I am new at gardening, and this is my first attempt with moonflowers. I live in North Texas, so I have hot, dry summers. I have a wrought iron gate that I want to cover with moon vine. The trouble I ran into is that the gate is actually over a concrete driveway (so there is no soil beneath it to plant the seeds), and I'd like to keep the functionality and allow the gate to swing open when necessary to get into my backyard. So, what I've done is attach a 36" long by 8" wide by 8" deep plastic flower box to the bottom of the gate. I planted 8 moonflower seeds in good quality potting soil in this planter, and, so far, I have 7 seedlings (the gate gets full sun). My plan is to help the seedlings twine up the gate when they are about 3" tall, and just keep them watered. My question is, will this planter, which currently has about 4" of soil in it, support the plants and their root systems when they mature? Is there anything else I should do?

Any advice would be greatly appreciated.

Thanks so much!