In the Garden Articles

Planting Beneath Oaks and Other Shady Places

by Joan S. Bolton

From afar, our native coast live oaks, with their sturdy branches held high, appear invincible. Many of these gnarled giants will live decades beyond any of us. Yet place a coast live oak (Quercus agrifolia) in a garden, and suddenly it may not be so indestructible.

Instead, common planting and watering practices can cause an oak's downfall.

Underplanting oaks with lawns, azaleas, fuchsias, ferns and other thirsty plants, or replanting colorful annuals beneath the canopy several times a year, make the trees vulnerable to pests and diseases.

That's because coast live oaks rely on our natural cycle of winter rains and summer droughts. Regular doses of summer water -- when the soil is warm -- encourage diseases to flourish at the same time that the oaks are in a resting stage and not as able to resist. In addition, our oaks don't like their many surface roots to be tampered with at any time of year.

Planting Solutions

Fall is the best time to plant the many native and water-conserving plants that will peacefully co-exist with our oaks.

You can irrigate to establish the plants over winter, when the oaks are accustomed to getting their roots wet. By next spring and summer, you can stretch out the watering intervals to every two to three weeks, or longer.

Natives that thrive in dry shade beneath oaks include hummingbird sage, Island alum root, California fescue and island snapdragon. If you must have ferns, look for coastal wood fern or California sword fern, both of which require little summer water. You can also get a lush, green look by planting evergreen currant or ceanothus, both of which need no summer water. Some natives experts suggest toyon and lemonade berry. But both shrubs grow 10 feet or taller, creating a dangerous fire ladder in rural gardens, and an overgrown look just about everywhere else.

If you can't live without high-impact, summer color, plant your water-loving begonias, fuchsias, impatiens and the like in containers, then make sure their saucers don't overflow. Clivia, with its showy, trumpet flowers, is a nice, shade-loving alternative. In the filtered light beneath an oak, it requires surprisingly little water. Yet its strappy, agapanthus-like leaves remain deep green all year.

Regardless of what you plant, tread lightly within 6 feet of the trunk and plan to keep the base of the oak dry.

Other Than Plants

Rather than planting beneath your oak, consider creating a seating area.

This can be as simple as setting out a bench or two amid the leaves, with an informal footpath leading the way.

For a larger conversation area, don't do anything that will compact the soil around the trunk, such as pouring a concrete foundation or laying tight-fitting bricks. Instead, lay loose-fitting flagstones, pavers or brick, then fill in the joints with sand or dirt. Or run bender board around the perimeter, then fill the interior with gravel. Any of these porous ground coverings will allow rain to soak into the earth and air to circulate into the soil and down to the roots.

Decks are another option. Use small concrete piers to avoid damaging many roots. Leave gaps between the deck planks so that when you sweep, any fallen leaves will drop to the earth below.

Minimize any trenching for irrigation or lighting.

Let the Leaves Lie

It's well-known that an oak sends down a tap root, anchoring itself deep into the soil. But it also sends out literally thousands of shallow roots that are equally critical to its survival. These roots grow mostly in the top 18 inches of soil and extend three times farther in every direction than the height of the tree. So if your oak is 30 feet tall, its roots may radiate 90 feet from the trunk, subject to rocks, streets and other obstructions.

To protect and enrich those rambling roots, leave the oak leaves where they fall.

If you rake, you'll take away natural compost material. In the wild, oaks have adjusted to recycling the nutrients contained in that compost. The leafy duff replenishes the soil, helps keep down weeds and dust, and moderates the temperature of the soil. It's a valuable resource that belongs right where it is.

What To Avoid

Rip out ivy, vinca or any other exceedingly thick ground covers. While they may flourish in dry shade, their extensive root masses make them lousy companions. They compete for the same minerals and nutrients, and block the exchange of air and moisture that the oak's roots need.

Over time, the oak will lose out. And in that weakened state, it will may succumb to diseases like oak root fungus (Armillaria mellea) and Phytophthora.

Oak root fungus typically produces clumps of small, honey-colored mushrooms at the base of the trunk. Above, the leaves may wilt, yellow and fall off. Phytophthora often shows up as slime flux, a dark liquid that oozes out of the higher branches of the tree. Down at the root crown, instead of a tan, fresh-smelling cambium layer just beneath the bark, the wood will be stained a dark burgundy and show signs of rotting.

Your best recourse is to beef up the tree's vigor, so that it can martial its defenses to wall off the disease. If there are thirsty plants in the vicinity, remove them. Mulch the soil and spray it with a mild dose of compost tea every couple of weeks to encourage beneficial organisms to multiply. If you've been running sprinklers, replace them with a soaker hose or drip system around the perimeter of the canopy. Run it only once a month, for a long, deep soak.

