Landscaping/
Urban Forestry
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Plant List and Procedures for Landscaping Under Native Oaks of the Central Valley California native oaks such as the valley oak (Quercus lobata), blueoak (Q. douglasii)), interior live oak (Q. wislizenii), andcoast live oak (Q. agrifolia) are some of the most significant naturalcomponents of our California landscape. Besides beauty they provide a multitudeof benefits, including wildlife habitat, shading, erosion control, windprotection, pollution reduction, and screening. Oak woodlands are sustainablelandscapes, requiring a minimum of resources if successfully incorporatedinto new developments. Even with the most careful attention to the architecturaldesign in oak woodlands, constructional alterations bring environmentalchanges that often lead to tree decline. By understanding the oak's habitat,physiology, growth habits and response to environmental changes, we candesign and manage landscapes near oaks which ensure their survival. The Oak Habitat, Growth Habits and Roots California native oaks have evolved under the cool, wet winters and hot,dry summers typical of our Mediterranean climate. Through adaptations suchas the development of deep and extensive root systems, various water conservingleaf characteristics, and a slowing of growth in the summer, these treesare able to survive the prolonged seasonal droughts which typify the CentralValley. California native oaks often produce sinker roots within severalfeet of the trunk, which grow deep into the soil profile, providing theoak access to water as the summer progresses. Lateral roots are commonlyshallow and extend well beyond the drip line of the tree. Maintaining thehealth of the oak root system is the key to successful landscaping aroundoaks. Tree root health is largely affected by mechanical injury, physical andchemical properties of soil, and pathogens. During landscape installation,oak roots may be directly injured by backhoes, trenchers, tillage equipmentor shovels, or indirectly by altering physical soil properties due to gradingand compaction. Frequent irrigation and compaction limit oxygen access tooak roots that favor pathogens, such as crown and root rot (Phytopthorasp.) and oak root fungus (Armillaria mellea). These fungi oftenlead to the decline or structural instability of California native oaks. Recommendations for Landscape Planting, Establishment and Maintenance As a general rule, strive to ensure that the environmental conditionsboth above and below ground are similar to those conditions under whichthe oak grows naturally. Therefore, an ideal landscape near most CentralValley oaks is one which will tolerate a minimum of maintenance and irrigationonce established. An appropriate plant palette may be composed of plantsnative to the local region or those plants which will tolerate the localenvironment (California natives or other Mediterranean plants). When establishinglandscapes near oaks, one should observe the following guidelines.
Saturated soils (especially near the tree trunk) when soil temperaturesare moderate create ideal conditions for the establishment of crown androot rot and oak root fungus, which can kill trees. Ensure that landscapegrading does not allow drainage to collect around the base of the trunk.French drains or other systems may be used to carry water away from thetree.
Avoid traffic and storage of equipment in the oak root zone. Installirrigation lines at the soil surface and cover them with mulch. Dig plantingpits by hand whenever possible to avoid root injury. Where equipment operationis essential in the oak root zone, install a 6" thick layer of woodchip mulch to reduce soil compaction.
Ideally, do not plant under the oak canopy or at least within ten feetof the mature tree trunk where buttress and sinker roots (critical to thehealth and structural stability of the tree) are located. The frequent irrigationrequired to establish landscape plants creates conditions favorable forthe fungi that can infect roots. Plant sparingly. Dense plantings can competewith oak root systems for water and nutrients. Therefore, use plants asaccents and specimens near oak trees.
The main goal of the irrigation for newly planted plants should be toprovide only as much water as required to establish them. The plants shouldbe irrigated in as small an area as possible. A drip or microsprinkler systemcan be used for this purpose. However, it should be operated in such a manneras to avoid saturated soils for long periods of time. Once the plants areestablished, they should be irrigated monthly, less often, or not at allduring the summer.
Select plants that tolerate the Central Valley climate, drought, theexposure to light as influenced by the tree, soils in the vicinity of thetree (pH, drainage, texture, etc.), and common pests. Many California nativespecies or species from Mediterranean climates are most suitable for plantingunder oaks. Consider the ultimate size of the plants selected and whetheror not they will interfere with the tree canopy, views or structures.
Avoid paving over the oak root system; the impermeability of these surfacesas well as the excavation and compaction associated with their installationdamage tree roots. If an area in the root zone of a tree needs to be paved,limit the paving to as small an area as possible and either install perviouspavers or decomposed granite on grade with a minimum of compaction (no morethan 80%) or utilize a deck with piers. In non-paved areas, a wood chipmulch is an ideal soil surface cover. Install the mulch by hand to avoidsoil compaction. A List of California Native Plants Suitable For Use Under Central Valley Oaks How To Use This List: Sketch the area to be planted, including the oak(s) trunk and drip line,other plants and structures. Indicate areas which receive full sun, partialshade, full shade or morning sun only. Choose plants from the list appropriateto the sun exposure in the area to be planted. Develop a planting designutilizing these species. Full Sun (tolerates south and west exposure)
Protect From Afternoon Sun (Partial shade)
Full Shade or Morning Sun
prepared and edited by Richard B. Standiford and Pavel Svihra |
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John Lichter
Horticultural Consultant/Certified
Arborist, Winters
Ellen Zagary
University Arboretum,
UC Davis
Reprinted from Hort Script, No. 11, March 1995, U.C. Cooperative Extension.
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