Answer: Finding a suitable plant for under citrus trees is a real challenge. Citrus have shallow root systems that have trouble competing with other plants, including weeds. Citrus roots likes ample moisture, and they don't like to be disturbed. Taking all this into consideration, the first choice would be an organic mulch rather than a plant. But, if you don't like the idea of a thick layer of bark or compost, you might try a well-behaved low-growing groundcover such as Ajuga (Carpet Bugle), Houttuynia, Vinca (Periwinkle), Gallium (Sweet Woodruff), or Phlox.
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