My suggestion would be to add cardboard or something up the sides to hold water. My garden did SO WELL, but it dried out easily. Also, in the individual holes, I planted herbs, green peppers and zucchini. They did excellent also!
How to build a rain garden designed to divert gutter water into an attractive planting bed that works like a sponge and natural filter to clean the water and let it percolate slowly into the surrounding soil.
Planning to use this to help create my new front flower bed this year. Burpee's "My Garden Designer". It's a mobile design tool made to take the guesswork out of flower color, height and compatibility.