Weeds can be a persistent problem in Hawai'i landscapes. An effective first line of defense is maintaining a healthy lawn and garden and using mulches where appropriate. However, since these steps rarely provide complete weed control, the occasional use of herbicides is often necessary.

Herbicides, or weed killers, are pesticides, potentially dangerous chemicals subject to strict government regulations. Before using any pesticide, read the instructions, along with safety information, methods of application and how much to mix and apply. Since many herbicides control specific types of plants, be very certain that the plants you want to keep will not be damaged during application. Herbicides kill plants, not just weeds.

The most effective herbicide weed control program involves preventing weeds from becoming established with pre-emergence herbicides, then eliminating any weeds that do grow 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, but since weeds constantly germinate here, they should be reapplied as needed throughout the year. Both granular and liquid pre-emergents can be applied over existing turf, ornamentals and groundcover if the label allows. However, many of these products are available only to landscape contractors or are sold in large quantities, which make them very expensive. Some garden shops have them in smaller quantities for homeowners, under brand names such as Amaze, Portrait and Preen.

Post-emergence herbicides kill weeds after they have germinated and started growing. They are most effective when applied to young vigorously growing weeds. They must be absorbed through the leaves, so liquid sprays are more effective than granular materials. Most will not provide any residual weed control. These post-emergence herbicides are either systemic (which result in a more complete kill) or non-systemic (which offer visible results more quickly but can allow regrowth from the roots).

Post-emergence products include both selective and non-selective formulations. Non-selective herbicides kill all plants, including desirable ornamentals and turfgrasses, so be careful to prevent over-spray. These products are used to clear an area 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's important to know that all non-selective herbicides kill any plant they contact and can cause unwanted injury to desirable plants. 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, broad- leaves or sedges, so it is 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. There are several Web sites available for plant identification, including Plants of Hawaii at www.hear.org or even Google Images.

Selective post-emergence herbicides for grass control can be used to eliminate any grass (weedy or turf) from broadleaf groundcovers and woody ornamental beds without injury to the desirable plants, but be careful to avoid over spray onto lawns because any of these products will seriously damage any lawn turf.

Selective broadleaf killers can be safely over-sprayed onto lawn areas without damage to most grasses, but be careful with seashore paspalum. These products kill weeds such as dandelion, chickweed, spurge, clover and oxalis (Google them) and are OK on mondo grass and lirope, but are not safe to use on most broadleaf ground covers,

The third category of selective weed killers include those that control sedges. The three most common weedy sedges in Hawai'i are nutsedge or "nutgrass," McCoy grass and kyllinga. They are often mistaken for a grass, but while grasses have a round flower stem, sedges have triangular flower stems and ridges that can be felt when gently twirled between two fingers. Two commonly available products for sedge control are Image and Sedgehammer. Both are safe to over-spray onto grass but may cause injury to some broadleaf ground covers, particularly wedelia.

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