I use liquid kelp as a foliar spray every couple of weeks after the plants are a couple of feet tall. This seems to help in their growth and here in the South it helps them to withstand the heat. I find they will also bloom earlier using kelp.

I don't feel it will be to rich for your plants either by mixing it in the soil or as a foliar spray if you follow the directions. Maxicrop seems to be the most popular brand so just Google it to see where you can purchase it.