are available to manage annual weeds. Preemergence
herbicides are absorbed by germinating seeds and seedlings
of annual grassy weeds such as crabgrass. Timing of
application is very important: The weed killer should be
applied before the crabgrass emerges from the soil. Many
preemergence crabgrass herbicides are available in
combination with lawn fertilizer at garden supply stores, so
crabgrass prevention and spring fertilization can be done at
the same time.
Crabgrass germinates when soil temperatures are
approximately 60°F at the quarter-inch level for five to
seven consecutive days. Other annual grasses germinate as
soils get above 60°F.
For central Illinois, mid- to late April is the suggested
time for applying a preemergence crabgrass herbicide. In the
southern portion of Illinois, make the application one to
two weeks earlier, and in northern Illinois, one to two
weeks later than the period suggested for the central
portion of the state. If the spring is very warm, use the
earlier dates; in cold, late springs these materials could
be applied later. Using forsythia flowering as a guide is
not a dependable method for determining application dates.
Some herbicides may be reapplied for extended control; refer
to the label for timing and rates. Core aerifying or
de-thatching should be done according to label instructions.
Preemergence annual grass herbicides, except siduron
(Tupersan), will interfere with seed germination of
desirable turfgrasses, whether you are seeding a new lawn or
overseeding into an existing lawn. Thus, siduron is the only
preemergence herbicide that can safely be used for spring
seeding or overseeding. Siduron is often combined with
high-phosphorus starter fertilizers and is available at many
retail garden outlets.