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applied
at a certain rate and a different material could have an eight-
week residual. The cost of the materials and the longevity could be
deciding factors. Conversely, the amount of labor to make more than
one application of any material may top the list of deciding factors.
These issues must be adjusted to accommodate each turf manager's
specific requirements.
Figure 4 represents the application of preemergence materials and
the expected length of time crabgrass will be controlled. This graph
shows how things are supposed to work. The X axis represents time
and the Y axis represents the concentration of preemergence herbicide
in the soil where the crabgrass seed will be germinating. For example
in the Mid-Atlantic re-
gion, typically crabgrass
germination starts some-
time in late April and
continues until about
mid-July. This is about 3
months (or 12 weeks or
90 days). The preemer-
gence material must
have adequate concen-
tration in the soil profile
in order to prevent the
crabgrass
seed from germinating. This is represented by the yellow line
on the graph. Any level of concentration of preemergence material
above the yellow line will prevent germination, and any level below
will not. The red line on the graph represents the concentration level
of a preemergence material applied once. The blue lines represent "split
applications" (two applications) of materials. As indicated on the graph,
when time passes, the concentration of preemergence material de-
creases in the soil profile but in both cases have lasted 90 days or more.
This should control the majority of crabgrass germination.
Product labels will have information about the residual (longevity)
of the product and application rates. If the preemergence control of
crabgrass has not been attainable, reevaluate your strategy. Review the
following questions: Has the correct rate of product been applied at
the correct time of the year? When was the last time you calibrated
you application equipment? Does the product have enough residual
to last for the season using the single application methodology? Should
split applications of product be incorporated into the program? Was
the product watered in per the label instructions? These are some of
the key questions to consider. Remember, you can make a plan —
but plan to change it frequently to adapt to nature.
Jeffrey Borger is an instructor, Turfgrass Weed Management, in the College
of Ag Sciences at Penn State's main campus in University Park, Pa.
Figure 4. A visual representation of the preemergence
herbicide concentrations in the top 1/4 inch of the soil
profile following an application in the spring of the year.
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