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 natural components of our California landscape. Besides beauty they provide a multitude of benefits, including wildlife habitat, shading, erosion control, wind protection, pollution reduction, and screening. Oak woodlands are sustainable landscapes, requiring a minimum of resources if successfully incorporated into new developments. Even with the most careful attention to the architectural design in oak woodlands, constructional alterations bring environmental changes that often lead to tree decline. By understanding the oak’s habitat,physiology, growth habits and response to environmental changes, we can design 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 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)

 

Latin name	Common name	Comments
Shrubs
Adenostoma fasciculatum	chamise	A low growing form is available
Arctostaphylos manzanita	manzanita
Artemisia tridentata	basin sagebrush	Needs dead branches removed with age
Ceanothus ‘Concha’ and ‘Dark Star’		Dense, dark flowered varieties
Ceanothus cuneatus	buckbrush	White flowers
Ceanothus megacarpus bigpod	buckbrush	Tree type ceanothus which is one of the earliest to bloom
Dendromecon rigida ssp. harfordii	bush poppy	Showy, yellow flowers
Encelia californica	encelia	Showy, yellow, daisy-type flowers
Ephedra species	Mormon tea
Eriogonum arborescens	Santa Cruz Is. buckwheat
Fallugia paradoxa	Apache plum	Ornamental fruits
Forestiera neo–mexicana	desert olive	Blue fruits are ornamental
Fremontodendron species and cultivars	fremontia	Large shrubs with golden, saucer shaped flowers
Heteromeles arbutifolia	toyon	Showy red berries around Christmas time
Isomeris arborea	bladderpod	Yellow flowers year-round. Some object to the odor of the leaves
Lupinus albifrons	silver bush lupine	Short-lived but self seeds.
Mahonia nevinii	San Fernando barberry	Spiny leaves, good barrier plant
Pickeringia montana	chaparral pea	Purple flowers in spring.
Quercus durata	leather oak
Rhus ovata	sugarbush
Rhus trilobata	squawbush
Romneya coulteri	Matilija poppy	Large “fried egg” flowers, spreads by underground runners
Salvia leucophylla	coastal white sage	Summer dormant without irrigation
Simmondsia chinensis	jojoba
Small Trees
Aesculus californica	California buckeye	Summer dormant
Prunus lyonii	Santa Catalina cherry
Ground Covers
Baccharis pilularis var. pilularis	dwarf coyote bush	“Pigeon Point” is the best cultivar for valley.
Eriogonum fasciculatum	California buckwheat
Salvia sonomensis	creeping sage	Short-lived, water monthly, needs good drainage
Zauschneria californica	California fuchsia	Many cultivars available, red, pink and white.
Zauschneria cana	island California fuchsia	Fine, silver-gray foliage
Perennials
Achillea millefolium	yarrow	Mow after bloom to remove flower stalks.
Dudleya sp.	live-forever	Can be used as sparse ground cover, may freeze back in winters.
Keckiella cordifolia	heart-leaved penstemon	Summer dormant
Salvia ‘Dare’s Choice’	hybrid sage	Evergreen perennial with purple-blue flowers
Viguiera deltoidea ssp. parishii	desert sunflower	Killed to ground in cold winters
Grasses and Accent Plants
Agave deserti	desert century plant	Accent plant.
Nolina sp.	nolina	Accent plant.
Stipa pulchra	purple needle grass	Water monthly. Summer dormant
Yucca whipplei	yucca	Succulent-leaved accent plant
Annuals
Clarkia sp.		Showy pink blooms, monthly watering, mow after seed set.
Eschscholzia californica	California poppy	Reseeds in disturbed areas, mow after seed set.
Lupinus sp.	lupine	Lupinus densiflorus and L. succulentus are reliableyear after year. Mow after seed set.
Nemophila sp.	baby blue eyes
Bulbs
Allium sp.	wild onion	Many sizes and colors/
Brodiaea sp.		Triteleia and Dichelostemma also. Summer dormant
Calochortus sp.	mariposa lily
Chlorogalum pomeridianum	soap plant	3-4 ft. flower spikes open in afternoon.

 

Protect From Afternoon Sun (Partial shade)

 

Latin Name	Common name	Comments
Shrubs
Arctostaphylos densiflora	Sonoma manzanita	Cultivars “Harmony”, “Howard McMinn” and “Sentinel”
Arctostaphylos rudis	shagbark manzanita
Carpenteria californica	bush anemone	Summer deciduous if irrigated
Ceanothus ‘Ray Hartman’	hybrid ceanothus
Cercis occidentalis	redbuds
Cercocarpus betuloides ssp. blanclzeae	mountain mahogany
Comarostaphylos diversifolia	summer holly	Ornamental red fruits
Eriogonum umbellatum var. polyanthum	sulfur buckwheat	Useful as loose ground cover or subshrub
Garrya elliptica	coast silktassel	Ornamental “tassels” in winter
Garrya fremontii	Fremont silktassel	Ornamental “tassels” in winter
Mahonia pinnata	California holly grape	Monthly watering
Prunus ilicifolia	hollyleaf cherry
Rhamnus californica	California coffeeberry
Rhamnus crocea	redberry
Ribes malvaccum	chaparal currant	Summer dormant, winter flowering
Ribes speciosum	fuchsia-flowered gooseberry	Summer dormant
Rosa californica	wild rose	Invasive if irrigated
Symphoricarpos rivularis	common snowberry	White fruits on winter deciduous branches
Small Trees
Lyonothamnus floribundus var. asplenifolius	Catalina ironwood	Fern-like, divided leaves
Umbellularia californica	California bay laurel
Vines
Aristolochia californica	Dutchman’s pipe	Winter deciduous, water monthly
Vitis californica	California wild grape	Deciduous, auumn red color forms available
Vitis girdiana	Desert grape
Grasses and Accent Plants
Lymus glaucus	blue wild rye	Monthly watering
Muhlenbergia rigens	deergrass	A large grass up to 6′ tall. Monthly watering
Festuca californica	Califoria fescue	Monthly watering
Ground Covers
Ceanothus ‘Joyce Coulter’	hybrid ceanothus	Medium blue flowers, good bloomers
Ceanothus maritimus	maritime ceanothus	Low shrub
Whipplea modesta	yerba de selva	Monthly watering
Perennials
Dinlacus aurantiacus	sticky monkeyflowers	Pinch to encourage new, bushy growth. Water monthly

 

Full Shade or Morning Sun

 

Latin Name	Common Name	Comments
Ground Cover
Ribes viburnifolium	evergreen currant	Best shrub for dry shade
Perennial
Heuchera maxima	giant alum root
Monardella sp.	deer mint, pennyroyal
Salvia spathacea	hummingbird sage	Large maroon and red flowers, water monthly.
Sisyrinchium bellum	blue eyed grass	Summer dormant without summer water
Shrubs
Arctostaphylos pajaroensis	pajaro manzanita	Best manzanita for our area.
Mahonia aquifolium	Oregon grape	Monthly watering.

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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.

prepared and edited by Richard B. Standiford and Pavel Svihra