and they grew OK. I was cutting them pretty regular and letting them come back and I let them go for a little over 3 months. The ones I planted in 18 gal. totes are still producing but I really have them crowded to much. When I pulled the ones out of the round plastic pots the roots had pretty much filled it up, but I do not recall seeing a tap root.
I plan to use them for brocollii this week coming at one plant per pot. provided I can find the seeds LOL

That gave me an idea. I don't get my milk in jugs, but with five cats I use a lot of plastic containers of kitty litter. If I cut off the top and poke some holes in the bottom, they will make perfectly fine containers.

And they won't "litter" a landfill, either.

Collard is not at all 'related' to chard; it is much closer to mustards and radishes than to the beet family. Collard is non-heading cabbage. It is a quite big plant when mature - much too big for a gallon container, so if grown in one it will not reach a large size.

I put these in a milk jug about 2 weeks ago and this is how it looks inside the jug

You'll get collards, but you won't get very much.

Just the right size to harden off and plant out. Or just consider them microgreens.

I was going to ask: how big do you want these to get? Are you going to use them small? My experience is that they can get very large and have a larger root system. But if you are going to use them small then your idea may be OK.