Planting Under Oaks
Sadly, there are too many stories like this one: New-comers to Sonoma County purchased a home with a magnificent spreading, 200-year-old oak tree. Not knowing any better, they planted a lawn around it for their children’s play. Some years later, they heard a stupendous crash and looked out to see that this magnificent tree had toppled over. The lawn and its associated water had killed the venerable giant in a matter of a few years.
To avoid such a catastrophic situation, it’s critical to partner oaks only with plants that associate with them in the wild or with others that thrive in dry shade. In short, plant choices for landscaping in and around oaks are limited. In our Mediterranean, winter-wet, summer-dry climate, only certain species are adapted to conditions that oaks enjoy. When soils are moist in winter, few disease organisms that could threaten oaks are active in the soil. But in summer heat, threatening soil pathogens are active when moisture is present.
Valley oaks (Quercus lobata) and Coast live oaks (Quercus agrifolia) are commonly found throughout rural areas of Sonoma County, particularly not far from the coast, and are frequently planted within new subdivisions. As a dominant species, these trees become a focus and concern for many gardeners, but all native oaks require care beneath their canopies.
Guidelines for planting under and around oak trees
- Find an alternative for digging or trenching that cuts roots when altering your landscape.
- Avoid compacting soil under the canopy, changing drainage patterns, or raising or lowering the soil grade, all of which limit oxygen supply to shallow feeder roots.
- If paving is necessary, use porous paving, such as brick or flagstone on sand, or decomposed gravel.
- Choose species that naturally occur with oaks, those with shallow roots that have an affinity for partial or complete shade and no need for summer moisture after becoming established.
- Keep all plants 4-6 feet away from the trunk.
- Keep mulch a foot away from the base of the trunk.
- Plant in the fall so winter rains help young plants become established, then need supplemental water for only one season.
- To help get new plants established, water cautiously by hand or with drip irrigation, never sprinklers.
- Leaf drop from the trees themselves is best left undisturbed on the ground.
Suggested plants for growing under evergreen or deciduous oaks
The following list of perennials, shrubs, grasses, and vines include species of varying sizes, some of which are suitable directly under an oak’s leafy branches; others may require more sun exposure and are more appropriately sited at or beyond the drip line where their roots may intertwine with the trees expanding root growth. Other species not listed here are also possible but must also fit the criteria for association with oaks.
|Aquilegia formosa (western columbine)||perennial|
|Aristolochia californica (California Dutchman’s pipe)||large vine|
|Carex species (sedges)||grasses|
|Carpentaria californica (bush anemone)||medium shrub|
|Ceanothus griseus & cultivars (Carmel ceanothus)||medium-large shrubs|
|Festuca californica (California fescue)||grasses|
|Heuchera maxima (coral bells)||perennial|
|Iris species (iris)||perennial|
|Juncus patens (California gray rush)||grass-like|
|Mahonia pinnata (California holly grape)||medium shrub|
|Mahonia repens (creeping mahonia)||groundcover|
|Ranunculus californica (California buttercup)||perennial|
|Rhamnus californica (coffeeberry)||medium-large shrub|
|Ribes viburnifolium (Catalina perfume)||groundcover|
|Ribes sanguineum (flowering currant)||medium-large shrub|
|Satureja species (yerba buena)||groundcover|
|Solidago californica (California goldenrod)||perennial|
|Salvia spathacea (hummingbird sage)||perennial|