Sonoma County Master Gardeners Helping Sonoma Gardeners

by Sonoma County Master Gardener Steven Hightower

Up the road and around the bend from my home is a 150+-year-old property known as Stone Tree Ranch (lots of petrified wood in the oak forest). Years ago the people that owned it had a magnificent spreading 200+ year old valley oak tree. Not knowing any better, they planted a lawn around it for their small daughter to play on. One year, some years later, they heard a stupendous crash, and looked out to see this magnificent old tree toppled over. The lawn, and its associated water, had killed the 200+ year-old giant in a matter of a few years.

To the west of my house, there is mixed oak and evergreen forest. A portion of it is full of understory plants, but the portion most visible from the house is fairly barren between trees. We wanted to add some lower plants to this area to give at a fuller look, and so I set out to do some research as to what we could interplant there that would look good and be successful, and very importantly, not negatively impact the native oaks. I was really seeking an answer to the question “what can I plant under, or near native oak trees, to obtain a bit more ‘landscaped’ look?”

First, I learned a couple of key issues: Certain plants occur naturally with oaks in the wild, and plants that are not normally associated with oaks generally don’t do as well. Secondly, oaks are native to our Mediterranean, winter-wet, summer-dry climate. During the winter, when the ground is wet from rain, the soil temperature is relatively low, and that lower temperature keeps disease from developing. However, if oaks get watered in the summer, when temperatures are high, oak root fungus (Armillaria mellea) can occur in the warm, wet conditions, to which the tree roots are very susceptible. This fungus enters the oak's roots and usually kills the tree slowly over several years. Even after a dead tree is gone, the disease can remain in the soil, attacking other susceptible plants. In addition oaks in warm wet conditions are susceptible to crown rot, caused by a soil-borne fungus, Phytophthora (another variant of which is responsible for Sudden Oak Death).  So summer water is a definite no-no for oaks. Which means that those things planted in and around oaks need to be able to survive the summer (after establishment) with no supplemental water.  And lastly, oaks do not like to have their soil or soil level disturbed or changed in any significant way.

So that leads to these general rules when planting under and around oak trees:

 

• It is important not to compact the soil, change the drainage patterns, or raise or lower the soil grade near native oaks.  If paving is necessary, use porous paving, such as brick or flagstone on sand, or decomposed gravel. 
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• Plant species that naturally occur with oaks—that have demonstrated an affinity for the microclimate.
• Plant species that require no summer water, nor fertilizer, after establishment. Watering near oak trees in the summer is inviting trouble. During the first summer, to get the plants established, water cautiously with drip irrigation, never sprinklers.
• Plant in the fall, so plants have the winter rains to help them get established, and need supplemental water for only one season.
• Don’t plant at all closer than 4-6 feet from the trunk.
• From 4-6 feet out, to the dripline, consider planting swaths of clumping native grasses-- Festuca californica (California  Fescue), Muhlenbergia rigens (Deer Grass), and Nassella pulchra (Purple needlegrass).  Or combine one or more Ceanothus (Wild lilac), Rhamnus californica (coffeeberry), Iris douglasiana (California iris), Salvia sonomensis (Creeping sage) and Huechera (Coral bells).
• At the dripline edge, and in the sunnier spots between trees, mix any of Arctostaphylos densiflora (Manzanita), Baccharis pilularis (Dwarf Coyote Brush), Mahonia aquifolium (Oregon Grape), Garrya elliptica (Coast Silktassel), Heteromeles arbutifolia (Toyon), Rosa gymnocarpa (Wood rose), Mahonia repens (Creeping Mahonia), Ribes viburnifolium (Evergreen Ribes) or Ribes sanguineum (Flowering currant).

 

Other plants that can work around oaks include:

Berberis (aka Mahonia) Pinnata

Salvia leucophylla (Purple sage)

Whipplea modesta.

Salvia spathacea (Hummingbird sage)

Santa Barbara daisy (Erigeron karvinskianus)

Narcissus (Daffodil)

Rhus Ovata (Sugarbush)

For a complete list see the Master Gardener Document Plants for Around and Under Oaks (but remember the above rules!) Also see the book Compatible Plants Under and Around Oaks—if you can find an out-of-print copy in a local bookstore, or your library (as of this writing, there are two used copies on Amazon.com).

 

©Sonoma County Master Gardeners