The War On Weeds: Postemergence Treatments

First, the average cubic foot of soil contains between 10,000 and 50,000 weed seeds, according to weed and soil scientists – no wonder LCOs have problems.

Second, weeds provoke bad vibes with clients. Customers are aware when a few dandelions are present, particularly if they are blooming. This isn’t the case with many turf insects or diseases.

Third, weeds establish easily in thin, weak grass stands. And if the turf becomes compact or damaged by drought or insects, many grass plants will die, leaving the turf open to weed invasion.

Skips and gaps in preemergence herbicide applications also can cause weed problems, leaving portions of the turf unprotected and making a postemergence herbicide application necessary. As a result, postemergence herbicides are becoming assets to LCOs because they are the only effective treatment for perennial broadleaves, such as plantain and white clover, and perennial grasses that are considered pests in sod-forming Kentucky bluegrass or buffalograss.

Even though the best defense against weed invasion is thick, healthy turf because it successfully competes with weeds for light, nutrients and water, postemergence herbicide applications can be necessary for proper control.

TIMING IS EVERYTHING. When controlling weeds, timing refers to the stage of the plant’s development, as well as the actual day of the year the postemergence application is made.

Control is most effective when applications target small, healthy, actively growing weeds. As they age, changes in the leaf surface, growth habit and physiological function occur. These changes result in reduced herbicide uptake and translocation in the plant. So, weeds should be sprayed when they are young and first observed in a landscape.

A basic strategy for enhancing postemergence control is applying the product when weeds have an adequate amount of leaf surface to absorb the herbicide. To accomplish this, let the weeds grow for a few days after mowing to create some leaf surface, then make the application. Avoid mowing for a couple of days afterward to maximize product absorption.

Accurate timing also involves spraying at the optimal time of the year. For perennial weed control, best results are achieved when postemergence products are applied in the fall. This is because most perennial broadleaves store carbohydrates and nutrients in their crowns and in underground storage organs (rhizomes, tubers, etc.) for the upcoming winter and spring. After application, the herbicide is likely to be translocated downward along with the food.

Also, newly germinated winter annual weeds (henbit, chickweed, etc.) are easier to kill than large, mature ones commonly seen in spring. And if weeds don’t die outright from the herbicide, they will become weak and more susceptible to winter kill.

Fall is also a good application time because neighbors with vegetable and flower gardens are less likely to worry about overspray. This also is the case with deciduous ornamentals, which lose leaves in the fall and are less likely to absorb potential drift.

Finally, fall tends to be cool, so there is less potential for injury to stressed-out turf due to herbicide phytotoxicity.

Another good opportunity for postemergence herbicide application is prior to total turf renovation, which is recommended when less than 60 percent of the turf area is comprised of desirable grasses (For tips on proper renovation steps, see Renovating Turf on page 84).

In addition to correct timing, effective use of liquid postemergence products can be greatly enhanced with the use of spray adjuvants – products added to the tank mix to improve performance, including spreaders, stickers, surfactants, anti-drift and foaming agents, and colorants (see Adjuvant-age below). Consult the pesticide label and pesticide suppliers for products that suit particular operations.

APPLICATION STRATEGIES. When considering effective use strategies for postemergence herbicides, start with the pesticide label, which contains information about application techniques and guidelines to improve control, such as optimal temperature ranges and wind speed restrictions.

Also, look for the following information:

• Recommended protective clothing to minimize applicator exposure
• Agitation requirements, if necessary
• Specific guidelines for the amount of product to mix with water
• Need for herbicide mixture circulation through the hoses and spray tank
• Size of mesh screen for spray system
• Restrictions on other products that may be perceived to enhance herbicide performance
• Application timing directions
• Suggestions for spray pattern indicators
• Indications for enhancement with adjuvants, crop oil concentrates or spreader-stickers.

Reading and understanding the label instructions is important, but following them is the key. Herbicide effectiveness is improved by implementing label recommendations and product guidelines.

GRANULAR VS. LIQUID. Formulation choice is important with postemergence herbicide applications. To be effective, most postemergence herbicides must contact the weed’s leaf surface. In the past, granular products performed poorly. Once granules are applied they tend to fall to the soil surface and work their way into the thatch, greatly reducing the herbicide’s effectiveness. Also, granules usually are applied to the entire turf area through a spreader, which places herbicide product on both weeds and turfgrasses. This is wasteful because herbicide also is applied to areas without weeds.

When using granular products, water the lawn to wet the weed’s leaf surface before making the application. Wet herbicide granules are more likely to stay in contact with the leaves. Recently, effective granular products have been developed that rival their liquid counterparts.

Still, liquid products offer significant advantages. Liquids cover a much greater percentage of the weed’s leaf surface, increasing herbicide absorption. Liquids also can be easily utilized for spot spraying. But granular products generally are easier to apply and create less potential exposure to the applicator during mixing and application. They also are less likely to drift.

NEW DEVELOPMENTS. Recent developments in postemergence products are limited. Most are the result of the reformulation of existing products in easier-to-use, safer packaging and delivery methods or increased effectiveness. Some new products can treat turfgrass species before or soon after germination.

Companies that reformulate existing products no longer available from manufacturers also are excellent resources for the turfgrass and ornamental market.