Preemergence Control Of Winter Weeds

Numerous winter annual weeds initiate their growth at the same time that Bermudagrass enters dormancy. Annual bluegrass, henbit, lawn burweed (spurweed), common chickweed and corn speedwell are prevalent throughout Georgia and other states. These species germinate in the late summer and fall months as temperatures cool and moisture becomes available. There is also some germination during the winter or early spring months as temperatures fluctuate. During the cold months of the winter, growth is slow, but as temperatures warm in February and March, winter annuals initiate rapid growth and development.

Besides being unsightly, dense infestations of winter weeds can be extremely damaging to warm-season turfgrasses during spring green-up. Competition for sunlight, soil moisture and plant nutrients can be intense during this time period. As these dense mats of winter weeds die, the warm-season turfgrass cover or stand can be severely stunted or have large openings that can easily be infested by summer annuals such as crabgrass.

OPTIONS. Numerous herbicides may be used to control winter annual weeds. Options include the use of preemergence herbicides applied in the late summer or early fall prior to winter annual weed germination or the use of postemergence herbicides on an as need basis. Traditionally, most LCOs have relied upon atrazine and simazine to control winter annual weeds in non-overseeded Bermudagrass. Both of these herbicides have both preemergence and postemergence activity on a wide array of winter annuals, such as annual bluegrass, corn speedwell, common chickweed, henbit, parsley-piert and lawn burweed. However, over the past five to seven years, several instance of triazine-resistant annual bluegrass have been found on southern golf courses on sites in which these herbicides have been used for several years.

Atrazine and simazine belong to the triazine herbicide family. Triazine-resistant populations of annual bluegrass have been known since 1977. Continued annual use of triazine herbicides can lead to the development of triazine-resistant weeds by selecting out populations that are naturally present but are inherently resistant. Rotating to a herbicide with a different mode-of-action is the only way to achieve acceptable levels of control. If triazine-resistant annual bluegrass populations are suspected, then rotation to pronamide, glyphosate or diquat for postemergence control would be necessary. Alternatively, a preemergence herbicide could be used in the late summer to early fall months.

Because of the application flexibility, low cost and the effectiveness of the triazine herbicides, traditional preemergence herbicides, such as oryzalin, pendimethalin, prodiamine, dithiopyr and oxadiazon have not been widely used for winter annual weed control on southern turfgrasses. However, research conducted at several universities has shown that preemergence herbicides do have a place for winter annual weed control. Research conducted in middle Georgia showed that when applied in mid-September to common Bermudagrass, oxadiazon, oryzalin, dithiopyr, benefin, prodiamine, pendimethalin, benefin + oryzalin and benefin + trifluralin provided excellent annual bluegrass control during the subsequent winter and spring months. Previous research has also shown that when applied at a similar time period, oryzalin, dithiopyr, benefin, prodiamine and pendimethalin would control common chickweed, henbit and corn speedwell. However, unlike the triazine herbicides, this group of herbicides is not effective for the control of parsley-piert and lawn burweed (spurweed).

APPLICATION TIMING. Correct application timing of preemergence herbicides is critical to achieving high levels of control. Annual bluegrass germinates in the late summer and early fall when daytime temperatures consistently drop into the mid-70s and nighttime temperatures are in the mid-50s for several days. Maximum seed germination occurs when sunlight penetrates the turfgrass canopy. Additionally, there is often continuous annual bluegrass germination during the winter and early spring months as soil temperatures periodically warm. Areas thinned out due to the drought, as well as wet or shaded areas usually have high annual bluegrass populations.

