Buckeye Yard and Garden onLine

BYGLers noted that it is time to plan for preemergence herbicide applications for the control of crabgrass and other grassy weeds in turf. In fact, some of the longer-lasting products are already being applied.

Crabgrass is by far the most common of the warm season weedy grasses on lawns. Unlike the other warm season grassy weeds, such as goosegrass, smooth crabgrass begins to germinate when soil temperatures fall within the range of 52-58F, or 55F at the 4" soil depth. The other warm season grasses germinate when soil temperatures are in the upper 60's to low 70's, which can be several weeks later. Proper grassy weed identification is critical to learning whether or not a lawn is riddled with early-germinating crabgrass or another grassy weed that requires a different management strategy.

As with most pesticides, timing is everything! Preemergence herbicides do not kill seeds; they derail the seed germination process. So, these herbicides must be applied and activated before the seeds of the targeted weed(s) begin to germinate. The full bloom of forsythia or eastern redbud trees are reasonably accurate phenological indicators that crabgrass seeds are nearly ready to germinate. Remember to read the herbicide label carefully to learn how to activate the herbicide. Many products require a significant application of water.

Of course, application timing is also critical relative to how long the herbicide will remain active. Once the herbicide is activated, the clock is ticking on how long the herbicide barrier remains effective in preventing successful seed germination. One of the reasons that we try to avoid applying the herbicide too early is to maximize the chance of the herbicide barrier lasting through the season. You can increase your chances of getting effective season long control by doing the following:

* Select a product with a long lasting active ingredient, such as prodiamine or dithiopyr. Both provide 90% control for up to 16 weeks. Dithiopyr also has early postemergence activity for those cases where some crabgrass may have already emerged.

* To increase the duration of control, make certain you are applying the full label rates, or make split-applications with each application at 1/2 the full labeled rate. Of course, READ THE LABEL(!) to find out if the product is labeled for split applications and to learn the timing between the two applications.

*Select products that have size granule numbers (SGN) in the range of 150‑200 (1.5‑2.0 mm diameter). This will provide approximately 1‑8 particles per square inch. Fewer particles per square inch increase the chances of reduced efficacy. Products with higher SGN values will most likely be cheaper but potentially less effective.

* Select products that will provide the appropriate amount of recommended active ingredient per acre. Some preemergence/fertilizer products today are formulated to apply preemergence herbicides at the low end of the recommended rate range or even below the traditional rate recommendation range. Established rate recommendation ranges for three commonly used preemergence herbicides are: Prodiamine (e.g. Barricade) 0.65 ‑ 0.75 lbs ai/A; dithiopyr (e.g. Dimension) 0.25 ‑ 0.50 lbs ai/A; and Pendimethalin (e.g. Pendulum) 1.5 ‑ 3.0 lbs ai/A

* Finally, uniformity of application and speed of applications with rotary spreaders are critical for optimum control. Ensure proper rotary spreader spacing/overlap for uniform applications.

What about overseeding turfgrass? Almost all of the preemergence herbicides on the market are very effective in suppressing the germination of desired turfgrass seeds! Remember that preemergence herbicides should not be used as the first line of defense against turfgrass weeds. A thick lawn is the first line of defense. If the choice is using an herbicide to suppress weeds in a thin lawn, or thickening the lawn by overseeding, the best choice is overseeding.

Alternatively, siduron is safe for use on seedling turf. Follow the label directions carefully. When used properly, siduron will reduce crabgrass, goosegrass, foxtail, and many summer annual broadleaf weeds by about 80%.