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 such as the development of deep and extensive root systems, various water conserving leaf characteristics, and a slowing of growth in the summer, these trees are able to survive the prolonged seasonal droughts which typify the Central Valley. California native oaks often produce sinker roots within several feet of the trunk, which grow deep into the soil profile, providing the oak access to water as the summer progresses. Lateral roots are commonly shallow and extend well beyond the drip line of the tree. Maintaining the health of the oak root system is the key to successful landscaping around oaks.

Tree root health is largely affected by mechanical injury, physical and chemical properties of soil, and pathogens. During landscape installation,oak roots may be directly injured by backhoes, trenchers, tillage equipment or shovels, or indirectly by altering physical soil properties due to grading and compaction. Frequent irrigation and compaction limit oxygen access to oak roots that favor pathogens, such as crown and root rot (Phytopthorasp.) and oak root fungus (Armillaria mellea). These fungi often lead 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 conditions both above and below ground are similar to those conditions under which the oak grows naturally. Therefore, an ideal landscape near most Central Valley oaks is one which will tolerate a minimum of maintenance and irrigation once established. An appropriate plant palette may be composed of plants native to the local region or those plants which will tolerate the local environment (California natives or other Mediterranean plants). When establishing landscapes near oaks, one should observe the following guidelines.

• Ensure that drainage from landscaped areas does not collect under oaks.

Saturated soils (especially near the tree trunk) when soil temperatures are moderate create ideal conditions for the establishment of crown and root rot and oak root fungus, which can kill trees. Ensure that landscape grading 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 the tree.

• Prevent tree injury and soil compaction during landscape installation.

Avoid traffic and storage of equipment in the oak root zone. Install irrigation lines at the soil surface and cover them with mulch. Dig planting pits by hand whenever possible to avoid root injury. Where equipment operation is essential in the oak root zone, install a 6″ thick layer of wood chip mulch to reduce soil compaction.

• Plant sparingly and away from the tree trunk.

Ideally, do not plant under the oak canopy or at least within ten feet of the mature tree trunk where buttress and sinker roots (critical to the health and structural stability of the tree) are located. The frequent irrigation required to establish landscape plants creates conditions favorable for the fungi that can infect roots. Plant sparingly. Dense plantings can compete with oak root systems for water and nutrients. Therefore, use plants as accents and specimens near oak trees.

• Irrigate landscape plants as infrequently as possible.

The main goal of the irrigation for newly planted plants should be to provide only as much water as required to establish them. The plants should be irrigated in as small an area as possible. A drip or micro sprinkler system can be used for this purpose. However, it should be operated in such a manner as to avoid saturated soils for long periods of time. Once the plants are established, they should be irrigated monthly, less often, or not at all during the summer.

• Plant appropriate species.

Select plants that tolerate the Central Valley climate, drought, the exposure to light as influenced by the tree, soils in the vicinity of the tree (pH, drainage, texture, etc.), and common pests. Many California native species or species from Mediterranean climates are most suitable for planting under oaks. Consider the ultimate size of the plants selected and whether or not they will interfere with the tree canopy, views or structures.

• Utilize an appropriate soil surface cover.

Avoid paving over the oak root system; the impermeability of these surfaces as well as the excavation and compaction associated with their installation damage 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 pervious pavers or decomposed granite on grade with a minimum of compaction (no more than 80%) or utilize a deck with piers. In non-paved areas, a wood chip mulch is an ideal soil surface cover. Install the mulch by hand to avoid soil 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, partial shade, full shade or morning sun only. Choose plants from the list appropriate to the sun exposure in the area to be planted. Develop a planting design utilizing these species.

Full Sun (tolerates south and west exposure)

Protect From Afternoon Sun (Partial shade)

Full Shade or Morning Sun

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.

prepared and edited by Richard B. Standiford and Pavel Svihra