Answer: It's difficult to keep plants happy beneath established oak trees because their extensive root systems use all the available water, to the detriment of other plants. And, additional watering required for new plants can compromise the health of the oaks.
To preserve the health of your oak trees, only drought-tolerant plants that require no summer water should be planted around oaks, and they should be planted no closer than six feet from the base of the tree. Avoid planting grasses, ivy, azaleas, ferns, rhododendrons or any other vegetation that needs summer watering.
The following plants do well beneath oak trees:
Ceanothus - These shrubs are most effective around oaks. Low-growing types such as C. griseus horizontalis and C. rigidus 'Snowball' reach 2 to 6 feet tall. Clusters of lavender-blue or white flowers appear in spring.
Coral bells (Heuchera; zones vary by species). Compact perennials with roundish leaves in shades of dark green to purplish red; dusters of coral pink, red, or white flowers in spring or summer.
Bamboo (Susa veitchii; hardy to 0 [degrees]). Dense, 2- to 3-foot-high ground cover; deep green leaves develop white edges in autumn. Needs water only during growth spurts. Can be invasive; contain planting with a plastic bamboo barrier.
Blue fescue (Festuca ovina 'Glauca'; all zones). Ankle-high clumps of bluegray foliage.
Dusty miller (Senecio cineraria; all zones). Woolly white leaves grow 2 feet tall.
Fountain grass (Pennisetum alopecuroides). Graceful 3- to 4-foothigh clumps of bright green foliage topped by pinkish flower plumes.
St. Johnswort (Hypericum calycinum). Ground cover with evergreen foliage, yellow summer flowers.
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