Research conducted in North Carolina dramatically showed the importance of preemergence herbicide application timing effects on the control of annual bluegrass. One herbicide at 2.0 pounds of active ingredient per acre (ai/acre) applied in mid-August, or mid-September provided more than 91 percent annual bluegrass control, but only 21 percent control if the application was delayed until mid-October. Another at 2.75 pounds ai/acre applied in mid-August controlled annual bluegrass only 49 percent, but the same rate applied in mid-September or mid-October provided more than 95 percent control. A third herbicide at 0.65 pounds ai/acre provided 89, 88 and 84 percent annual bluegrass control with mid-August, mid-September or mid-October applications, respectively. The research indicates that on this site annual bluegrass germinated between mid-September and mid-October as evidenced by the poor control with a mid-October application. In the Piedmont of Georgia for winter annual weed control, I would apply the preemergence herbicide sometime during the first two to three weeks of September (by September 20). In north Georgia, the last week of August up to about September 15 would be the preferred time. In south Georgia, the application should be made during the mid-September to mid-October time frame.

FORMULATIONS. Most preemergence herbicides are available as a sprayable or dry formulation. Dry formulations consist of the herbicide impregnated on an inert carrier such as clay or various analyses of fertilizer. Herbicide/fertilizer carrier products have become extremely popular among LCOs and golf course superintendents. Applying a herbicide/fertilizer product is convenient and enables two operations to be conducted at the same time. In general, sprayable and granular formulations of preemergence herbicides are equally effective in controlling susceptible weeds. But keep in mind that regardless of the formulation, herbicides must be uniformly applied to the site for acceptable control. Uniform coverage is usually easier to achieve with a spray than with a granular application.

Several factors impact the results obtained with a herbicide formulated on a fertilizer carrier. Of these, application uniformity and percent load of the herbicide are the most critical. Application uniformity is determined by particle size, uniformity of particle size and application equipment. Particle size and uniformity of particle size are determined by the manufacturer or formulator. As particle size decreases, the density of particles per unit area increases. Uniform particle sizes are equally important to prevent ballistic segregation. Research conducted in Mississippi showed that southern crabgrass control increased to a point then leveled off as particle size of a dry fertilizer/herbicide product decreases. In other words, extremely small particle sizes were not necessary to achieve high levels of control. This research concluded that with dithiopyr and oryzalin, a particle size 465 particles per gram or greater was necessary to achieve high levels of control. For prodiamine, a particle size of 165 particles per gram or greater were equivalent in activity on southern crabgrass.

Another key factor to effective performance of the fertilizer/herbicide product is the percent load of the herbicide. High load products usually are applied at lower amounts of total material per acre than a low load product. Research conducted in North Carolina showed that prodiamine formulated on a 0.29G product controlled smooth crabgrass better than when formulated as 0.5G product. The increase in smooth crabgrass control was attributable to the better coverage with 0.29G product.

KEYS TO SUCCESSFUL CONTROL. Key factors to getting successful control of winter annual weeds with preemergence herbicides are:

• Apply the product at the recommended time. Weather varies from year to year and it may be necessary to apply earlier or later than normal. Reference to 30-day weather forecasts can help with this decision.



• Apply the product before rain is expected or water it in with ½ inch of irrigation water. Numerous instances of poor weed control occur each year because of the lack of rain or an irrigation event with seven days of preemergence application. Additionally, irrigating in the herbicide is an excellent method to prevent lateral herbicide leaching. Turfgrass preemergence herbicides essentially do not leach in a downward direction beyond a depth of 2 to 3 inches due to binding to soil colloids and organic matter. But they can move laterally, particularly if heavy rainfall occurs shortly after application. Thus, irrigation will usually improve weed control and will help to prevent lateral movement.



• Calibrate all application equipment. Uniform application is critical to achieving good weed control.



• If fertilizer/herbicide formulations are to be used, select a product that has uniform particle size and a sufficient number of particles that will ensure even, uniform application. Also, be sure that the herbicide load is sufficient to apply the recommended rate of the product. There is good data that indicates that dithiopyr rates can be reduced if applied on a dry granular carrier. However, with most other preemergence herbicides the amount of active ingredient applied per acre should be the same either for sprayable or dry formulations.