Q: I’ve heard you can’t build a raised flower bed around a tree because you will kill the tree. Is this true? — Bonnie Peterson, Emory, TX
A: Yes, it is. The vast majority of a tree’s feeder roots are in the upper foot or so of soil, and those roots are responsible for taking in most of the moisture and nutrients the tree receives. By adding another foot of soil to create a garden on top of the existing soil, you basically smother those small but essential feeder roots. You can also rot the bark (and eventually the trunk) by placing moist soil up against it.
There are other possibilities for landscaping beneath the canopy of a tree. Tuck small, young, shade-tolerant perennials into the soil, starting about 18 inches out from the trunk in all directions. If they’re well-suited to the site, they’ll increase in size from year to year, but their roots will grow among the tree’s roots without damaging or overwhelming them. Shade-tolerant ground covers are another good choice.
Any plants you grow under a tree will compete with the tree for moisture and nutrients, so water them periodically—especially during hot, dry weather—and fertilize them each spring.
You could also put large pots of flowers under the tree, though they’ll require more maintenance than plants that are growing directly in the ground.