Use Preemergent Herbicides to
Prevent Crabgrass
Allan Robinson: Ask Master Gardener
Sunday, February 17, 2019
Q: When is the best
time to apply preemergent herbicide to help control crabgrass? Johnathan W.,
Tulsa
A: Preemergent
herbicides are definitely helpful in preventing weed establishment. They are
used in early spring (mid-February through mid-March) to prevent crabgrass and
other summer weeds. They also are useful in the fall (mid-August through
mid-September) to prevent winter weeds such as henbit. Many of the herbicides
will need a second application in spring 60 days after the first application for
complete coverage of crabgrass. The product label will indicate if this is
needed.
Many people are reluctant to
use herbicides of any sort for weed control. That is a reasonable choice for
those who are willing to tolerate some weeds. The best preventative is to
maintain a healthy, well-established lawn, as this will help prevent much of
the weed invasion. A healthy, thick lawn depends on good soil, proper turf
grass for the area, adequate sunlight and supplemental irrigation. Most lawns
need some fertilizer, and there are organic and synthetic sources available for
nutrients.
The Tulsa Master Gardener
website contains several helpful lawn maintenance calendars indicating what to
do, what to use and when to use it for Bermuda and fescue lawns. Specifically,
see the “Turf” section of the Master Gardener website for complete details.
Master Gardeners are often
asked if there are any “organic” preemergent herbicides, as opposed to
commercial or “synthetic” ones. Unfortunately, while there are other organic pesticides,
there is no effective organic preemergent herbicide.
Corn gluten is an organic
sold as crabgrass prevention. Some reports state that if it is applied during a
narrow window in spring, there may be some benefit. OSU turf grass specialists
cite studies that show little benefit.
For those wishing to use a
synthetic pre-emergent herbicide, OSU has some recommendations. While there are
several varieties of preemergents available on the market to prevent weeds,
especially crabgrass, OSU feels that one of the many commercial brands
containing the chemicals dithiopyr, pendimethalin or prodiamine are good
choices. These preemergents cost a bit more than other types but last a lot
longer and, in many cases, can kill crabgrass and other weeds after they have
sprouted.
The labeled directions of all
such products must be followed. These herbicides usually come on a dry particle
such as fertilizer or other inert material. They may also be found less often
as liquids. They must be washed onto the soil with at least ½ inch of water
after application. After washed onto the soil, they form a barrier for weed
prevention, which may last for months if undisturbed.
One of the benefits of these
three products is that they are not soluble in water and, thus, do not leach
into groundwater or spread from where they are applied. They are broken down in
nature by sunlight and soil microorganisms.
Garden tips
• Now is a good time
to cut back your perennial ornamental grasses, such as Pampas grass. Cut back
to remove the dead grass, but avoid damaging new buds and early green growth at
the base.
• Begin planting
blackberries, raspberries, strawberries, grapes, asparagus and other perennial
garden crops this month.
• Most bare-rooted
trees and shrubs should be planted in February or March. The roots of these
plants are easily damaged and should never be left exposed to air. Plant them at
the same depth as in the nursery and make sure good root and soil contact is
made by gentle tamping and irrigation after planting.
• Finish pruning shade
trees, summer-flowering shrubs and hedges. Spring-blooming shrubs, such as
forsythia and azaleas, may be pruned immediately after flowering (not before).
Do not top trees or prune just for the sake of pruning.
• Applying
preemergent herbicides earlier rather than later may be desirable to prevent
crabgrass and other summer weeds.
