Fizzy Foliage: Cork oaks, known for their thick, spongy bark, are good street trees because they require no irrigation once established and have noninvasive roots.




How and when to water depends on many factors

By Tony Tomeo

The most common question I am asked is "How often should I water?" Because I am very experienced in horticulture, I always have the correct answer: "I don't know."

There are too many factors to consider and correct irrigation can only be determined by observation. Every species has specific requirements, which vary with maturity and growth rate. Temperature, humidity and wind affect how much water plants use. Soil conditions are also important considerations, as are irrigation methods.

Some plant species have very high evapotranspiration rates. Translated into English, this means they lose considerable amounts of moisture to evaporation from foliar surfaces. Willows, for example, like dry swampy conditions, as they consume large volumes of water. Although they can survive in less than swampy conditions, they are easily damaged if the soil dries out. It is therefore often necessary to give willows extra water to compensate for loss through the foliage.

Large, mature willows have more extensive root systems and are more resilient than young trees. Of course, during the winter when the trees defoliate, they draw no moisture from the soil, and only require enough to keep the roots from desiccating.

Warmer temperatures, low humidity and increased breeze all enhance evapotranspiration, increasing the need for water. All three factors affect foliar surfaces similar to laundry, which dries best on warm, dry days with a breeze.

Soil is best when well-drained, but with reasonable moisture retention. Sandy or gravely soils may drain well, but dry out very quickly. Heavy clay soils may hold too much moisture even when the surface indicates irrigation is necessary, since lower strata may be too moist to support healthy root growth.

Development of healthy roots requires a good ration of moisture and air, as well as nutrients. Dry soil causes wilt. Saturated soil will drown roots and provide an environment ideal for root rot. If upper soil frequently dries, most roots will grow at lower levels. Conversely, if lower soil is too saturated for the roots, or irrigation only moistens upper soil and leaves lower soil too dry, surface roots will develop.

In most situations, it is best to irrigate generously, but as infrequently as possible. This will allow for good air penetration between irrigation and for enough water to reach lower soil. If plants are wilting, frequency should be increased.

Lawns and a few plant species, such as rhododendrons and azaleas, are exceptions because almost all of their roots are near the surface. In this case irrigation may need to be more frequent than for the rest of the landscape. Mulch helps keep the top soil moist between watering.

Sprinkler irrigation is essential for lawns, and usually effective for most parts of the landscape. Although not as thorough, drip irritation is useful for conserving water. Slow application of water, as from drip emitters, usually moves it vertically through the soil faster than horizontally. Roots outside the volume of soil affected do not get any water. High volume emitters also conserve water and are a good compromise because they flood a basin, distributing water over a wider area, but do not irrigate the area outside the most dense part of the root zone. Spray emitters also affect a wider area than drip ones, but require more maintenance and often clog.

If irrigation is automated, it is best done at night or in the morning, when less water is lost to evaporation. Roses or other plants sensitive to fungal diseases may require that water be applied about dawn so that leaves and surroundings dry as quickly as possible. Some people may prefer to water later in the morning when everyone is out of the house, to avoid plumbing noise or lack of water pressure.

Tree of the Week: Cork Oak

During the French colonization of Algeria, many small groves of cork oak, Quercus suber, were lost to supply corks for the wine industry of France. Eventually, the French were able to grow the cork on plantations. (Now if only they could make wine comparable to that of California.)

Because the cork oak is native to the Mediterranean, it is well-suited to the climate of the Santa Clara Valley. Once established, they require no irrigation. They grow at a moderate rate and can get to 80 feet tall and wide. The leaves are about two and a half inches long with gray undersides. The canopy is not very showy, but the trunk makes a bold statement in the landscape. The thick, spongy bark is grayish tan and deeply fissured.

Cork oaks have well-behaved root systems and are excellent street trees. They are not commonly available, but can be found with persistence.

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Horticulturist Tony Tomeo can be reached at 408.358.2574.

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