Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityMarch, 1995
Preemergence Herbicides for Landscape and Nursery
Keywords: herbicides, weed control, nursery, landscape, field production, container production
P reemergence herbicides prevent weeds from germinating, or kill them as they emerge from the soil. Most preemergence herbicides will not kill existing weeds; therefore these products are most effective when applied to weed-free soil. All preemergence herbicides must be 'activated,' - moved into the weed germination zone by rainfall, irrigation, or mechanical incorporation. Some herbicides will sit on the soil surface and wait days or weeks for activation, whereas some will be lost if not incorporated immediately. Some products may be safely applied over the tops of ornamentals, and some must be carefully applied as a directed spray. Be sure to know the difference!
Preemergence herbicides generally do not provide season-long control, nor does any one herbicide control all weed species. Most preemergence herbicides can be grouped into two categories: 1) products that provide good control of broad-leaf weeds and some control of grasses 2) products that provide good control of grassy weeds and some control of broad-leaf weeds.
Broad-leaf herbicides. Products that work best to control broad-leaf weeds include Princep, Goal, Ronstar, and Gallery. Princep may be used on established field-grown nursery stock. Nurserymen have relied on this product for many years. The EPA has called for a review of this product; in three to five years we may see greater restrictions on its use. Do not use more than four pounds of active ingredient per year. Lower labeled rates should be used on sandy soils; higher labeled rates should be used on heavy, clay soils. Princep may be less effective on weeds in low pH soils.
Goal is labeled for use on conifers in the field and in containers. Goal has an added benefit of providing some postemergence control of weeds under four inches high. This light-activated product forms a barrier on the soil surface that controls weeds as they emerge. Covering treated soil with a mulch or cultivating after application will reduce its effectiveness. Rout and OH-2 are two granular products that contain Goal; these may be used on a wide variety of ornamentals. Where mulch is used with these two products be sure to apply these products on top of the mulch (in contrast, most other preemergence herbicides are better applied under the mulch.)
Ronstar is a chemical similar to Goal; Do not cover with a mulch or mechanically incorporate this product. Ronstar has a much wider landscape and nursery label than Goal, and can be used in field, container and landscape ornamentals. It is also safe to use on some groundcovers. The granular formulation is gentler on desirable ornamentals; the wettable powder formulation can burn foliage, so use it as a directed spray.
Gallery has become quite popular in the few years that it has been available because it may be sprayed over the tops of quite a few ornamental plants, including some perennials, groundcovers, and bulbs. It may be used in both landscape and nursery, container and field.
Grass Herbicides. Preemergence herbicides that are most effective on grass weeds include Pennant, Devrinol, Barricade, Pendulum, Surflan, and Treflan.
Pennant provides good preemergence control of nutsedge in addition to annual grasses. It is labeled on many woody ornamental species and also some ground covers, bulbs, and herbaceous plants. To reduce the chance of injury, wash this product from the foliage of desirable ornamentals when applied over the top. Image is a new product that also controls nutsedge by pre- and early postemergence applications. However, it has a limited label for landscape ornamentals.
Devrinol. Devrinol is a good choice for fall herbicide application because warm temperatures and light reduce its effectiveness and longevity. However it may be used in the warm season if followed by adequate irrigation or incorporation. Devrinol may be broadcast over the tops of liners, container and field stock, as well as some ground covers and flowers. In addition to grassy weeds, Devrinol provides good control of chickweed.
Barricade, Pendulum, Surflan, and Treflan are related chemicals (dinitroanilines) that are most effective on grasses, but will control some broadleaf weeds. The dinitroaniline herbicides are all yellow in color, and inhibit root and shoot development of emerging seedlings. Where these chemicals are labeled for use in new plantings, the soil or media must be firmly settled-in around the plant before application because of the potential for root inhibition.
Barricade is the least soluble of the four, and has the longest residual. It may be broadcast over landscape plantings of many woody ornamentals, perennials, and wildflowers. Factor and Endurance are identical products to Barricade, but have a nursery label. Pendulum may be used over the tops of many landscape, container and field-grown ornamentals, groundcovers, annuals, perennials, and ornamental grasses. Southern Weedgrass Control is another commercial formulation that contains the same active ingredient Pendulum. Surflan has a very wide landscape, field, and container label that includes ornamental bulbs. All of the dinitroanilines mentioned above will wait for rain at least one week without losing potential effectiveness. In contrast, Treflan is a volatile substance, and should be incorporated by irrigation or incorporation immediately after application. The granular Treflan formulation is safe on many woody ornamentals, flowers, and groundcovers.
Broad-spectrum weed control. For optimal weed control, when making preemergence herbicide applications, combine one of the broad-leaf herbicides listed above with one of the grass herbicides. For convenience, commercial products are available that are premixed combinations of grass and broadleaf herbicides. Snapshot, Derby, OH-2, and Rout are all premixed combinations that contain a grass and a broad-leaf herbicide. Each is available in a granular formulation; Snapshot is also available in a sprayable formulation. Derby contains Pennant, thus is the only combination product which controls nutsedge. OH-2 is labeled for use in field and container nurseries. The other three combination products are labeled for the landscape in addition to field and container nurseries. XL is another granular combination product that may not give as good of control of broadleaf weeds, but it has a very wide label, and is the only granular product that may be used on some annual flowers. Apply granular products to dry foliage and irrigate-in shortly after application to activate and wash particles from leaves; do not use them on plants with whorls of foliage that capture the granules.
Kerb and Casoron are special-purpose products that provide broad-spectrum control of both grasses and broad-leaf weeds in established field-grown ornamentals. Kerb and Casoron are highly effective and long-lasting when applied during fall or winter; however weed control will not persist long into the warm season. In addition to their preemergence activity, these products are root-absorbed, and will control established perennials and winter annuals. Both chemicals require activation by rain or snow. Kerb is the best material for eliminating fine fescues. Kerb is a restricted-use chemical. Casoron should not be used on hillsides because it can move down a slope and injure non-target plants.
Weed Control Program. Preemergence herbicides should be the backbone of any chemical weed control program since prevention is much easier in the long run than cleaning up after the weeds appear. An effective, safe herbicide program should be built around two preemergence herbicide applications per year, spring and fall. The overall strategy is to prevent germination of summer annuals with the spring application, then prevent winter annual germination with the fall application. Make your spring herbicide application in late March to mid April. In Late August or early September, use postemergence herbicides to clean up existing weeds, then apply the fall preemergence herbicides. Don't wait until late fall after winter annuals have already germinated.
Because of hectic spring schedules, many nurserymen and landscapers find it difficult to get their herbicide applications on until May; - this is a problem since weeds have begun to germinate! If this dilemma describes you, in the fall of the year, use the higher herbicide application rates that appear on the herbicide label. Herbicide longevity is largely a function of application rate, length of time, and temperature, so using higher labeled rates will extend your preemergence weed control later into the spring.
Always consult the herbicide label for application rates, pesticide compatibility, and specific crop instructions!
