We are all interested in unthirsty plants these days. When I design gardens under our native oaks there are several principles that I follow that I want to share with you. Drought tolerant plants are a must in this situation. Evergreen oaks even have special needs requiring a different plant palette than deciduous oaks like blue and valley oaks.
Most oak roots are in the upper 3 ft of the soil. The roots which take up water and nutrients are in the upper 12". The critical root disease zone for an oak is within 6-10 ft of the trunk. Do not irrigate plants or disturb the soil in this area. Outside this zone but within the drip line one deep watering per month is allowable for your plantings.
The leaf mulch that accumulates under a large coast live oak increases soil moisture and available nutrients, improves soil structure and moderates soil temperature. It’s a valuable resource that can be used in other parts of the garden. Never remove the entire insulating layer of leaves from under a trees canopy and don’t allow leaf mulch to cover the trunk.
One of the showiest California natives for planting under a coast live oak is carpenteria californica or Bush anemone. This evergreen shrub is resistant to oak root fungus and grows in sun or shade. Although this plant can get by with no summer water after becoming established, occasional summer water will help maintain a fresher look. The compact variety ‘Elizabeth’ produces a profuse display of white flowers with as many as 20 flowers per cluster. Cut back the top 1/3 to 1/2 of the stems after flowering to keep it compact.
Another deer resistant plant I like to use under oaks is Centennial ceanothus. This gopher resistant low spreading groundcover grows 12" high and 8 ft wide. Cobalt blue flower clusters cover the plant each spring. They also attract birds.
Western redbud can be grown as an accent shrub or small tree under coast live oaks. Large bluish grey rounded leaves have heart-shaped base making quite a show among smaller leafed plants. The magenta sweet-pea shaped flowers cover the branches in spring. Scattered flowers may also appear in late summer and fall. Flattened seedpods rustle in the wind in late summer and fall. And if that’s not enough to love about western redbuds they brighten the landscape with a fall color display of yellow or red leaves. Western redbuds respond well to pruning. Thin the oldest trunks each year to keep them growing vigorously. You can also cut the entire clump to the ground to rejuvenate it. This is truly a four season plant.
Combine any of these plants with coffeeberry, native irises, pink-flowering currant, toyon, berberis aquifolium, snowberry, hummingbird sage or yerba buena to complete your woodland garden and keep your oaks happy too.