MANHATTAN, Kan. - When forsythia and redbuds reach full bloom, crabgrass will be sprouting soon.
"Generally, you get about two weeks between those events. So, since spring seems determined to get an early start this year, crabgrass is likely to germinate several weeks earlier than normal. Most Kansans should think about using April 1, not April 15, as their deadline for spreading preventer on their lawn," said Ward Upham, horticulturist, K-State Research and Extension.
With few exceptions, crabgrass preventers are simple pre-emergence herbicides. They have to be in place before crabgrass seeds germinate. They require a follow-up application about eight weeks later (May 27), to pick up any late starters.
Once applied to the soil, though, these preventers gradually begin to lose strength. So, applying them too early can be a time and money waster, Upham said.
"Products vary, of course. But, most are fairly ineffective after about 60 days," he said.
The major exceptions are season-long preventers: Barricade (prodiamine) and Dimension (dithiopyr). Nowadays, both are available under their trade and/or their chemical name. (Herbicide labels list the product's active ingredients, as well as instructions for its safe use.)
Homeowners usually can apply Barricade in November, to provide crabgrass control the following spring. Still, Barricade's no exception to the lawn-seeding rule: Pre-emergence herbicides are hazardous until new turf has grown enough to merit mowing two to four times.
March 1 is the earliest to apply Dimension. Upham said, however, it's the better choice in two situations:
* You missed fall's and spring's application deadlines. Dimension is the one preventer that also can kill young crabgrass until it exceeds the two- to three-leaf stage.
• You've seeded. Dimension is uniquely kind to young cool-season turfs. In fact, it's safe to use two weeks after tall fescue, perennial ryegrass, or Kentucky bluegrass germinates.