Last week's article discussed pre-emergence herbicides that can be used to decrease weed invasion, and the nonselective herbicides, such as Roundup, used as a spot spray to kill weeds in areas with no closely intermingled desirable plants. There are other types of herbicide to consider.

A special problem is created when weeds begin to appear in the lawn. It's too late for a pre-emergence product to do any good, and using a nonselective will kill the weeds but also the grass. In this case you have two choices — hand weeding or treating with a group of pesticides known as selective post-emergence herbicides. These products are formulated to control only one of three main groups of plants, broadleaf plants, grasses and sedges.

Selective post-emergence herbicides for grass control will eliminate any grass from broadleaf groundcovers and woody ornamental beds without injuring plants. Avoid overspray onto lawns because many of these products will seriously damage lawn turf. Some of these products are not readily available at garden shops because of restrictions or large-quantity purchase requirements. Trade name examples are Fusilade, Ornamec, Poast and Acclaim. Some labels such as Ortho and Weed B Gone simply market these products as Grass Killers.

An even more selective variation of grass killers is the crabgrass killer which contains a common active ingredient called MSMA, a poisonous arsenic derivative. Examples are Weed Hoe, Trimec Plus and Weed B Gone Crabgrass Killer. They are safe to use on bermuda grass and zoysia lawns to remove crabgrass, however they may burn seashore paspalum lawns. Products containing MSMA kill any wide-bladed grass, such as centipede grass and St. Augustine grass.

Selective broadleaf killers can be safely oversprayed onto lawn areas without damage to the grass. Again, be careful with seashore paspalum, which is sensitive to many types of herbicides. These products will kill weeds such as dandelion, chickweed, spurge, clover and oxalis. They should not be sprayed on broadleaf ground covers, but are OK for mondo grass and lirope. Common products available at most garden shops are Weed B Gone Broadleaf Killer and Trimec for Southern Lawns.

Some companies market products that are particularly effective on hard-to-kill weeds. Ortho Chickweed and Oxalis Killer is effective on these two troublesome weeds but will also kill many other broadleaf weeds and groundcovers.

The third type of selective herbicide is the sedge killer. Hawai'i's three most common weedy sedges are nutsedge or "nutgrass," McCoy grass and kyllinga, which often are mistaken for grass. A key identifying feature of sedge is its triangular flower stem. The ridges can be felt when gently twirled between two fingers. Grasses have a round flower stem.

Two commonly available products for sedge control are Image and Manage. Both are safe to overspray onto grass but may cause injury to some broadleaf ground covers, particularly to wedelia, and Image will also injure seashore paspalum. Image works better than Manage for controlling kyllinga and McCoy grass and is most effective if followed by light irrigation within seven days.

Do follow mixing and application directions and safety instructions, which include protective clothing and the safe re-entry interval. The re-entry interval is the number of hours after application that is necessary before anyone can be in the treated area without protective clothing. Most herbicides approved for home use have a re-entry interval of 2 hours or less.

A good reference on weed control in turf is the UH College of Tropical Agriculture and Human Resources publication "Chemical Weed Control for Turfgrasses in Hawaii" (www.ctahr.hawaii.edu, click on free publications). To identify weeds, go to Google Images, type in the name of the weed you want and you'll find pictures.

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 state administrator for the certified landscape technician program sponsored by the Landscape Industry Council of Hawaii. Send questions to deputy@hawaii.edu.