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Skip to content Skip to navigation. Herbicides are chemicals that kill or alter the normal growth of weeds. They can be divided into two main groups: Selective herbicides are those that control the target weed s without damaging desirable turfgrass species.
Nonselective herbicides kill all vegetation including turfgrasses and are used in lawn renovation or on weeds not controlled by selective herbicides. Herbicides can be further divided into preemergence and postemergence categories.
Preemergence herbicides are applied prior to germination and emergence of weeds. These are typically used for controlling annual weeds. Postemergence herbicides are used for controlling weeds that have already emerged from the soil. They are either contact or systemic in nature. Postemergence-contact herbicides affect only those plant parts that they contact and are not translocated to other portions of the plant. Postemergence-systemic herbicides are translocated throughout the plant; hence they are effective in controlling perennial weeds that can generate new foliage from underground vegetative structures.
Herbicides can be applied to foliage or soil. Postemergence herbicides are usually foliar applied, whereas preemergence herbicides are soil applied. A foliar-applied herbicide must contact and be absorbed by foliage, and is less effective if washed off the leaf surface by rainfall or irrigation.
Soil-applied herbicides can be applied as either liquids or granulars; they should be watered into the soil during or following application. Summer annual grass weeds are usually controlled with preemergence herbicides. These herbicides act by forming a chemical barrier in the soil prior to seed germination or emergence.
The barrier prevents grass seedlings from emerging and developing normally. You can use several preemergence herbicides to control summer annual grass weeds in Pennsylvania. Table 1 lists the chemical generic and trade names of some commonly used preemergence herbicides.
Several factors should be considered in choosing a preemergence herbicide. The first is the safety of the chemical for turfgrass species and cultivars. For example, oxadiazon may injure fine fescues, but it is generally safe on Kentucky bluegrass, ryegrass, and tall fescue. Most preemergence herbicides have long residual activity in the soil and may affect newly seeded turfgrasses. Siduron is the only material that can be safely used during or immediately following seeding.
For maximum effectiveness, preemergence herbicides should be applied uniformly at the label recommended rates. These herbicides are more efficient when watered-in within two to three days of application. The timing of preemergence herbicide applications is the most critical component of an effective chemical control program.
As a general rule, the best time to apply preemergence materials is approximately 10 to 14 days prior to the expected germination period in spring. Forsythia flower petal fall is not a consistently reliable means of determining timing of crabgrass herbicide applications.
Normally, preemergence crabgrass treatment in Pennsylvania should take place as follows:. Depending on the product, time of application, and location, reapplication within 60 days may be required for season-long control. Consult product labels to determine if two applications are allowed or needed. Poor weed control may occur with late applications. In these cases, postemergence herbicides may be required. Goosegrass germinates later than crabgrass.
Preemergence herbicide applications to control goosegrass should take place three to four weeks after the normal dates for applying crabgrass control materials. Postemergence control of crabgrass and some other summer annual weeds involves the use of chemicals that kill growing plants after they have appeared in the turf.
Postemergence herbicides can be used to treat only those areas where summer annual grass weeds have emerged. Preemergence herbicides, on the other hand, are usually applied over the entire turf area since the applicator does not know where seeds are or if they are present. For postemergence herbicides to be effective, crabgrass must be uniformly covered. Thus, these compounds should be applied only when crabgrass is visible in the stand. Table 2 lists the chemical and trade names of some commonly used postemergence herbicides for annual grass control.
It is important not to water turf for 24 hours after application. MSMA is effective in controlling crabgrass under both adequate and low soil moisture levels. Fenoxaprop-p-ethyl is a postemergence herbicide that is slowly translocated within the plant. It can effectively control tillered crabgrass with a single application. It is relatively safe on cool-season turfgrasses but may injure some Kentucky bluegrass cultivars, especially at high temperatures early in the season.
It should not be applied if cool-season turfs show signs of drought stress. Fenoxaprop-p-ethyl is less effective when tank-mixed with phenoxy-type herbicides such as 2,4-D and MCPP. Dithiopyr acts as a preemergence and postemergence herbicide. It provides postemergence control of crabgrass only up to the one-tiller stage of development, but it can be combined with fenoxaprop-p-ethyl when two or more tillers are present.
