IN THE GARDEN DALLISGRASS WILL PERSIST, BUT IT CAN BE CONTAINED.

Byline: JOSHUA SISKIN

Q: My home was a rental for 15 years. When I actually moved in here a few years ago, I noted that dallisgrass had taken over the front lawn. I first plied a little trowel and hand hoe - neither of which made a dent, of course. I eventually graduated to using a large, pointed shovel upon which all 110 pounds of me jumped with both feet, attempting to dig it out - unfortunately, this didn't dent the insidious stuff, either.

My husband then applied a commercial weed killer and allowed it to work. We then had the entire yard deeply rototilled and planted new grass from seed, which came up healthy and looked lovely - until the dallisgrass returned. Attempts at digging it out are futile. We applied commercial weed killer again; however, our lawn clearly looks blighted. We have read that over seeding is one way to combat the nasty stuff, which we'll try after the weed killer has had time to do its job.

Are we on the right path, or do you have any other suggestions for us?

- Nancy Chattle, Winnetka

A: Dallisgrass, named after a 19th-century American farmer named A.T. Dallis, may best be described as crabgrass with rhizomes. Dallisgrass is easily identified by its square-edged seedheads, which resemble those plastic lanyards you made at summer camp. Like crabgrass, dallisgrass germinates and proliferates when the weather warms. Crabgrass, however, is an annual weed, which means that it dies completely with the onset of cold nights in late fall. In order to control crabgrass, you need only apply pre-emergent herbicide as soon as winter is over.

Dallisgrass, on the other hand, has perennial, fleshy roots known as rhizomes. Dallisgrass persists from year to year so that, to be eradicated, it must be dug up or sprayed. Still, it is advisable to use pre-emergent herbicide each spring to keep dallisgrass from returning to your property. If you do see dallisgrass start to grow again here or there, pull or spray it immediately before rhizomes or flowers (and seeds) can form.

You might want to consider planting an aggressive lawn grass such as rhizomatous fescue, Bermuda or Kikuyu, any of which could eventually outgrow dallisgrass. Keep in mind that dallisgrass is purposely seeded by ranchers as a pasture or forage crop. So, if you decided to turn your yard exclusively into dallisgrass, you could use a pet sheep instead of a lawnmower to keep it trimmed.

Q: My leyland cypress trees (Cupressocyparis leylandii) are dying. Is there something I can do to save these trees? I planted them as a screen because they are very fast- growing and tall enough to screen my back yard from the apartment building behind my house. If these trees die and Leyland cypress are subject to some killing disease, what can I plant to replace them? I need a fast-growing plant, 20 to 30 feet tall, but not something that will take up a lot of space, more vertical than wide. Any suggestions?

- William Langham, Van Nuys

A: There is no tree more beautiful than the Leyland cypress. It has a perfect pyramidal shape and flat, lustrous blue-green foliage. It grows five feet a year, reaching more than 60 feet at maturity. As a bonus, it never needs to be pruned. There is just one problem: In southern California, it seldom lives more than eight or 10 years, and it may die in half that time or less. The disease agent is a fungus that produces lesions or cankers on the bark, leading to dieback of stems. Where stems begin to die, they should be cut an inch or two below the blighted area. Disinfect pruning shears between cuts with rubbing alcohol or a 10 percent chlorine bleach solution (1 part bleach, 9 parts water).

It is best to prune now, during the dry season, because spores of the fungus are easily splashed onto cut surfaces during rainy weather.

Leyland cypress trees become sick due to stress. Too much or not enough water are equally dangerous to their health, although they do have a reputation for drought tolerance - I once saw a magnificent 50-foot Leyland cypress tree at the Peter Pitchess Honor Ranch in Castaic. This remarkable specimen was not growing in an irrigated landscape and seemed to thrive on neglect, although I did see it getting soaked, courtesy of a hose, from time to time.

To quickly create a tall hedge, I would plant yew pine (Podocarpus gracilior), Carolina cherry (Prunus caroliniana), white paperbark (Melaleuca quinquenervia) or Indian laurel fig (Ficus nitida). Be aware that any of these could lift up adjacent paved surfaces - the Ficus will definitely do so and therefore must be at least 30 feet from all sidewalks, driveways, patios and pool decks.

TIP OF THE WEEK: Consider planting bamboo as an alternative to trees where a tall screen along your property line is desired. Make sure you select noninvasive, clumping types as opposed to running bamboo varieties. To becoming acquainted with the various kinds of bamboo and their care, browse the Web site at www.bambooselect.us.