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QUERCUS


545


a quarter of its length. Mildew- resistant. Can live more than 600 years. Q. berberidifolia. SCRUB OAK. Semievergreen shrub or tree. Zones 5, 7–10, 14–24. Native to California chaparral from 300–5,000-ft. elevation. Dense growth 6–15 ft. tall and wide. Leaves variable in shape, with wavy edges; may have smooth, toothed, or spiny mar- gins. Medium green on top, gray-green below. Oval acorns to 1 in. long enclosed by up to one-half in knobby cap with spi- ral pattern. Good unthirsty clipped hedge, background shrub, informal screen. Long confused with Q. dumosa, which is native only near the Southern California coast. Q. bicolor. SWAMP WHITE OAK. Deciduous tree. Zones A3; 1–3. Native to eastern North America. Slow to moder- ate growth to 50–60 ft., rarely taller, with equal or greater spread. Bark of trunk and branches fl akes off in scales. Shallowly lobed or scalloped leaves are 3–7 in. long, a little more than half as wide, shiny dark green above, silvery white beneath. Fall color is usually yellow but sometimes orange, fi ery red, or purple. Oblong to egg-shaped, 1-in. acorns, one- third enclosed by rounded cap with hairy scales. Tolerates wet soil; also thrives where soil is well drained. Q. buckleyi. TEXAS RED OAK. Deciduous tree. Zones 3b, 6–12, 18–22. Native from cen- tral Texas to northern Okla- homa. Grows 30 ft. tall and wide, often with multiple trunks. Bark can be smooth gray or black and furrowed. Green leaves grow up to 4 in. long and wide, with deep pointed lobes,


turning red in fall. Egg-shaped, 1⁄2-in. acorns have a cup-shaped cap. Native to bottomlands, it does well in rich, deep soil. Q. chrysolepis. CANYON LIVE OAK, MAUL OAK, GOLDEN- CUP OAK. Evergreen tree. Zones 3–11, 14–24. Native to foothills and desert mountains from southwestern Oregon to Baja California and east into Ari- zona. Handsome, round-headed or somewhat spreading tree to 20–60 ft. tall and wide. Bark is smooth and whitish when young, checked and gray with


How to Grow Oak Trees


Homeowners acquire oaks either by planting them or by inheriting trees that were present before their land was developed.


Planted oaks usually thrive with no special care, but spe- cies from summer-rainfall areas appreciate moderate to regular summer irrigation during dry spells. After the Mediterranean natives are established, they can take irri- gation or leave it, and Western U.S. natives are better off with little or no extra water. It takes about two years for a small oak to become established, and up to seven years for a tree planted from a 7-ft. box. Old native oaks from dry- summer regions must be kept dry during the warm season, since they often succumb to fungal root diseases if given routine summer watering. But don’t hesitate to give all kinds of oaks monthly soakings in winter if rains fail (and do this every winter in places that get less than 20 in. of rainfall per year), applying water at the drip line.


SPECIAL TREATMENT FOR EXISTING NATIVE OAKS Though highly prized mature oaks may look indestructible, they are easily killed by root compaction, grade changes, and overwatering. But because oaks take a decade or more to die, few gardeners connect the tree’s thinning, dying can- opy with the events that caused it. Here’s how to pre- vent trouble. 1. Don’t pave, excavate, or change grade level between trunk and drip line without fi rst consulting an arborist. All those things risk smothering (or killing outright) feeder roots that only thrive in well- aerated soil close to the sur- face. Compaction from driving heavy equipment or vehicles over the same area puts the same roots at risk with the same effect. One arborist spe- cializing in native oaks told us that it isn’t enough to tell a contractor not to drive across the root zone: you have to


surround the protected area (everything under the tree’s drip line) with rented fencing until the project is done and workers are gone. 2. When necessary, apply supplemental water over only the outer third of the root system—never over the roots closer to the trunk—to keep roots moving outward. Irrigate until soil is damp 8–12 in. deep (drip irrigation and soaker hoses make this easy). Never let water pool near the trunk. Though you should water recently planted trees weekly through summer, the most critical watering will be from March through May to support the tree’s new growth. 3. To safeguard oaks, only use underplantings that require little or no summer water, and keep even these several feet from the trunk. In dry-summer parts of the West, the heavy summer watering needed to keep thirstier underplantings happy can acti- vate oak root fungus (see “Pests & Diseases,” below). 4. Old native oaks benefi t from periodic grooming to remove dead wood, but don’t cut thick branches without good reason. To avoid stimulat- ing out-of-season new growth that will be susceptible to mil- dew and other problems, prune trees only when they’re dormant. With evergreen oaks in the far West, this means pruning during the dry sea- son—in mid- to late summer.


PESTS & DISEASES Any of a number of sucking and chew- ing insects and mites feed on oaks, but such creatures are usually kept in check by other insects, mites, and birds. Occasionally, though, an out- break of some organism gets bad enough to require artifi cial control.


Oak moths. Oak moth


cater pillars can defoliate trees and rain droppings on every- thing below. Severe attacks for 2 or more consecutive years can weaken or even kill a tree. However, oak moth cater pillar populations tend to


be cyclical, peaking every 7 to 10 years, then diminishing on their own. If control becomes necessary, consult an arborist or tree service; oaks are usu- ally too large for the limited spray equipment available to home gardeners. Galls. Most oaks produce


galls of various colors, shapes, and sizes on twigs or leaf sur- faces. The galls are swellings that form after insects (mostly wasps) lay eggs in the plant tissue; the larvae develop inside. Depending on the oak species and the type of insect involved, the galls may resem- ble apples, potatoes, mush- rooms, dunce caps, or other objects. They do little harm and normally do not warrant treatment. Fungi. Many fungi, both


benefi cial and pernicious, are associated with oaks. Oak root fungus (Armillaria mellea) occurs naturally in the roots of many oaks, but usually does not harm them unless they are under stress for other rea- sons. Quercus ilex and Q. lobata are especially resistant. The organism is particularly common in neighborhoods that once contained oak or walnut groves. It produces mushrooms at the base of the trunk and causes dieback in the canopy. This fungus is nor- mally held in check in summer by drought and in winter by cool temperatures, but it grows rapidly in the warm, moist conditions caused by summer watering, especially close to the trunk—and also attacks a wide range of other plants (resistant species are mentioned throughout this book). Consult an arborist about infected trees.


Sudden oak death (Phy- tophthora ramorum). This relatively recent disease, com- monly called SOD, has killed over millions of susceptible plants in coastal California and Oregon (see “Sudden Oak Death,” page 547). In the spe- cies descriptions on these pages, both susceptible and apparently immune oaks are noted.


Q


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