By Kim Wilson, UC Master Gardener
The majestic oaks throughout California are a defining feature of our native landscape. California has 15 species of oak, 8 of which reach tree size. The deciduous Valley oak, Blue oak, evergreen Coast Live oak and Interior Live oak are the most common. Oaks can be successfully incorporated into the home landscape. Selection of compatible plants placed under and around the oaks requires knowledge of environmental needs of the tree.
Root System and Irrigation
California native oak trees are long lived and resilient trees. They have evolved under cool wet winters and dry summers. The majority of the tree’s root system is an expansive network of roots in the top 3 feet of the soil, extending well beyond the drip line. The roots are crucial to the health of the tree and any root injury or change in the soil oxygen or moisture can impair root function and increase susceptibility to disease. The root crown is especially vulnerable and excess moisture in this location can result in disease.
Although native oaks do not require irrigation, urban planted oaks may need irrigation under well-defined circumstances.
Guidelines for Irrigation of Urban Oaks:
- Newly planted oak trees need regular irrigation until they are established after 2-3 years.
- Irrigation should never occur near the base of the tree, but halfway between trunk and drip line to 10-15 feet beyond. Irrigate to a depth of 18-24 inches.
- Established urban oak trees need little to no supplemental irrigation. During droughts with dry winters, supplemental irrigation in the spring and once or twice during summer may be helpful to the tree. Irrigation should be slow and deep, in the outer 2/3rds of the root zone. Soil should dry for a month or two between these drought season irrigations.
- Common oak tree root pathogens are present in the soil but are inactive during dry conditions or cool weather. These same pathogens proliferate under warm wet conditions as would occur with frequent summer irrigation. This overwatering can cause oak root rot or crown rot resulting in decline, structural instability and death of a tree. Summer irrigation of an oak tree should occur only under drought or water stressed conditions and should be carefully planned and monitored.
Basic Rules for Landscaping Around Oaks:
- Suitable plants require little to no maintenance with no summer water requirements and, ideally, should be chosen from those which are part of the native oak ecosystem. The area under the tree canopy should remain undisturbed; avoid injuring the roots or disturbing the soil. Plant compatible plants 10 feet from the mature tree’s trunk.
- Plants can be placed sparingly around oaks, as accents. Avoid dense growing vines or ground covers which would interfere with the normal exchange of air and moisture.
- Plant in fall to allow new landscape plants to benefit from winter rain. Water new plants with drip irrigation for first year to get established but avoid continually saturated soil.
- After the first year, the plants should be watered at most once per month or not at all. Plants that will require regular summer irrigation should be avoided.
- Ensure proper drainage to avoid saturated soils near tree trunk. Do not raise or lower the surrounding soil lines of mature oaks. Do not compact soil.
- Allow oak leaf litter to remain undisturbed and use 2-4” of organic mulch.
- Mature oaks, like most native plants, do not need fertilizer, as long as the normal leaf litter is allowed to remain. Do not fertilize stressed trees. Poor growth is usually an indication of poor growing conditions and not nutritional deficiencies.
References
Hagen, Bruce, Barrie Coate, and Keith Oldham. 2007. Compatible Plants Under And Around Oaks. PDF California Oak Foundation. WebBookCompatPlnts.doc
Lichter, John, and Ellen Zagary. "Plant List And Procedures For Landscaping Under Native Oaks Of The Central Valley". UC Oak Woodland
UC Oaks. UC ANR. Welcome - UC Oaks | Oak Woodland Management
Living Among the Oaks: A Management Guide For Landowners. PDF. UC ANR. ANRCatalog - Living among the Oaks: A Management Guide for Landowners and Managers - ANR Catalog
Plants For Around and Under Oaks. 2016. PDF. UC Master Gardeners Sonoma County. Planting Under Oaks - UC Master Gardener Program of Sonoma County
Tietje, Bill. “Native Oaks Stressed By Drought May Recover”. ANR Blogs. 2014. Native oaks stressed by drought may recover - Forest Research and Outreach - ANR Blogs