Have you ever considered keeping them in pots even while they are outside?I have done this with other plants to much effect. It also lets you control the soil they are in. Just keep the container a little ways out of the dirt, and it also helps protect them from other pests. If you use 5gal, you can grow them to any size you want(I'm sure you know this), or the rubbermaid trashcans work well. Also, have you tried using the red film on the ground underneath? It actually works with tomatos. For a few dollars, it is worth a try i would think.I know a little about gardening, alot of older tricks i am sure everyone knows.
See, I am not always an ass.