Never again apply water within 10 feet of the trunk, any time of year. Also, ignore the folks who insist that their oaks prosper with extra water. The thing is, their oaks may very well look fantastically lush. But if oak root fungus shows up -- and it can be completely unpredictable -- you've set up a perfect storm.

# # #

Perfect Partners

The following native plants will thrive under coast live oaks and in dry shade.

Manzanita (Arctostaphylos). Evergreen shrubs with beautiful red bark. Beware of taller species that won't easily fit beneath the canopy. Plant anywhere except in deepest shade.

Dwarf Oregon grape (Berberis aquifolium 'Compacta' or Mahonia aquifolium 'Compacta'). Four-season plant. Green foliage all year; yellow flowers in spring; attractive berries in summer provide food for birds and other wildlife.

Bush anemone (Carpenteria californica). Evergreen shrub. Bears white, camellia-like flowers in spring and summer. Resists oak root fungus.

California lilac, ceanothus (Ceanothus). Evergreen shrubs. Look for lower-growing varieties. Plant along fringes, in half-day sun.

Coastal wood fern (Dryopteris arguta). Difficult to find, but worth the search. Terrific under oaks, other dry shade areas.

California fescue (Festuca californica). Short, clumping, perennial grass.

Island snapdragon (Galvezia speciosa). Sprawling, evergreen shrub with dainty, shiny green leaves. Scarlet, tubular flowers attract hummingbirds.

Island alum root (Heuchera maxima). Forms foot-tall mounds of large, dark green, maple-like leaves. Produces dainty spires of cream-colored, tiny bell-shaped flowers. Prefers shadier spots.

Pacific Coast iris (Iris - Pacific Coast hybrids). Flowers in shades of purple, bronze, yellow and white. Strappy leaves are said to be evergreen, although mine -- which are not summer irrigated -- tend to vanish by August.

Canyon Prince giant rye (Leymus condensatus 'Canyon Prince'). Silvery blue, evergreen grass. Provide absolutely no water. Otherwise, it may take off running.

Sticky monkey flower (Mimulus aurantiacus). Rambling, informal perennial. Grow along fringes, in at least half-day sun.

California polypody (Polypodium californicum). Goes dormant in summer; rounded-tip leaves curl under to conserve moisture.

Leathery polypody (P. scouleri). Forms knee-high, evergreen clumps. Prefers lots of leaf litter.

California sword fern (Polystichum munitum). Most prolific evergreen fern in western states. Forms v-shape in deepest shade; more upright with more sun.

California coffeeberry (Rhamnus californica). Evergreen shrub. Green berries mature to red, then black. Leaves curl up when rains fade.

Pink flowering currant (Ribes sanguineum var. glutinosum). Deciduous shrub. Dangling pink flowers in winter and early spring usually appear before the neat, rounded leaves.

Evergreen currant (Ribes viburnifolium). Low-growing, evergreen shrub. Spreads 10 to 12 feet.

Hummingbird sage (Salvia spathacea). Fragrant, plump magenta flowers. Blooms best beneath fairly open canopy.

Yerba buena (Satureja douglasii). The "good herb" grows flat, smells like mint.

Snowberry (Symphoricarpos mollis). Low growing. Spreads slowly, making colonies. Deciduous at end of summer.

Meadow rue (Thalictrum polycarpum). Perennial. Delicate, fern-like leaves. Dormant in late summer if let go dry. Smells skunky.

# # #

From an Acorn

In nature, an acorn falls from a tree, sinks through fallen oak leaves and duff, and makes contact with the soil. Under the best of circumstances, the acorn sprouts, sends a tap root deep into the earth and a tiny, pink-tinged set of leaves emerges above ground.

It's easy enough to replicate.

Gather a handful of acorns and dump them into a bucket of water. The ones that float may be viable. But the sinkers are better.

Lay the acorns on their sides on the ground wherever you'd like a tree to grow. Cover each with half an inch of soil. The roots and shoots will orient themselves naturally.

Also consider shielding the spot with a little dome of aviary wire to protect your emerging seedling from birds or squirrels.

You can sow acorns in containers. But be sure to transplant the seedlings after the first few leaves have sprouted, so that the fast-growing tap root does not stretch so long that it begins to twist around the inside of the container.

Likewise, if you discover any oak "volunteers" in your garden that you would like to move, transplant them while they're still small. Dig way down around each seedling to avoid disturbing or severing the roots.

# # #

Seeds of Wisdom

The various native plants that live beneath our coast live oaks provide valuable habitat for birds. Nuthatches, creepers, oak titmouses, thrashers and thrushes eat the berries, sip the nectar, use the thorns for protection and scavenge twigs and detritus for nesting material.