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Timing pre-emergence herbicides

Thursday, March 15, 2012

After last year's impressive crabgrass presence, control was on the minds of many this spring. Recently an in-depth article on crabgrass control for professionals was posted at http://turf.unl.edu/pdfcaextpub/LCOProSe..., but we've had a number of questions on timing of preemergence herbicides.

Traditional timing for preemergence herbicides is normally April to early May. It has been well-established in states east of Nebraska that applications made in the very late fall or early spring can be as effective as those made at the traditional mid-April to early May timing. Control from late fall or early spring applications will never be better than from the traditional application timing, but the primary advantage to very late fall or early spring applications is that they can help with managing the time and labor during the spring mowing period. Very late fall or early spring applications may also help control early germinating weeds like prostrate knotweed which may be missed by late April applications. The reason that the late fall or early spring timings work is because preemergence herbicides are broken down primarily by microbes that become highly active once soil temperatures reach 50-55F. Thus these herbicides do not break down significantly in cold or frozen soils. However, these herbicides do break down to a lesser extent by UV degradation and hydrolysis, so it is critical to use the highest labeled rate if applying in fall or early spring. The late fall or very early spring strategy is most efficient if using sequential applications. The sequential applications would still occur at the traditional late May to early June timing. A good example of this is shown in Table 1 highlighting data that will be appearing in an article in the June issue of Journal of Environmental Horticulture. A single November application of Barricade applied at 0.65 lbs ai/A (a medium rate) provided only 12 to 40 percent control by August depending on the year and location. However, applying a sequential application of 0.25 lbs ai/A in late May increased control to a more respectable 69 or 85 percent, given the crabgrass pressure on the sites. We currently have research studies examining application timing of preemergence herbicides.

The take home messages:

* If you do not have to balance time demands for mowing and preemergence weed control, applications in April to very early May still make sense.

* If you traditionally have struggled with the time demands for mowing and preemergence herbicide application, consider a November or early spring application.

* Regardless of when you apply, best season-long control results from the higher labeled rates.

* Regardless of when you apply the initial application, sequential applications in late May/early June will maximize season-long control.

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