I would wait until the last killing frost. Since I live in Hawaii I have no idea when that will be. No frost here, so you know your local conditions. I would wait until the crop you are going to hide them in gets at least a couple of feet high.

Remember to adjust your indoor plants to the outdoors slowly. If you live close to the plants and you don't run the risk of getting seen or leaving trails, go ahead and ammend the soil. It will help your plants grow big. Use the same stuff they are currently in.

Good Luck