September in St. Louis generally marks the end of perennial planting. This is due to the fact that unlike trees and shrubs, perennials need quite a bit of time to establish a strong root system before going into dormancy. So if you’re still hoping to get some herbaceous perennials in the ground… do it now!
When shopping this time of year look for healthy plants with a significant amount of fibrous roots. In fact, you can even get away with a somewhat root bound plant as long as the top growth doesn’t look stressed. Just tease apart the roots a bit to stimulate new root growth and plant it in the proper soil conditions. Another good tip is to apply a root stimulator with mycorrhizal fungi at the time of planting. This will help produce large root systems very quickly, and also helps to ensure new plantings get maximum water and nutrients from the soil to minimize transplant loss.
Another bonus… you can usually score some really good deals at the end of the season!