Weed Control for Home Lawns
Postemergence Herbicides
Postemergence herbicides are intended for use on weeds that have germinated
and are visible. They are applied directly to emerged weeds. Established
perennial weeds, both grasses and broadleaves, must be controlled with
postemergence herbicides.
General guidelines for postemergence applications are small weeds, good soil
moisture, and air temperatures between 60 and 90 degrees Fahrenheit. Weeds that
are small (two- to four-leaf stage) and actively growing are much easier to
control with postemergence herbicides. Control is improved at this stage because
young weeds readily absorb and translocate herbicides. Early weed control also
provides an opportunity for turfgrasses to fill in the bare areas left by dying
weeds.
Do not apply postemergence herbicides during the green-up (transition from
winter dormancy to active growth) process of warm-season turfgrasses. The risk
of injury from postemergence herbicides is greater during the green-up process
than when the turfgrass is fully dormant or actively growing.
Weeds that are stressed due to dry weather, heat, or other environmental
factors are more difficult to control with postemergence herbicides. Applying
herbicides such as MSMA, DSMA, 2,4-D, mecoprop, dichlorprop, and dicamba at
temperatures above 90 degrees Fahrenheit increase the risk of turfgrass injury.
The resistance of postemergence herbicides to wash-off by rainfall or
irrigation varies among products. Typically, a rain-free period of 6 to 24 hours
is sufficient to avoid a reduction in effectiveness. Even if rain falls soon
after application, some degree of reduced control will be achieved.
Mowing can affect performance of postemergence herbicides. Avoid mowing three
to four days before application for greater leaf area to intercept the spray.
Delay mowing three to four days after spraying to allow the herbicide to be
absorbed and translocated.
Rather than a single rate, a range of postemergence herbicide rates for a
product usually is given. Repeat applications of the low rate are generally more
effective than a single application of the higher rate. The follow-up
application is timed to be 7 to 14 days after the first or when regrowth
appears.
Also see
Herbicide Safety Precautions.
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