Preemergent herbicides, also known as soil-residual herbicides, are applied to the soil before the weeds appear in order to prevent their germination. In some cases, they will also inhibit young weed seedlings from growing. After application, the herbicide is activated when incorporated into the soil or when water is applied (irrigation or rain). The residual effect of the herbicide will last from several weeks to a number of years, depending on the particular herbicide used, the rate of application, and soil characteristics. The more common preemergent herbicides will persist in the soil from 1- 12 months. Some herbicides must be activated within 24 hours of application while others can wait up to 21 days. Best results are obtained when a half inch of water is applied to the area immediately after application.
Preemergents can be applied either as liquid or granular with a fertilizer spreader. They can also be applied over the top of existing ornamentals and groundcovers if the label permits. In Hawaii, preemergent herbicides may not last as long as the label indicates since they generally degrade faster under wet, warm conditions.
In choosing the right herbicide, first determine the type of weeds that need to be controlled. Since many homeowners do not know the names of the various weeds, at least determine whether the weeds are broadleaf or grassy weeds. This is essential since herbicides are categorized as to whether they kill broadleaves or grasses. Some herbicides crossover and control both types of weeds. It is important to read the label carefully since it will tell you what weeds the product will control.
Next, determine where the herbicide will be applied - turf, flower beds, edible plants, around trees or shrubs. Situations differ between herbicides. Some are allowed in vegetable gardens while others are not. Some are labeled for turf, and others are not. Again read the label carefully.
The following are some preemergent herbicides that are registered in the State of Hawaii.
- · Ronstar (oxadiazon) - provides preemergent and early postemergent control of a variety of annual broadleaf and annual grassy weeds (greater activity on broadleaf weeds). It may be used on established perennial turf such as bermudagrass, perennial ryegrass, St. Augustine grass, seashore paspalum, and zoysiagrass, and around woody ornaments, shrubs, vines and trees.
- · Impede (oryzalin) – provides control of a variety of annual grasses and broadleaf weeds in flower beds, groundcovers and around trees and shrubs. As with many of the preemergent herbicides it will not work on weeds already growing. It lasts up to 4 months and is not registered for use around edibles or lawns.
- · Preen (Trifluralin) – for use in flower and vegetable gardens and around trees and shrubs. Not for use on lawns; for control of a variety of both grassy and broadleaf weeds.
- · Gallery (isoxaben) – for the control of a variety of broadleaf weeds in turf, ornaments and groundcovers.
Additional choices are available for use in woody ornamental plantings. They are Casoron, Rout, Ornamental Herbicide 2 and Snapshot TG. These should not be used in herbaceous ornamental beds; they have the potential for injuring herbaceous plants.
For more detailed information, download the CTAHR (College of Tropical Agriculture and Human Resources - University of Hawaii-Manoa) publication Weed Control Options in Landscape Beds and Groundcovers.
