Answer: It's often difficult to get plants to grow under trees that cast a lot of shade. In the case of citrus, it's probably not a good idea to plant directly under the dripline (measured from the trunk to the tips of the outermost branches) because citrus trees develop very shallow feeder roots. The roots won't deal well with competition for moisture and nutrients from other plant roots. To keep your citrus happy, use an organic mulch to help suppress weeds and hold in soil moisture. If you want to plant outside of the dripline, most any annual or perennial will grow well, be attractive, and not compete with the citrus roots.
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