NORTH FLORIDA — Timing of preemergence herbicide applications to prevent warm-season lawn weeds such as crabgrass should be from mid-February to March 1 when day temperatures reach 65 to 70˚ F for four to five consecutive days.
This generally coincides with the blooming of azaleas and dogwoods.
A preemergence herbicide must be applied just before the weed seedlings emerge. Correct timing is critical.
If your lawn has a history of warm-season annual weeds, one control option is to apply a preemergence herbicide at the correct time.
Weeds can be divided into broad categories. It’s important to know which category your weeds fit into in order to maximize control efforts. We have weeds that are cool-season annuals, warm-season annuals, perennials (some cool season, some warm season, and some persist year-round), biennials, and sedges.
Sedges look like grasses, except they have triangular-shaped stems, unlike the round stems of true grasses.
The weeds you’re seeing in your yard now are not warm-season annuals. They are annual winter weeds. Warm season annuals aren’t up and growing yet. Warm-season annual weeds will germinate and emerge from seed in spring 2023, then grow and become larger during spring and summer. Next, they produce flowers, followed by seeds which will be killed months later due to frosts and freezes in the fall and winter of 2023.
Seeds from warm-season annual weeds are still dormant now, awaiting warmer spring temperatures to germinate and emerge.
Warm-season annual weeds include crabgrass, Florida pusley, sandspur, old world diamond-flower, spotted spurge and chamberbitter. There are others.
When targeting chamberbitter, apply a preemergence herbicide that contains isoxabin in April, as this weed requires much warmer soil temperatures to germinate and emerge.
Season-long control may require a second application about six to nine weeks after the initial application, based on the herbicide label directions.
Not every lawn needs an application of preemergence herbicide. If your lawn has had no problem with annual summer weeds, there’s probably no need to apply a preemergence herbicide to prevent non-existent seedlings from emerging.
Only use preemergence herbicides on lawns that have been established for at least a year. They can severely injure freshly seeded or sprigged turfgrass. Most preemergence products interfere with lawn grass seed germination, delay reseeding 6 to 16 weeks after application.
Always follow label directions and precautions when using any pesticide, including herbicides!
More information on Florida lawns is available at https://edis.ifas.ufl.edu/entity/topic/lawns or from the UF/IFAS Extension office in your county.