Weed Out Weeds

Early spring is a great time to control summer annual grass weeds like crabgrass. The most effective way to minimize crabgrass is through good cultural management practices, such as proper mowing, adequate fertilization and weed control. However, in spite of our best efforts, herbicides will be needed, especially in high pressure areas next to drives and sidewalks, south facing slopes, and heavily trafficked areas.

PREEMERGENCE PRODUCTS. Preemergence herbicides must be applied prior to crabgrass germination. As the crabgrass germinates, it absorbs the herbicide, thus killing the plant. Applying preemergence herbicides accurately and evenly across a turf site is important. Preemergence herbicides are immobile in the soil and thus a uniform application is necessary to ensure that the germinating crabgrass will come into contact with the herbicide. Using a preemergence herbicide incorporated on a large fertilizer granule might make applications easier, but they might not provide a dense enough pattern to insure consistent herbicide application.

TIMING TIPS. There are many rules of thumb concerning the ideal time to apply a preemergence herbicide in the spring. Most labels say the product should be applied two weeks prior to the expected germination period for crabgrass, while practitioners apply products when the soil temperatures reach 50 F for five consecutive days or when the Forsythia are blooming. Timing of preemergence herbicides was more important years ago when the herbicides that were available did not have the duration of control like today’s products. Research at Purdue University and other universities across the Midwest has shown preemergence herbicides applied in March still provide season-long control. Many turf managers have even started applying preemergence herbicides with the final fertilizer application in November and have had excellent success. Late fall and early spring applications work because low soil temperatures minimize microbial breakdown of the herbicide, which is the primary method of degradation. A side-benefit of applying preemergence herbicides early in spring is that LCOs have a better chance of controlling early-emerging summer annual broadleaf weeds like prostrate spurge or knotweed. A tip to remember is that being too early with preemergence herbicide product applications is better than being too late.

SEQUENTIAL APPLICATIONS. For areas with extremely high crabgrass pressure or with the possibility of goosegrass, consider a sequential application of preemergence herbicides. An early spring application will provide acceptable control into early summer, but a second application will boost the activity of the herbicide in the soil to extend the duration of control throughout the growing season. Most labels will recommend 2/3 to ¾ of the high label rate in the first application followed by second application of 1/3 to ¼ of the total label rate in late May or early June. Purdue research showed that using sequential applications of an active ingredient different than what was used in initial applications will not give satisfactory results, appearing that concentrations of different herbicides are not additives in the soil. So, instead of boosting concentration of the initial herbicide in the soil with a second application, LCOs are merely adding a second herbicide at a concentration that will probably not give season long control. As stated earlier, research has shown that early spring-approved preemergence herbicides or even those applied in very late fall can provide season-long control of crabgrass when used at higher rates.

BROADLEAF WEED AWARENESS. Postemergence broadleaf herbicides are just the opposite of preemergence herbicides and it’s better to apply these later in the spring. Applications at or after flowering of dandelion are more effective than earlier applications. Ester formulations are more effective than amines in cold weather. Our data show that ester formulations of 2,4-D become effective on dandelion at 100 GDD (base: 50 F) whereas amine formulations become effective at 150 GDD (base: 50 F). Early spring applications simply burn off the leaves, and dandelions quickly regerminate. However, the new herbicide ingredient carfentrazone has shown improved early spring control of broadleaf weeds, according to Purdue University research.

The author is associate professor, extension turfgrass specialist, Department of Agronomy, Purdue University, West Lafayette, Ind., and can be reached at 765/494-9737 or zreicher@purdue.edu.