HIBISCUS FLOWERY FAVORITE IN VALLEY GARDENS.

Byline: Joshua Siskin

This week's column focuses on two e-mails received during the past month.

Q: I was wondering if you could tell me anything about the hibiscus. I own one but don't know much about it. I would like to plant it instead of keeping it in a pot.

- Shanin

A: The common Chinese hibiscus (Hibiscus rosa-sinensis) to which you probably refer has long been one of the most popular ornamental plants in Valley gardens. Dozens of varieties are available with giant five-lobed star flowers, up to 8 inches across, in red, pink, yellow, orange or white. There are also recently introduced so-called connoisseur varieties whose flowers are multicolored with psychedelic patterns.

Hibiscus blooms virtually year around. Prune it lightly, to eliminate crossing branches and allow air and light into the interior. Do not grow hibiscus as a hedge since flowers are produced on shoot terminals. If you trim it too often, it simply will not bloom since you will be snipping the new growth from which flower buds are produced. Most varieties of hibiscus must be allowed to reach a height of 6 to 8 feet in order to be fully appreciated.

Under the best of circumstances, hibiscus foliage is another asset, providing a polished, deep green background for the vivid, tropical colors of the flowers. Be prepared for this foliage to turn yellow and drop, however, when there is a sudden change in temperature. Do not take this development personally; the plant will return to glory within a month or two.

Your desire to plant your hibiscus in the ground is sensible. In our climate, hibiscus does not do well as a container plant. After a year or so, foliage growth becomes spindly and flowering diminishes. In this regard, hibiscus resembles the bougainvillea, another tropical plant which, although it grows with reckless abandon when planted in the ground, struggles to grow at all as a container-bound plant.

Since 1992, Southern California has been infested with giant whitefly, which finds Chinese hibiscus to be one of its favorite host plants. For this reason, some people have ceased to plant hibiscus altogether. However, if you inspect your plants weekly you can prevent the giant whitefly from getting a foothold by rubbing off the eggs, clearly visible as squiggly white concentric circles on leaf undersides, before they hatch.

If the giant whitefly has established itself in your plant, and you see hairy white ``fur'' hanging from the leaves, you will have to prune radically, removing nearly all leaves and green shoots. Pruned back to its woody stems, the hibiscus will regrow soon enough, but you will have to carefully keep out whiteflies by rubbing eggs off new leaves or keeping leaves hosed off on a regular basis.

In its habitat, hibiscus grows in rocky, volcanic soil. This type of soil drains well, as should the soil in your garden where hibiscus is planted. In the Valley, hibiscus does best in half-day sun, preferably morning sun, where water and fertilizer requirements are modest. It will require more water and fertilizer if grown in full-day sun.

Q: When is the best time to move rhubarb?

- Brandi Brandeberry

A: The best time to move rhubarb, a perennial that grows from semi-underground stems known as rhizomes, is in the late winter or early spring, just prior to its first growth flush of the year after winter dormancy.

But since rhubarb must be planted in the shade to survive Valley heat, it could be moved now without undue concern about its survival, especially if it is suffering from over-exposure to the sun. Bear in mind that rhubarb has a short harvest period that is confined to the spring. By the time hot weather sets in, care of the plants is confined to watering and fertilization, both of which should be done generously.

Although poisonous, the large leaves of common rhubarb are highly decorative during most of the year. (Only the leaf stalks are fit for consumption). Even more worthwhile for garden accents are the ornamental rhubarbs (Rheum australe and Rheum palmatum). Deeply lobed leaves are up to 3 feet wide, with wavy or straight edges, and red veins or red foliage, depending on the variety. Magnificently tasseled, 2-foot-long inflorescences appear in white, pink, or red.

TIP OF THE WEEK: As July sets in and summer begins in earnest, make sure mulch is in place to minimize water loss from the soil and to keep roots cool, reducing plant stress. It is also recommended to hose off leaves of annuals and perennials to remove dust. Dust blocks sunlight from reaching leaves, interfering with photosynthesis, and encourages nesting of insect pests.