One crabgrass plant can produce 150,000 seeds!
is a warm season annual that germinates from seed as the soil warms
in spring. Whether in lawns or ornamental beds, pre-emergence
herbicides are the easiest way to control crabgrass. These products
prevent the seedlings from germinating and developing normally.
The timing of pre-emergence herbicide applications is
critical to successful control because most of these products do not
control crabgrass that has germinated and is actively growing. They
should be applied 10-14 days prior to expected germination.
Crabgrass begins to germinate when soils are moist and the
temperature in the top inch of soil is 55-58 degrees at daybreak for
four or five consecutive days. There is a saying among turf managers
that you should apply crabgrass control when forsythia blooms. This
is an example of a phenological indicator, because the temperatures
and day length that push forsythia into bloom are close to those
that push crabgrass seed to germinate.
Preen (trifluralin) is one of the most widely
available pre-emergence herbicides for home gardeners to use in
shrub borders and flowerbeds. There is a long list of established
plants that safely tolerate the use of Preen Ė be sure the plants in
your garden are on the list before applying it. Be sure to remove
any growing weeds prior to application and water or lightly rake it
in after application. Like many pre-emergence herbicides,
trifluralin breaks down on exposure to sunlight. You should also
avoid digging or otherwise disturbing treated areas because that
disrupts the chemical barrier in the soil, which could allow some
crabgrass seed to germinate.
As for the large,
established plants in your beds now, hand weeding is the best
option. You are correct that Round Up (glyphosate) will injure or
kill the desirable plants in them. And since they will die with the
first frost, it is almost a waste of money to spray them now. The
biggest benefit is that you are removing the seeds that will cause
you grief for years to come.