GARDENING : ORNERY ORNAMENTALS PUT A DEATH GRIP ON VEGETATION.

Byline: Joshua Siskin

For years, my sons and I had been climbing up Hastain Trail at the south end of Franklin Canyon in the Santa Monica Mountains, but had never seen that vining and twining plant with the bright yellow flowers and glossy green leaves there before.

To the uninitiated, the first reaction to encountering this plant in the wild is, ``Wow, why don't we see more of that in our gardens and landscapes?'' For those familiar with this eye-catching botanical specimen, however, the sight of it in the Santa Monica Mountains is troubling. The plant in question is German ivy (Senecio mikanoides) which, like a wehrmacht blitzkrieg, takes no prisoners in its invasion of new territory.

German ivy is one of a list of ornamental plants, available in nurseries and costing good money, that can quickly become weeds in Los Angeles gardens and open spaces, and are virtually ineradicable. A ``buyer beware'' disclaimer should be attached to such species.

In the right situation, the selection of some of these plants is justifiable. For instance, if you have a slope to plant or heavy, poorly drained soil, you may wish to consider naturalizing it with - that is, having it taken over by - one of these plants. Do not plant any of these in close proximity to other species, for these invaders are ruthless with the competition, which they eventually outgrow, strangle and smother at will. Each of these plants must be allowed lots of room - or be restricted to the confines of a container - to be appreciated.

Certain kinds of ornamental asparagus are on this list of pesky plants. The popular Asparagus densiflorus ``Sprengeri'' is a springy billowly plant with long shoots of light green, needly leaves. But once you have planted it, you may never be able to get rid of it, should you decide to renovate your garden. This asparagus has tubers, any of which can spring up into a giant prickly green mass years after you thought it had been eliminated from your garden.

Asparagus setaceus, or fern asparagus, is a vining specimen with the laciest, most feathery leaves imaginable. This soft foliage, which is used in cut flower arrangements, does a wonderful job of camouflaging wicked triangular thorns that sprout from wiry stems. Because of its thorns, fern asparagus discourages extrication from the surrounding plants into which it grows.

Have you ever seen ivy kill a tree? This is not an uncommon occurrence in side yards or in out-of-the-way corners of large back yards. What begins as a romantic notion - how pretty the neat green ivy looks climbing up a bare tree trunk - ends in disaster when the ivy arches and spreads over the tree's foliage, depriving it of light and life.

Of all invasive ornamental plants, the most seductive, by far, is Hall's honeysuckle (Lonicera japonica ``Halliana''). It flowers on and off throughout the year with fragrant flowers that can change color from gleaming white to deep yellow. This plant grows with a zeal unmatched by any other vine or ground cover. It is an excellent choice for hiding a chain-link fence where water is scarce, since it quickly grows up to 15 feet high and requires no more than a single monthly soaking once established. Just don't plant any other species in its vicinity; it will literally bury any neighbors under its rapidly proliferating shoots and leaves.

Common fennel (Foeniculum vulgare) is recommended for gardens since it attracts beneficial insects and swallow tail butterflies. However, it self-sows with rapidity and can quickly take up residence in adjacent parks or conservation areas, choking out more delicate native plants.

Tip of the week: Several readers have requested sources for unusual plants mentioned in this column. First of all, establish a good relationship with a neighborhood nursery, which should be able to special order many plants that strike your fancy but are not regularly brought in. You also might look at an excellent publication, Plant Search, which appears every two months and lists virtually every species grown in Southern California by the wholesalers that grow it. Contact the listed grower to find out which retail nurseries buy his plants, and then visit or special-order from those retail nurseries. A one-year subscription costs $24.50, sent to Vista Ventura Inc., P.O. Box 1115, Vista, Calif. 92805. You can also access the information in Plant Search through the Internet, at www.igin.com.

Visit botanical gardens having plant sales not only to find exotic plants but to meet the people who grow them. Lastly, you might want to consider attending a Southern California Horticultural Society meeting. Its members know more about plants, and where to get them, than anyone else in this city. The Society meets one evening a month in Friendship Hall in Griffith Park. For more information, call [phone].