Seems to me it might be a lot easier just to put the fabric pot in a plastic trash bag. That would hold in the moisture at the edges. You could cut off the bottom of the trash bag if you're concerned about drainage.

Frankly, I'm interested in the bags because I've already got them lying around, and buying the number of hard pots I'd need would probably cost me $100 or more.

I've never had a problem with peppers becoming rootbound, although I've grown them in smaller hard pots before and felt that the size of the pot was limiting the plants' growth. I can see that this could be an issue with many other types of plants.

I've never had a problem with excess water retention. As I said, I have the opposite problem: that in hot weather I can't keep the soil sufficiently moist without watering everything more than once a day.

Lining the sides with plastic is certainly a solution except that it might be a bit awkward/a bit of a hassle keeping it in place while filling up the pot. if you can find plastic bags with the same diameter as the pot, it would be easy to cut off the bottom of the plastic and then insert in the grow bags/fabric pots. You can of course staple polythene sheets to the sides to keep them in place as well.

Again, putting plastic INSIDE the bags is big trouble. You need to empty the bags out entirely, and fill them back up again, destroying your plantings, and hoping the lining doesn't cave as you refill. Much easier to just line the OUTSIDE of the bags with plastic, like with a garbage bag, preserving whatever you have planted in them. The moisture in the bag will be held in just as well in either case. Of course, the plastic on the outside will be exposed to UV, and will degrade in a season or two. No big deal. Just put on a second layer.

Would these solutions cause the pots to heat up inside more in Summer?

Not sure this is really a big "problem" you can improve on much. Water twice a day. Some sort of organic mulch on top. You could try putting the bags on plastic saucers that hold water and hope the water wicks into the bag. Buy a couple of plastic pots and compare results with the fabric bags next year.

If you used a white plastic garbage bag to surround the fabric pots, the fabric pots would heat up a lot less. Putting black plastic over a dark colored fabric pot wouldn't really make that much of a difference. No question that either strategy would prevent the outer two inches of soil from drying out. That's a lot of growing medium.

@getgoing100_7b_nj, I figure I'll line the inside with a piece of waxed paper and hold it in place with one hand while I toss in a little soil with the other. Then the soil will hold the paper in place while I fill the pot. I've done similar things before for other purposes.

@daninthedirt (USDA 8b, HZ9, CentTX, Sunset z30, Cfa), I think you misunderstood what I proposing. I'm transplanting these babies to larger containers, whatever else I do. I'll probably use 7 gallon grow bags, about a foot across and 14 inches high. (They're in 1.5 gallon bags now.)

I've wondered about the black fabric bags heating up too much in the sun. I've tried putting one or two of them inside slightly larger pots so that sunlight doesn't hit the sides, but I'm not sure whether it made a difference. I didn't consider putting a moisture barrier on the outside of the pot; it would eliminate the black exterior, but I'd have to find a material other than wax paper because it would it would curl and tear after a short time.

@robert567, comparing fabric bags to plastic pots is a good idea, and I'm going to do that. I have a couple of plastic pots about the right size... not nearly enough to use them for the whole job, though.

Maybe I'm still misunderstanding, but what I'm suggesting is to put your grow bags in plastic garbage bags to prevent surface evaporation. You can get fifty 15 gallon white garbage bags inside of which your larger 7 gallon grow bags will easily fit, for a few bucks. You were talking about lining the inside wall of the grow bag with plastic before planting. I'm saying that lining the OUTSIDE of the bag in this way is far easier to do and more manageable. In transferring to a larger container, many people just plant the grow bag. Most roots will penetrate the grow bag, and expand into the larger container. If you have plastic on the inside wall of the grow bag, that won't work. If you have plastic on the outside wall, it's easily removed, and will then work fine.

@daninthedirt (USDA 8b, HZ9, CentTX, Sunset z30, Cfa), I understand what you mean now. I'm reluctant to do that because it would seal the bags on the bottom as well as the sides... like using a pot with no drainage hole.

Dan covered that above.

“You could cut off the bottom of the trash bag if you're concerned about drainage.”

Or just poke a few holes. If things are drying as quickly as daily, it shouldn't be much of a concern.