Quinclorac is a postemergence herbicide effective in controlling crabgrass and some broadleaf weeds. Optimum control of crabgrass is achieved when quinclorac is applied before development of the second tiller or when crabgrass plants have five or more tillers.
In some cases, quinclorac does not provide complete control of crabgrass at the two-, three-, and four-tiller stages of development. In these situations, a second application may be required for complete control. Quinclorac can be mixed with other herbicides, including pendimethalin and phenoxy herbicides, to improve weed control.
For best results, apply quinclorac in combination with a methylated seed oil according to directions on the label. Postemergence herbicides can be combined with preemergence herbicides to ensure that late-germinating summer annual grasses will be controlled along with weeds that have already emerged.
Studies at Penn State have demonstrated improved control of crabgrass with postemergence-preemergence applications over postemergence applications alone. Be sure to follow label directions when considering combinations of herbicides. Annual bluegrass is a tenacious and difficult-to-control winter annual grass weed in turf.
The following should be used only as a guide for determining which products and methods are available for controlling this weed. Before attempting to manage this species, realize that complete eradication is nearly impossible and that acceptable control may take several years to achieve. A chemical control program will usually be ineffective without a well-designed cultural control program that favors desirable turf species over annual bluegrass.
Chemical control of annual bluegrass can be attempted with preemergence herbicides, herbicides that have both preemergence and postemergence activity, and chemical growth regulators.
Several commercial preemergence herbicides can be used for annual bluegrass control. Application should take place in September just prior to the peak period of annual bluegrass germination.
Since seeds of this species germinate at different times of the year, complete control with a single application of a preemergence herbicide is unlikely. Although repeat applications will improve control, newly developing roots and rhizomes of desirable turfgrass species may be inhibited.
Only the annual subspecies of annual bluegrass can be controlled with preemergence herbicides. Be sure to follow label precautions if overseeding with turfgrasses. This herbicide can be applied to perennial ryegrass and Kentucky bluegrass, and is labeled for use on creeping bentgrass fairways. At least two applications of ethofumesate are required for successful control. Be sure to follow label directions as rates vary depending on the turfgrass species present.
The best results occur with both spring and fall applications. Both products reduce growth and cause yellowing of annual bluegrass for up to five weeks after application. The use of growth regulators for annual bluegrass control requires a great deal of care and a fundamental knowledge of golf course management. Most perennial grass weeds cannot be controlled with selective herbicides in turf. Glyphosate is an especially effective herbicide for perennial grass weeds since it is translocated through stolons and rhizomes and leaves no harmful soil residual.
This herbicide is most effective when applied to actively growing plants. Be sure to follow label directions and note restrictions for nontolerant species.
Broadleaf weeds are usually controlled with selective postemergence herbicides. The most common broadleaf herbicides used in turf include 2,4-D, 2,4-DP dichlorprop , MCPP mecoprop , dicamba, clopyralid, and triclopyr. There are many different commercial formulations and mixtures of these compounds see Table 3. It is extremely important to identify the weed s to be controlled before selecting one of these herbicides.
All have the potential to damage trees, shrubs, flowers, and vegetables if they contact the foliage. Trees and shrubs are particularly sensitive to dicamba since this herbicide is mobile in the soil and can be taken up by tree roots. Liquid postemergence broadleaf herbicides are available as salts and esters.
The most popular salt formulation is the amine salt. The amine salt is soluble in water and suitable for spray applications. Amine formulations are nonvolatile, but they can react with calcium and magnesium ions present in hard water and fertilizer solutions to form insoluble salts. Esters are insoluble in water but soluble in organic solvents and oils; hence, they are formulated as emulsifiable concentrates.
When added to water, ester formulations form emulsions and do not react with calcium and magnesium ions. Esters provide better penetration of the thick waxy coating on leaves of some weeds when compared with salts.
Esters can be classified as short- or long-chain molecules. Short-chain esters are highly volatile and can damage sensitive nontarget plants. Volatility increases as temperatures increase and therefore esters are usually used during cool weather.