GARDENING : CACTUS-COVERED HILL REMAINS A HORTICULTURAL MYSTERY.

Byline: Joshua Siskin

By now, any plant watcher who regularly drives west on the Ventura Freeway (101) toward Camarillo should have wondered about the cactus growing between the Camarillo Springs Road and Pleasant Valley Road exits.

If by some chance you have not yet noticed, there is a steep hill, immediately north of the freeway, that rises several hundred feet. Covering this gargantuan hill are thousands upon thousands of opuntia or beaver tail cactus plants. You can't help asking yourself how a cactus could have spread over such an enormous area.

``The Great Saguaro Book,'' by Susan Hazen-Hammond (Ten Speed Press, 1997), may help solve this mystery. The saguaro (pronounced sa-WAH-ro) is the famous candelabra or tree cactus - the one with the distinctive arms. Its sole habitat is the Sonoran Desert, which occupies southern Arizona, a small piece of southeastern California and Sonora, Mexico. It is the most charismatic of the large cactuses, reaching 60 feet in height and living for up to 300 years.

For dissemination of its seed, the saguaro (Carnegiea gigantea) relies completely on desert animals. Coyotes, peccaries and wood rats, as well as a variety of birds - such as purple martins, cactus wrens and brown towhees - feast on the sweet saguaro fruits. Undigested seeds from these fruits are then dispersed with the animal's droppings.

It seems entirely possible that seeds near our own freeways are dispersed the same way. Then again, since these opuntias are growing up and down the slopes of a hill, movement of the fruits downhill, whether simply by gravity or in rivulets of rainwater, could also explain the cactus's proliferation.

Contrary to popular opinion, the saguaro is not an endangered species, with as many as 1 billion plants alive today. Still, like our own local oak trees, if a developer wants to build in an area where saguaros live, the plants must be carefully dug and transplanted. When replanted, care is taken to orient the cactus so that the same side that faced the sun previously does so in the new location. Otherwise, life-threatening sun scald could result. Often, though, no matter how carefully they are handled, saguaros do not survive being moved.

This death-by-transplant syndrome is also a problem with Joshua trees - which are agaves, incidentally, not cactuses - that we see throughout the Antelope Valley and Mojave Desert. In fact, many desert plants, no matter how robust they seem, have difficulty recovering from being uprooted, even when transplanted a short distance from their original homes.

I have yet to see saguaros planted in Los Angeles, even though, according to the ``Sunset Western Garden Book,'' our climate should support their growth. Perhaps we lack the patience to follow their progress; it takes a saguaro no less than 35 years to reach a height of 6 feet. Hazen-Hammond informs us, ``At an age (about 65) when human beings think about retirement, the plant has just reached maturity and is putting on its first flowers.'' But, if we can wait for the flowers - which only bloom for a day - and their fruits, we will not be disappointed.

The author tempts us with the ultimate gustatory dare: ``Taste the sweet red pulp of the saguaro fruit, laced with thousands of tiny black seeds, and you will hunger the rest of your life for more.''

Until your saguaro is big enough to plant in the garden, try growing it in a container, either outdoors in a frost-protected location or indoors in bright light. Potting media should consist of 50 percent sand or perlite and 50 percent houseplant soil mix. From spring to fall, fertilize every two to three months with a low-nitrogen fertilizer such as 5-10-10.

To order saguaros through the mail, call King Nursery at [phone] or write to Phoenix Desert Cactus Nursery at 2619 E. Lynne Lane, Phoenix, Ariz. 85040.