If I had to think of ONE thing, just one thing that I get asked in the spring about lawns, it’s “when do I put on my preemergence for crabgrass?”. In most years, people are antsy to put it on, partially because they have spring fever after a long, cold Nebraska winter and they’re really wanting to do something outdoors in the landscape, and partially because they desperately want to prevent weed invasion in their lawns. Both are valid reasons.
So, when is the best time? It’s mostly based on soil temperature, as this is the driver for warm season grassy and broadleaf weeds such as crabgrass, foxtail, spurge and goosegrass. In order for these undesirables to germinate and grow, soil temps at the 1” deep level need to be 55 degrees F or greater for a few day in a row, which usually occurs in late April or in early May in some years. If you’re using a “4 step program”, this is step 1.
As with all products that control weeds, fungi and insects, it’s important to read the label thoroughly. It will tell you how much product to apply over each 1,000 sq. ft. section of turfgrass, how to apply it evenly and what to do after it is applied. Watering it off the turfgrass blades and into the soil where the target pests exist is important, as is using a leaf blower to move product particles off of sidewalks, driveways and streets back to the target area (the lawn), where it will achieve the goal of weed prevention.
In many cases, the second question that I’m asked goes something like this – “wait a minute, well, I don’t know…do I really need to apply it…my neighbor said I should, but then I got to thinking that I really haven’t seen any weeds in the past few years, and I don’t want to put it down if it’s not needed…what do you think?”. This question is well intended and demonstrates good intuition. In this specific case, if the lawn does not have a history of heavy weed pressure, then, no, it doesn’t need a preemergence herbicide application. If however, the weed species mentioned above are present year in and year out in big numbers, then it’s certainly a good idea to control them. As well, if the lawn is in pretty good shape, but has some bare spots, then a starter fertilizer with the active ingredient mesotrione application makes sense as it will prevent undesirable weeds from growing but encourage ryegrass, bluegrass or fescue seeds to germinate and become established….coupled with a regular premergence product containing prodiamine or pendimethalin for the other parts of the lawn, depending on need is a reasonable approach.
