Answer: Sounds like a wonderful project! Since the planter is so deep, I'd fill the bottom with something other than potting soil, and then finish filling it with potting soil. Perennials will be happy with 10-12 inches of potting soil for their roots. I fill plastic bags with styrofoam peanuts and set them in the bottoms of my overly large planters. Don't just dump the styrofoam into the planters or they will float up to the surface. Instead, place them in plastic bags and seal the bags, then set them in the bottom of the planters. Top it off with about 12 inches of potting soil and then transplant your mums and other perennials. Enjoy your new planters!
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