Weed Control for Home Lawns
Preemergence Herbicides
Sequential or Repeat Applications of Preemergence Herbicides
• Advantages of
Preemergence Herbicides
Preemergence herbicides are the foundation of a turfgrass weed management
program. Preemergence herbicides are applied to the site before weed seed
germination. After being activated by rainfall or irrigation, these herbicides
form a herbicide barrier at or just below the soil surface. When the roots or
shoots of germinating seeds come in contact with the herbicide barrier, their
growth is inhibited. Most preemergence herbicides are cell division inhibitors
affecting the emerging root and shoot which are sites of rapid cell division.
Weeds that have already emerged (visible) are not consistently controlled
because their growing point has escaped contact with the herbicide. The primary
target of preemergence herbicides is annual grasses, but some small-seeded
annual broad leaves will be controlled.
A variety of factors affect the performance of preemergence herbicides. These
include timing of application in relation to weed seed germination, soil type,
environmental conditions (primarily temperature and rainfall), target weed
species and biotype.
Ideally, preemergence herbicides should be applied just before weed seed
germination begins. Applying too early may result in reduced control or no
control due to leaching and/or normal herbicide degradation. However, there is a
good deal of research that indicates preemergence summer annual grass control
applications may be made as early as January. The reason this works is that
during cool weather the rate of herbicide degradation is slow and most of the
preemergence grass herbicides do not leach readily. Preemergence herbicides must
be in place and activated before weed seed germination begins. Activation of
preemergence herbicides requires 0.25 to 0.5 inch of rainfall or irrigation. For
optimum performance, rainfall or irrigation should occur within 24 hours of
application to move the herbicides into the upper layer of the soil. The
critical period between application and activation by rainfall or irrigation
varies with herbicide, rate, and environmental conditions.
Crabgrass germinates in the spring (late March-April) when soil temperature
at the 4-inch depth reaches 53 degrees Fahrenheit. Alternating wet and dry
conditions at the soil surface as well as light encourage crabgrass germination.
Sequential or Repeat Applications of Preemergence Herbicides
In warm weather, herbicides begin to degrade soon after application
eventually reaching a level at which weed seed germination can occur.
Preemergence herbicides will degrade to the point of ineffectiveness from 6 to
16 weeks after application. For this reason, repeat or sequential applications
are needed for full season control. Make sequential applilcations 60 days after
the initial treatment.
Advantages of Preemergence
Herbicides
- Susceptible weeds are never seen.
- Most trees, shrubs and flowers are tolerant to preemergence
herbicides. Many of these products are approved for use in selected
ornamentals. Atrazine is an exception.
- Postemergence herbicides often cause temporary injury whereas
preemergence herbicides will not injure established turfgrasses.
- Preemergence herbicides are typically available as granules.
Granules are easier to spread and not susceptible vapor drift.
Also see
Herbicide Safety Precautions.
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