The red clusterberry, part of the large Cotoneaster genus, can be grown as a hedge or as a small tree with arching branches. Birds enjoy eating its berries throughout winter.

Photograph by Shari Kaplan

Winter irrigation needs differ for trees and plants

By TONY TOMEO

The first and last storm of the season gave us only about an inch of rain at the nursery and about half an inch at my Los Gatos home. It seems almost everyone enjoys the first rain, regardless of the mud and the mess it can produce. Rain is especially pleasurable for those in the nursery production industries. An inch of rain certainly is less than substantial, but enough to justify suspension of the irrigation cycle.

When the rain ended, the irrigation did not immediately resume as it would during warm summer weather. Because it is now autumn and the weather is cooler, irrigation was postponed until several days afterward. Also, the frequency of irrigation has been decreased and will stay that way until the weather begins to warm in spring. Less irrigation is necessary because plants are currently not as active as they are during warm weather and because moisture does not evaporate as rapidly.

Most plant species are beginning their dormancy cycle now that weather is cooler. The quantity of moisture necessary to sustain foliar activity during warmer weather is no longer required. Deciduous species abscise their foliage and will not require substantial water until new foliage is produced in spring. Evergreen species need only to sustain existing foliage but not active growth.

Although foliage remains partially active among many evergreen species, consumption of moisture is greatly inhibited because weather conditions cause the stomata (pores on foliar surfaces) to remain closed much of the time. Evapotranspiration (evaporation of moisture from foliar surfaces) is consequently inhibited as well.

Even species that insist on remaining active do not use as much moisture, simply because it cannot evaporate as it would during warm, dry weather. Anyone who took Mr. Sharpe's physics class at Prospect High School or who dries laundry on the line knows that cool air cannot absorb as much moisture as warm air. Humid air that is already partially saturated with absorbed moisture will absorb even less. Air that is both cool and humid is Maytag weather.

Irrigation should always be relevant to weather conditions. Superfluous water is not only wasted, but can cause or enhance serious diseases. Many fungal diseases and some bacterial diseases proliferate in moist or nearly saturated conditions. Soil saturation does not only affect roots within the soil, but may affect any plant part in direct proximity to ambient humidity caused by saturated soil below. Such diseases are more apparent in the endemic, mild climate because of the preferred combination of cool, moist weather and mildly warm weather through winter.

Soil type, drainage, exposure and irrigation requirements of plant species present should also be considered with weather conditions when adjusting irrigation frequency and duration. Sun exposure changes with the seasons, too, as the sun is out fewer hours of the day than during summer and at a lower angle in the sky (which means its rays pass through more of the atmosphere). In contrast to spring, it is usually safer to risk inadequate irrigation than to irrigate too generously.

Berry of the Week: Red clusterberry

Appreciation of the red clusterberry, Cotoneaster lacteus, is an acquired taste. Resembling the firethorn--mentioned in this column Oct. 31--it is usually more attractive to overwintering birds than to garden enthusiasts who encounter "volunteer" specimens inadvertently installed by the aforementioned birds. It can be espaliered or sheared into a formal hedge but is most attractive in its natural form: exhibiting long, arching limbs, loaded with 2-inch-wide clusters of red berries.

It is best among unrefined landscapes or in areas that are only minimally landscaped. Although the berries are dull relative to those of the firethorn, they last into spring without the deleterious influence to the birds that feast on them. The leathery, 2-inch-long leaves are forest green with white, tomentose undersides. The arching limbs may reach 8 feet high and wide.

The red clusterberry prefers full sun exposure, and it tolerates most soil types if it is well-drained. Refined garden situations with regular soil disturbances and irrigation may be somewhat stressful for it. Red clusterberry is not commonly available in nurseries, but may become a concern as it often appears in or near established gardens.

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Horticulturist Tony Tomeo can be contacted at 408.358.2574 or at LGHORTICULTURE@aol.com.

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