I'm thinking about starting a small native plant nursery with an emphasis on potted milkweed that I can sell locally. After some failures, I've gotten pretty good at getting milkweed to sprout from seed. However, the problem I've run into is that my milkweed will sprout and grow to about 3 inches in a soil-less potting mix, but then it stops growing. I did some experimenting this summer with different mixes and found that my milkweed sprouts would not progress beyond 3 inches in any bagged mixes, even a heavier vegetable mix. In desperation, I planted them in some backyard soil that I had mixed with compost and humus, and suddenly they took off. The same was true of all the other natives I've grown (coreopsis, blackeyed susans, liatris, mistflower)--they would sprout in potting mix, go dormant, and then "wake up" as soon as I put them in native soil from my yard.

So, my question is: does anybody know how I could design a heavy, water-retaining mix that would somewhat imitate my backyard soil? I need something more standardized and weed and disease-free. I live in the Gulf Coast region, and my soil is black gumbo. I'm aware that many people will tell me not to use a rich soil for potted plants, but this was the only thing that worked for me.

Thanks for your help.