Answer: A general rule of thumb for soil depth is: 4-6 inches for annual flowers like impatiens and lobelia, 12-18 inches for larger annuals like tomatoes and squash, and 24 - 36 inches for small shrubs and dwarf trees. The larger the container, the easier it will be for the plant to stretch its roots and find moisture and nutrients. If you have only four-inches of soil depth, stick with annual flowers and tender herbs like basil. You can have a wonderfully colorful and fragrant garden if you stick to smallish-plants, or those known to be especially hardy like marigolds and nasturtiums. Experiment a little and you may be pleasantly surprised what you can grow!
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