Answer: It depends on what you are planting in them. Most of the root systems of annuals, perennials, cacti and succulents are in the top 12-16 inches or so of soil, so you need to make sure they have at least that and a little more. Because plant roots are confined to the container, which heats up considerably in summer, it's important to make sure they won't "cook." If your pots are considerably deeper than that, I know a container gardener who places styrofoam peanuts in the bottom of large pots. If you are planting, for example, shrubs or a dwarf citrus tree, their deeper root systems need the deeper soil base that a big pot offers. Good luck with your containers!
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