Weeds are a continual problem in Hawai'i landscapes. An effective first line of defense is maintaining a healthy lawn and garden, and using mulches where appropriate.

However, these steps rarely completely control weeds, so the occasional herbicide use can be necessary.

Herbicides, or weed killers, are among the general class of chemicals known as pesticides. All pesticides are potentially dangerous, and their use is under strict state and federal regulation. Before using any pesticide, read and follow all of the instructions on the label.

Since many herbicides control specific types of plants, it's also important to read the label to make sure the herbicide doesn't exterminate plants you want to keep.

The most effective herbicide weed-control program involves two approaches. First, prevent weeds from taking root in your yard with pre-emergence herbicides and second, eliminate weeds that do emerge with post-emergence products.

Pre-emergence herbicides affect germinating seeds and control the establishment of annual grasses and many broadleaf weeds that spread by seed dispersal. A pre-emergent will not kill existing weeds. This type of herbicide should be applied to the soil or turf surface and watered in well. Pre-emergents remain active for several months or more. Since weeds constantly germinate in Hawai'i, the herbicide should be reapplied as needed throughout the year. Both granular and liquid pre-emergents can be applied over existing turf, ornamentals and ground cover if the label allows.

However, many pre-emergents — such as Ronstar, Snapshot, Gallery and Surflan — are available only to landscape contractors or are sold in large quantities. In a quick survey of garden shops around town, I found several others available in smaller quantities — Amaze, Portrait and Preen.

Post-emergence herbicides kill already-growing weeds. They are most effective on young weeds. These herbicides must be absorbed through the leaves, so liquid sprays are more effective than granular materials. Most will not provide any residual weed control.

Post-emergence herbicides are either systemic or nonsystemic. Systemic types are absorbed through the leaves and green stems and circulated throughout the entire plant, resulting in a more complete kill of tops and roots. They are usually slower acting than nonsystemics, which may quickly kill the tops of weeds but can allow regrowth from the roots.

Post-emergents include selective and nonselective formulations. Nonselective herbicides kill all plants — including desirable ornamentals and turfgrasses — so be careful to prevent over-spray. These products are used when an area is to be cleared of all vegetation or as a spot spray in places without desirable ground cover, such as under hedges and shrubs or on mulched areas. It is important to understand that all nonselective herbicides kill any plant they contact. Common trade names for these products include Roundup, Kleenup, Finale, Ace Grass and Weed Killer, and Ortho Vegetation Killer.

Selective herbicides are formulated to control only one of three main groups of plants — grasses, broadleaves or sedges. It is therefore helpful to be able to identify these three main classes of plants. Most of us can recognize a grass and broadleaf plant but sedges are often mistaken for grasses. You can learn plant identification on several Web sites, and one of the best is www.hear.org.

Selective post-emergents for grass control can eliminate any grass (weedy or turf) from broadleaf ground covers and woody ornamental beds without injuring the desirable plants. Be careful to avoid over- spray onto lawns — many of these products will seriously damage any lawn turf.

Some companies simply market post-emergents as "grass killers." Ortho Grass Killer and Weed B Gone Grass Killer are found in most garden shops.

You can find more information on weed control and other lawn and garden topics at www.ctahr.hawaii.edu. Click on free publications.

Jay Deputy is an education specialist in landscape horticulture and turf at the University of Hawai'i-Manoa, Department of Tropical Plant and Soil Sciences, and Hawai'i state administrator for the Certified Landscape Technician Program sponsored by the Landscape Industry Council of Hawaii. Got a lawncare or turf question? Write to deputy@hawaii.edu.