Q: What can I plant under the coast live oak in my front yard to fill in the sandy area below and around the canopy? — Ginny Kemper, Nipomo
A: You have lots of colorful and interesting choices in companion plantings for oak trees. Hummingbird sage will give you a wide-spreading groundcover of bright green leaves with pink flowers. Carpenteria californica is an evergreen shrub with beautiful white anemone-like flowers. In a sunnier section under the tree, plant California fuchsia for gray-green foliage with red tubular flowers attractive to hummingbirds.
When planting under oaks, however, there are a few basic rules to keep your tree healthy. First, any companion plant should be well-adapted to our Mediterranean climate of hot, dry summers and cool, wet winters. Our venerable coast live oak (Quercus agrifolia) is very susceptible to soilborne diseases, such as crown rot and oak root fungus, that favor the combination of moisture and warm temperatures from summer watering. Many of our colorful California natives are drought tolerant and will not require summer watering once established.
Be careful where and how you plant. A mature oak feeds from a network of roots 6 to 18 inches below the soil and extending out from the tree half again the distance from the trunk of the tree to the edge of its canopy (drip line). Damaging or excessively disturbing this vulnerable network can be harmful to the tree. Don’t plant anything within the first 6 feet from the trunk. Instead, leave the leaf mulch, but rake it away from the bark so the tree does not become buried in deep leaf litter. Be careful how you plant in the rest of the root zone. As you dig a hole for a new plant, try to avoid cutting through thicker roots you may unearth. It’s better to fill in the hole and dig a new one a little bit away than it is to damage your tree.
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Water your new companion plants with care. It’s best to plant them in the late fall, so that they can become established over winter, during the natural rainy season. There’s a chance that they may need some extra water during those first two years, especially if rainfall is low. Drip irrigation is a good choice in this circumstance, providing water directly to the companion plants without saturating the soil under the rest of the canopy.
A free UC Publication is available for instant download: “Living Among the Oaks, A Guide for Landowners and Managers” by Douglas Mc-Creary at http://anrcatalog.ucdavis.edu/pdf/21538.pdf.