Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityApril, 1995
Postemergence Herbicides for Landscape and Nursery
Keywords: herbicides, weed control, nursery, landscape, field production, container production
C ombined with timely applications of preemergence herbicides, postemergence herbicides contribute to an effective weed control program. Postemergence herbicides by definition control existing weeds; those that are labeled for use in ornamentals have little or no soil residual, and thus have little effect on weeds that germinate after application. Postemergence herbicides are very useful tools for site preparation and selective applications to existing weeds in ornamental plantings.
Postemergence herbicides may be contact or translocated in their mode of action. The contact herbicides cause a rapid phytotoxic response at the cellular level. Cells lose their integrity and die within hours or minutes. We speak of a 'burndown' with contact herbicides because of the rapidity and appearance of the symptoms. Adequate coverage is important when using a contact herbicide. Obviously, protection of desirable plants is also essential.
Translocated herbicides are absorbed through leaves or roots and move systemically. Absorption through the leaves is the primary entry for postemergence herbicides with ornamental labels. Allowing adequate time between application and rainfall or irrigation is critical for success with these chemicals. These chemicals kill plants by targeting and interrupting one or more physiological processes within the plant. In contrast to contact herbicides which induce an acute toxic response, translocated herbicides may require ten days to two weeks for weed mortality. This disadvantage is far outweighed by the primary advantage of translocated herbicides: their systemic mode of action. Where contact herbicides will burn down the foliage of perennial weeds but leave underground propagules intact, the translocated herbicides move down into below-ground structures and kill the entire plant. Typically, effective control of the problematic perennial weeds will require the higher labeled rates of translocated herbicides relative to annual weed control.
The translocated herbicides are most effective when applied to actively growing, intact weeds; in this state translocation occurs rapidly and throughout. Avoid making applications when plants are under stress; control of weeds will be poorer and the risk of injury to desirable plants will be greater. Translocation is most active prior to flowering, in the bud stage; this is the time to spray problematic weeds. As a rule of thumb, late summer to early fall is also good timing for control of perennial weeds, since the plants are transporting carbohydrates to their root systems at this time of the year.
The postemergence herbicides may be divided into two broad groups. The non-selective herbicides indiscriminately kill green plants, including your desirable ornamentals if you are not careful. Selective postemergence herbicides kill some plants and leave others unharmed. Some of the selective postemergence herbicides may be used over the tops of ornamentals, but other must be used as carefully directed sprays.
Non-selective postemergence herbicides. Gramoxone Extra (paraquat) is a restricted-use, contact herbicide that is labeled for non-crop areas, site preparation, and carefully directed sprays in field-grown ornamentals. It provides a very rapid burndown of vegetation. Add a non-ionic surfactant to the spray mixture at 0.25-0.50 % (v/v). In the nursery, avoid using this herbicide around trees with a thin or pigmented bark.
In contrast, Sharpshooter is a mild contact herbicide used for spot treatments in the landscape. This product also may be used safely beneath greenhouse benches. It controls small annual weeds, under 4 inches in height.
Non-selective translocated herbicides include Roundup, Touchdown, and Finale. Roundup is an excellent product for site preparation or selective application in the nursery and landscape. In ornamental plantings, directed sprays or wipe-on applications may be used to control weeds. Usually a 1% spray solution is adequate for annual weeds, and 2% for perennials. Where a wipe-on application is intended, use a very concentrated solution of 20-30%. At this concentration, only a very small portion of the weed need be treated with the Roundup for control. Avoid using Roundup on thin or pigmented-bark trees; trunk injury may occur that may not be visible for a few years.
Ranger and Rodeo are products with the same active ingredient as Roundup. Ranger has additional surfactant in the product which increases efficacy. It is labeled and is highly effective for site preparation, but is not labeled for use in ornamentals. Rodeo has an aquatic label, and is a safe product for controlling vegetation in and around ponds where wildlife are a concern.
Touchdown is labeled for site preparation and field-grown conifers. This translocated herbicide works much like Roundup, and similar precautions should be taken. A surfactant should be added to improve spray coverage. Finale may be used for site preparation as well as directed sprays in the landscape, field, and container nursery. This product is something of a hybrid between a contact and a translocated herbicide. The chemical is absorbed into the plant, but translocation is limited because the plant is killed rapidly. Finale kills weeds more rapidly than Roundup, but is less effective on perennial weeds with below-ground storage organs. As with Roundup, avoid getting the material on immature bark.
The selective postemergence herbicides are selective in that they control either broadleaf weeds or grasses. The herbicides that are primarily active on broadleaf weeds are Garlon, Stinger, and Basagran. Garlon may be used in non-crop areas, or as a directed spray in field-grown conifers. It is an excellent product for the control of difficult woody perennial weeds. It also may be applied to brush in the dormant season as a basal bark application. It is much safer to use in the vicinity of desirable plants than other, soil active herbicides used for brush control. Stinger also has a conifer and non-crop label. It may be used safely over the tops of quite a few actively growing Christmas tree species, and provides excellent control of Canada thistle, vetch, and other perennial weeds.
Basagran T&O is a broadleaf herbicide with a landscape as well as a nursery label. It does a good job on nutsedge in addition to the broadleaf weeds. Addition of a crop oil concentrate improves weed control. Basagran works primarily by contact action, so spray coverage is important. This chemical can burn foliage, so use directed sprays around ornamentals. Injury occurs more frequently in hot and humid weather. Conifers are more susceptible to injury than other plants. While most plants should be shielded from the spray, Basagran may be sprayed over the top of English ivy, pachysandra, and liriope. Image also controls nutsedge in the landscape as well as wild onion, but has a limited ornamental label. Image has some preemergence in addition to postemergence activity.
The last group of herbicides are the selective grass herbicides. Fusilade (Ornamec), Vantage (Poast), Prism, and Acclaim are all translocated herbicides that control annual and perennial grasses. Unlike the broadleaf herbicides, these may be applied safely over the tops of many ornamental plants. They have no residual activity in the soil. Fusilade and Prism require the addition of a non-ionic surfactant to spray solutions at 0.25% v/v. The other two grass herbicides have surfactant included. Prism may be used under greenhouse benches.
All postemergence herbicides are subject to loss with rainfall or irrigation. The length of time required for absorption varies. (see table 1).
Table 1. Length of time required for adequate absorption of postemergence herbicides.
Acclaim 1 hour
Basagran T&O 8 hours
Finale 4 hours
Fusilade 1 hour
Garlon 4 hours
Gramoxone Extra 30 minutes
Prism 1 hour
Roundup 6 hours
Stinger 6 hours
Touchdown 6 hours
Vantage 1 hour
Always consult the herbicide label for application rates, pesticide compatibility, and specific crop instructions!
