Preemergence vs. Postemergence Herbicides

Tony Michaelsen - noemail@earthwormjobs.com — Wed, 10 Mar 2010 20:00:00 GMT

Which is better?

Prostrate knotweed is difficult to control, as it is tolerant of compacted soils.
Photos by John Fech, UNL.

For many turf managers, weeds are the bane of their existence. They’re ugly, compete with turf for water and nutrients, decrease turf density and cause clients to raise questions about the effectiveness of their pest management program.

If you’re having weed problems and want to use an herbicide as part of the overall control plan, which is better, pre or post? The answer: It depends.

There are many factors to consider. As with other pests, several cultural practices are just as important as using control agents and should be utilized as part of an overall approach.

Target weeds

For some weeds, preemergent treatments are more consistently effective. These include crabgrass, foxtail, goosegrass, henbit and prostrate knotweed.

For other weeds, postemergent treatments offer better control. These include white clover, ground ivy and wild violet.

Certain weeds can be effectively controlled with either approach, such as dandelions.

Timing

Regardless of the mode of action of the chosen product, application timing is critical for successful control.

As with most pest species, an optimal period for control treatments exists. For preemergent control of summer annual grassy weeds like crabgrass and foxtail, it’s when soil temperatures warm in the spring such that they remain in the 55 to 60-degree range for three consecutive days. For grassy sandbur, soil temperatures should reach 60 degrees before application. Prostrate knotweed is best controlled preemergence when soil temperatures are slightly above freezing in late winter/early spring. The bottom line for preemergence control of any weed is to apply prior to germination and as late as possible to improve season-long control. Application mid-fall is the preferred time for postemergence products for many perennial broadleaf and grassy weeds, as the leaf cuticle is thin, the plants are translocating nutrients to the roots and other underground storage organs like rhizomes, corms and bulbs and will move the herbicide along with them. Also, at this time, less opportunity exists for damage to desirable nontarget plants. In addition, if the target weed is not killed outright in the fall, it will enter the winter in a weakened state and be more likely to winter-kill.

Perennial grassy weeds are difficult to control under any circumstances, as the target weeds are closely related to the turfgrass species surrounding them, which has traditionally limited the capacity of the product to control the weed, but not the turf. However, recent developments in product chemistry have facilitated the entry of new products to the market that offer a reasonable degree of control. From a timing standpoint, younger grasses are easier to control than old, well-established ones.

Bottom line: Even if you choose the right product type, if it’s applied too early or late, it’s not going to work.

Newer products

Several newer products are now available that have expanded the list of choices for turfgrass managers. Here is a quick rundown:

Mulch will suppress most annual grasses, or in this case, allow newly germinated crabgrass to be pulled more easily.

Tenacity—Tenacity can be used as a preemergent and postemergent and has unique chemistry with unique selectivity. It is safe on all cool-season grasses except creeping bentgrass, when used as directed. University of Nebraska-Lincoln (UNL) trials have shown good to excellent control of creeping bentgrass, nimblewill and windmillgrass in Kentucky bluegrass and good to excellent control of a laundry list of broadleaf weeds, crabgrass and foxtail. Another unique property of Tenacity is safety at seeding. Tests for applications at planting for safety on Kentucky bluegrass, perennial ryegrass and tall fescue have shown excellent results. Work at other universities has shown control, albeit inconsistent depending on application timing, for annual bluegrass. Tenacity is currently labeled for sod farms and golf turf with a residential label expected shortly.

Certainty—Certainty is safe on all warm-season grasses, including buffalograss, and the cool-season grasses Kentucky bluegrass and creeping bentgrass. It is an excellent yellow nutsedge product and will also selectively remove rough bluegrass, tall fescue and quackgrass from Kentucky bluegrass. For turf managers wishing to maintain pure stands of Kentucky bluegrass, Certainty offers refined selectivity for difficult to control perennial grasses to achieve this goal.

Revolver and Monument—Revolver is a postemergent that recently received expanded label use for buffalograss, and is an excellent tool to selectively remove cool-season grasses from warm-season grasses. UNL researchers have also tested it on an array of broadleaf species with good to excellent results. Monument is a product similar to Revolver (both are sulfonylureas). Monument has a greater susceptible weed list on its label, including sedges.

The reduction of canopy in this perennial bed allows for rapid germination of annual weeds after pruning/renovation of the yarrow.

Dismiss—In addition to both pre and post activity on yellow nutsedge, Dismiss is effective on a multitude of broadleaf weeds and has been reported to have good activity on goosegrass. Weeds susceptible to Dismiss show injury relatively quickly, often within 12 to 24 hours after application. It is a postemergent product.

Onetime and Drive XLR8—Onetime, a combination product containing dicamba, MCPP and quinchlorac, offers postemergent, broad-spectrum broadleaf and annual grass weed control. Tests in Nebraska have shown good to excellent activity on ground ivy, as well as dandelion and, as expected, clover, crabgrass and foxtail, offering significant advantages for many turf species, including buffalograss. The exclusion of 2,4-D results in excellent safety and broad-spectrum weed control. Drive XLR8 is an improved formulation of Drive (quinchlorac) with quicker response and rainfastness in less than one hour.

RoundUp PROMAX—Offers the security and confidence of the industry standard nonselective herbicide glyphosate in a more concentrated formulation and smaller packaging, decreasing storage and shipping requirements. UNL testing has shown equal or better efficacy than RoundUp PRO.

Factors that affect product success

When applying preemergent herbicide products, keep in mind that various factors can enhance or reduce the efficacy of the application. Slope, lack of rain, too much rain, timing, inadequate infiltration (often limited by heavy clay soils), excessive adsorption to soil particles, residual effect of the product and photodegradation can all compromise efficacy.

Many factors can limit or improve the effectiveness of postemergent applications, as well. Excessive rainfall washing the product off the target, volatilization (change from liquid to gas) of the product to the atmosphere, and off-target drift resulting in reduced efficacy and increased nontarget injury are possible negative factors. The use of an appropriate spray adjuvant can go a long way in avoiding many problems associated with postemergent applications.

The efficacy of both pre and postemergent applications can be severely limited by numerous other factors, including carrier volume, improperly calibrated equipment and mixing incompatible materials, such as other pesticides or fertilizers, in a spray tank. The pesticide label is an excellent resource to help the applicator avoid costly mistakes.

Overall weed management

The best defense against weeds is a thick, well-managed turf. A vigorous turf will successfully compete with weeds for light, nutrients and water. Weeds become established most readily in thin, weak stands of turf. In most situations, just spraying weeds does not produce satisfactory, long-term results. Although herbicides can be used in an integrated weed control system, proper management can do much to encourage a dense, vigorous turf and discourage weeds.

10-Mar-10 2:00 PM

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Preemergence vs. Postemergence Herbicides