Potted plants seem to be much more common in gardens of western Washington, perhaps because many plants would simply become overwhelmed by rampant growth of other species if installed directly into the garden. Or perhaps potted plants are simply not as common in California because they are not as easily grown in semi-arid climates. Summer weather in western Washington is considerably cooler and more humid than summer weather of the Santa Clara Valley. Likewise, intensity of solar exposure is slightly diminished at that latitude (distance from the equator).

Irrigation of potted plants either in the garden or home can be somewhat tricky. Roots are confined to relatively minimal volumes of soil and cannot be as selective of environment as roots of plants grown directly in the soil. Unlike mature trees in the garden, potted plants are unable to disperse roots into deeper strata where environmental conditions are somewhat stabilized. If surface soil of the garden becomes desiccated to a depth of approximately eight inches, deeper roots are not damaged. If the upper eight inches of soil of a potted specimen becomes desiccated, and the pot is only eight inches deep, there will be a serious problem.

Roots confined to containers will, of course. be very sensitive to any adversities within the limited environment, including inferiority of the media (soil), salinity, alkalinity and desiccation or saturation. Potted plants are consequently more appreciative of regular applications of fertilizer but also require drainage so that salinity and alkalinity can be leached from the media. Irrigation must be regularly applied to avoid desiccation, but cannot be so generous that saturation is maintained.

Potting media, which rarely contains real "soil," is typically of good quality, but eventually decomposes and collapses, inhibiting drainage. As the level of the media becomes lower, addition of media at the surface may promote crown rot of the potted specimen. The container should instead be "stuffed" by sliding out the entire root ball, adding media to the bottom of the container and then placing the root ball into the container above the added media. Because most containers are tapered, additional media will likely be necessary around the root ball. A small amount of additional media may be added to the surface if roots have become exposed, but the base of the trunk or main stems should not be buried.

Roots prefer to grow toward gravity (positive geotropism) and are happy to grow into soil added below the root system, but might otherwise be disoriented by media added above the surface. Muddy soil that might fall away from the bottom of the root ball while it is stuffed has been too saturated for roots to survive in and should be discarded because it likely contained a profusion or fungal organisms associated with decay.


Flower of the Week: Orchid rockrose

Orchid rockrose, Cistus purpureus, probably does not perform well in western Washington because it prefers warmth and dry air. Humidity or frequent overhead irrigation during warm weather causes severe leaf-spot. Good drainage is necessary if irrigation is regularly applied. Rockrose is tolerant of heat, minimal irrigation and inferior soil. Full sun exposure is necessary for bloom.

The purplish pink flowers are approximately 2 1/2 inches wide. The base of each petal is marked with a dark red spot. Individual leaves are approximately 1 1/2 inches long and somewhat raspy. Growth is typically limited to four feet high and wide. Other species and cultivars of rockrose are also available.

Horticulturist Tony Tomeo can be contacted at 408-358-2574 or at LGHORTICULTURE@aol.com.