GARDENING : LETTING NATURE TAKE ITS COURSE AT THE GRASS-ROOTS LEVEL.

Byline: Joshua Siskin

``Once, while walking through my garden, I saw a hummingbird jab its beak into a leaf where a horde of whiteflies had nested. The whiteflies scattered in a cloud. Seconds later, the hummingbird was opening its beak to devour the airborne insects.''

Kathleen Brenzel, gardening editor of Sunset Books, recalled this experience during a recent conversation in connection with the just published ``Western Garden Problem Solver.'' This newly minted volume, released only last month, is meant to serve as a companion to the perennially popular ``Western Garden Book.'' Both are published by Sunset Books.

The hummingbird-whitefly incident was meant to illustrate Brenzel's laissez faire philosophy on pest control, which is encompassed by the timeworn but ever true maxim: ``Let nature take its course.'' To use pesticides for the sake of a few aphids, mealy bugs, or whiteflies is to cancel invitations to insect-eating birds, which should be welcome guests at your garden party. It would be wiser to invest in a birdbath or a birdhouse - since birds eat several times their weight in insects every day - than in a shelf full of insecticides.

In letting nature take its course, sound gardening practices still need to be followed. For Brenzel, such practices include: using plenty of compost to create a healthy soil, locating each species in the right spot where it gets the proper amount of sun or shade, and adhering to the principle of diversity in your selection of plants. The more diversified your selection, the wider the spectrum of beneficial, pest-consuming birds and insects your garden will attract.

Brenzel's favorite plants for attracting beneficial insects - such as parasitic wasps, lace wings and hover flies - are: common fennel, of lacy leaves and anise fragrance; rue, the finely cut, if foul-scented, blue-leafed herb; cosmos, the frilly leafed, somewhat gangly annual with white, pink and mauve petaled daisy flowers; coreopsis, the yellow-orange petaled daisy; achillea, better known as yarrow; nemophila or baby blue eyes, suitable for bright shade; Lychnis coronaria, a drought-tolerant, gray-leafed plant with brilliant contrasting blooms in red or pink; Agrostemma, or corn cockle, an ornamental weed with purple-spotted, magenta-pink petals. A bonus in growing these plants is that all have self-sowing tendencies; allowed to go to seed, they may sprout in corners of your garden where you would never think of planting them.

Turning to the subject of roses, I asked if there were any new trends that would simplify the matter of pest control among nature's most beautiful and most pest-susceptible plants. She pointed to the increased use of species roses, such as Rosa rugosa, and old roses that, as a group, are more pest-resistant than hybrid teas.

There is an outstanding example of an easy-care rose garden mentioned in the new book. It is the Victorian Rose Garden, in our own back yard at the Los Angeles County Arboretum in Arcadia. Here, prevention is key. Copious amounts of organic matter were worked into the soil before the roses were planted, and additional organic mulches and fertilizers are added throughout the year.

Another preventive measure is found in the roses themselves - all strong, disease-resistant varieties. In her own garden, Brenzel has found the David Austin hybrid Graham Thomas, a wonderfully fragrant yellow rose, to be impervious to pests. This should come as no surprise to Los Angeles gardeners, who are familiar with one of Graham Thomas' parents - the popular and durable white Iceberg cultivar. Both Graham Thomas and Iceberg can even tolerate a certain amount of shade.

On the subject of companion plants, Brenzel was less sanguine. A companion plant is supposed to be a sturdy type that keeps pests away from more sensitive species. For example, there is a certain amount of anecdotal evidence to suggest that chives, when planted near roses, will deter aphids. Yet, when Brenzel planted white flowering Chinese chives next to her own roses, the chives - to say nothing of the roses - were soon covered with black aphids.

To keep out weeds when growing vegetables, Brenzel recommends landscape fabric. Just roll out the felt-like fabric as though it were carpet and punch holes wherever you wish to plant. Place soaker hoses or drip emitters under the fabric for easy irrigation and put gravel or some other mulch on top of the fabric.

Tip of the week: To minimize insect pests and diseases in the vegetable garden, Brenzel recommends that you choose disease-resistant cultivars and that you rotate your crops. You can still have tomatoes every year. Just make sure that tomatoes do not grow on the same patch of ground more often than once every four years.