Lecture 7: Postemergent Weed Control in Landscapes
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- Selective Herbicides
- Non-Selective Herbicides
Effective weed management programs will reduce the amount of hand weeding required but there will always be some hand weeding.
Postemergence herbicides labeled for use in landscapes include selective and non-selective products. Selective herbicides kill some plants but not others. For example: we have herbicides available that kill grasses but do not damage most broadleaf ornamentals. In landscape beds we have several selective herbicides available for controlling grasses but few for controlling broadleaf weeds. Non-selective herbicides are, well…., non-selective; they will kill or damage most any plant. Non-selective herbicides must be used carefully to avoid damage to ornamental plants. Directed applications to the base of woody plants, avoiding contact with green tissues, will control weeds without damaging the ornamentals. Another “selective” use for non-selective herbicides is to make applications with a wiper. In this application a concentrated herbicide solution is wiped onto weeds using a sponge-like or paint-roller device. Such wiper or wick applicators are available from several sources. An understanding of where you may or may not use postemergence herbicides will aid in understanding how these tools may be integrated into a landscape weed management plan. The following table presents landscape sites where these herbicides have been successfully used.
Suggested use sites for postemergence herbicides
|Herbicide||Tree & Shrub beds||Ground-covers||Annual Flowers||Perennials|
|Fusilade, Vantage, Envoy, Acclaim||Yes||Yes||Yes||Yes|
Selective Postemergence Weed Control
- Grass control
- Sedge control
- Limited broadleaf weed control
Postemergence Grass Control in Landscapes and Nurseries
Annual and perennial grasses can be selectively controlled in most broadleaf crops and landscapes using postemergence herbicides that control only grasses – chemicals often referred to as “postemergence graminicides”. There are four graminicides labeled for use in horticultural crops – fenoxaprop, fluazifop-p, sethoxydim and clethodim. Each graminicide is systemic (translocated) and has short-term soil residual (about 2 weeks). Although each herbicide kills grasses in the same way (acting upon the same site of action), they differ in their effectiveness on grass weeds, safety on crops, and labeled uses.
FOPS & DIMS
The four postemergence graminicides can be grouped into two chemical classes sometimes referred to as the “FOPS” and “DIMS”. Fenoxaprop and fluazifop-p are chemically similar and are often referred to as the “FOPS”. Sethoxydim and clethodim, the “DIMS”, are chemically similar to one another, but quite different from the FOPS. It is important to know to which chemical class the herbicides belong because crop selectivity differs between the classes.
- Fenoxaprop – Acclaim
- Fluazifop-p – Fusilade DX, Fusilade II, Ornamec
- Sethoxydim – Poast, Vantage, Take-Away
- Clethodim – Select, Envoy
Acclaim is labeled for postemergence control of summer annual grasses in cool-season turfgrasses and landscape beds. It is most commonly used to control crabgrass in fescue or bluegrass turf. It is not as effective as the other three graminicides on perennial grasses. While safe on most broadleaf crops and ornamentals, it has been shown to severely injure certain cultivars of prostrate junipers and some azaleas.
Fusilade II is labeled for annual and perennial grass control in ornamentals, a few vegetable crops and some fruit crops. Ornamec is a special label for use in landscape ornamentals. Fusilade II controls perennial grasses including bermudagrass and johnsongrass, but is weak on fescues. Under certain conditions Fusilade II can be applied to tall fescue to control other weedy grasses, but high rates can kill tall fescue. Like Acclaim, Fusilade II can be safely applied over the top of most broadleaf plants but has injured certain cultivars of prostrate junipers and azaleas.
Poast / Vantage control both annual and perennial grasses. Poast is labeled for many vegetable and fruit crops whereas Vantage is labeled for use in ornamentals. Poast / Vantage has controlled crabgrass better than Fusilade II or Envoy / Select, but is generally considered to be weaker on perennial grasses. Varietal differences in crop susceptibility have not been reported with Poast / Vantage.
Select / Envoy is the only postemergence graminicide that controls annual bluegrass. It is generally considered to be one of the better graminicides on perennial grasses including bermudagrass and fescues. The newest of the graminicides on the market, Select / Envoy is not currently labeled on many crop species but the lists of labeled crops is expected to be expanded in the near future to include many vegetable crops and woody ornamentals.
Getting the most out of graminicide treatments:
Use the right product at the right time. As stated above, the herbicides differ in their effectiveness on several weeds. Select / Envoy is the only one that controls annual bluegrass. See the table below for a comparison of post-graminicides on some common grass weeds. The graminicides are more effective when applied to young, actively growing weeds, and less effective when applied to large, mature weeds. For example: Acclaim is most effective when applied to young plants – 1 to 3 tillers in size. At larger growth stages higher doses and repeated applications might be required. Similarly, Fusilade II and Select / Envoy are more effective on bermudagrass in early summer (with 6 to 8 inches of new growth) than in late summer when plants are flowering.
Table 1. Postemergence graminicides compared
Control rankings: E = excellent, G = good, F = fair, P=poor
Table 2. Recommended optimum times for treatment with
postemergence graminicides for several common weeds.
|Annual bluegrass||mid – fall||mid-spring|
|Downy brome||mid – fall||mid-spring|
|Crabgrass||1-tiller||up to 5 tillers|
~ 6 inches new growth
~ 6 inches new growth
|Summer when actively growing|
~6 in. new growth
Table 3. Ornamental plant safety
|Bar Harbor Jun.||No||No||OT||OT|
OT= Over the Top; Dir = Directed appl.; * = on certain varieties
- Define selective and non-selective.
- What is a directed application?
- What postemergence graminicide controls annual bluegrass?In a ground cover juniper bed what postemergence graminicides would you NOT use and why?
- List the 4 selective graminicides and classify them as FOPS or DIMS.
- When is the optimum time to treat bermudagrass with Fusilade?
- Which postemergence graminicides provides the best control of crabgrass?
- Which postemergence graminicide is used primarily for summer annual grass weeds?