Timing of a preemergence herbicide application for summer annual weeds such as crabgrass should be during mid-February to March 5 when day temperatures reach 65° to 70°F for four to five consecutive days. This generally coincides with when azaleas and dogwoods first begin to bloom. Note: This is not true for chamberbitter. Chamberbitter requires warmer soil temperatures to germinate. Apply a preemergence herbicide during April to May 1 when battling chamberbitter.
Most preemergent type herbicides won’t work when applied after weeds are visible. The product must be applied just before the seedlings emerge.
The weeds growing now in local lawns are not summer annuals. Summer annual weed seeds are still dormant awaiting warmer spring temperatures to germinate and emerge.
Most of the weeds in yards now are winter annuals. A few include annual bluegrass, chickweed, henbit, hop clover, lawn burweed and wild geranium. A preemergence herbicide should have been applied during October to help prevent these weeds.
A few common summer annual weeds include crabgrass, Florida pusley, chamberbitter, sandspur, spotted spurge and doveweed.
If your lawn has a history of summer annual weeds, one control option is to apply a preemergence herbicide. Timing is critical in order for preemergence herbicides to work.
Some preemergence herbicides to look for include oryzalin (Surflan), benefin (Sta-green Crabgrass Preventer, Hi-Yield Crabgrass Preventer), pendimethalin (Pre-M, Pendulum, Turf Weedgrass Control, Halts Crabgrass Preventer), benefin + oryzalin (XL), DCPA (Dacthal) and bensulide (Green Light Betasan Crabgrass Preventer).
For season-long weed control, a second application may be needed about six to nine weeks after the initial application. To activate some products, irrigation or rain may be necessary following application. Because preemergence products may interfere with lawn grass seed germination, delay reseeding six to sixteen weeks after application.
Overuse of some types of preemergence herbicides can cause a lawn to produce short stubby weak roots. So only apply the product if there is a pest to control – in this case, if you have had a history of summer annual weeds. Otherwise, save your money and time. Use preemergence herbicides only on lawns that have been established for at least a year. These products can severely injure newly planted lawns.
It is the user’s responsibility to read and follow all label directions and precautions when using any pesticide, including herbicides.
For additional information on lawn weed control, contact the UF/IFAS Extension Office in your County or access the following sites. http://edis.ifas.ufl.edu/ep141
Larry Williams, UF/IFAS Extension Agent, Okaloosa County