Weed management in the landscape is often a difficult task for homeowners. Deterring weeds from taking root is the first line of defense against weed invasion into any part of the yard. The best way to accomplish this is to maintain healthy plants by avoiding over watering and over fertilizing — moderation is the key. Too much water and fertilizer can give weeds an advantage. Mow grass and prune shrubs frequently enough so that you are trimming off no more than one-third of the new vegetative growth. A thick, healthy lawn will crowd out many weedy grasses.

Shrub beds and ground covers often require different approaches than those used in grassy areas. Weeds can be suppressed in areas around the base of trees and shrubs and in ornamental planting beds by using mulch. Place a circle of mulch at least several feet in diameter around trees. It's best to mulch the entire soil surface in ornamental planting beds and under hedges. Mulching can define and accentuate the design of a site and give it a finished look.

The best mulches are relatively coarse-textured with a low water-holding capacity. Mulch helps reduce competition from grass and weeds and helps keep the soil moist and cool. They work well in suppressing annual weeds but do not work as well on perennials. It is important to keep the mulch at least several inches away from the base of trees and shrubs. Covering the base with mulch or loose soil can result in the bark staying too wet and eventually rotting. This will girdle the trunk and the plant will slowly decline and die.

Mulches are classified as organic (bark, wood chips, compost, leaves), inorganic (crushed rock, crushed coral, gravel) and synthetic (black plastic, landscape fabric).

The best organic mulching material is medium size (one or two inches) pieces of wood chips or chunks of tree bark. This woody material will last several years and it's easy to blow leaf and other types of yard debris off of it. Larger pieces of mulching material allow for better water and air flow to the underlying soil. Leaves or compost (such as Menehune Mix) doesn't work as well — it breaks down more rapidly to finer particles that hold water and become inviting to weed seeds.

Inorganic and synthetic mulches last longer and provide better weed control than organic mulches. Inorganic mulches such as gravel or stone are initially more expensive than organics, but are more stable over time and allow better drainage and air flow. Medium-sized bluestone gravel and crushed volcanic cinder or lava rock are the most commonly used inorganic mulches. Crushed coral can be used but it will gradually increase the underlying soil's alkalinity. Use coral only in areas that contain plants that can tolerate high salt and alkalinity.

A 3- to 4-inch-deep mulch layer is recommended for most situations in Hawai'i. One cubic yard of mulch covers an area of 100 square feet when applied at a thickness of 3 inches.

Installing a commercial weed cloth under the mulch will help keep weeds from coming up from below. Make sure the weed cloth allows water penetration for adequate drainage. Do not use solid black plastic, which keeps water and air from the root zone. Porous black plastic and other geotextile fabrics are now available on the market. Some of these products are impregnated with a preemergence herbicide.

Although mulching is effective in controlling weeds around the base of trees, under hedges and in ornamental shrub beds, it is not foolproof. Some weeds may grow on top of the mulch but hand weeding or spot spraying with an herbicide easily eliminates them.

Weed control in turf and many ground covers is a much different situation. These areas will often require the occasional use of herbicides, and this will be the topic of my next article.

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? Send it to deputy@hawaii.edu.