Crabgrass is one of the first warm-season grassy weeds to arrive in the springtime and this is due to its ability to germinate at a lower soil temperature than other warm season grassy weeds (around 58 degrees Fahrenheit). Note: the existence of crabgrass is a symptom of poor fertility, soil, and watering problems. Correct these underlying issues and the pre-emergent herbicide may not be needed.Pre-emergent herbicide applications are used to prevent the pop-up of grassy weeds. One of the most common warm season grassy weeds is crabgrass.

Crabgrass may begin to appear in the middle to lower transition zone as early as mid-April. It will first appear in areas, which warm up quick – along sidewalks, driveways and parking lots. If crabgrass is starting to appear and you have yet to put control down, do not worry. Crabgrass that germinates early is often killed by late-season frost. However, germinating crabgrass is your call to action. Put a pre-emergent herbicide down now.

Large crabgrass appears coarse, lighter in color, and grows at a faster rate than surrounding desirable turf.

How do pre-emergent herbicides work?

Pre-emergent herbicides are the most effective when applied before a seed germinates. While weather does determine when to make these applications, the first week of April should be a target date (especially, if the first week of April is not unusually cold or warm).

You do not want to put pre-emergent herbicide applications down too early; stay as near to that first week of April as possible. Why? After putting down a pre-emergent herbicide application, the herbicide barrier will start to degrade (quickly in the beginning, and then slower as time passes). When the herbicide has broken down to a minimum level in the soil, weeds will start to break through. Put down an application too early and the barrier will have more time to deteriorate.

Pre-emergent herbicides are available on a granular fertilizer carrier and spread with a broadcast spreader, or in a sprayable form. Typically, either of these requires watering-in or application ahead of a rain event.

Always check your product label for specific application instructions.

Increase the probability of obtaining effective control throughout the growing season by following these guidelines:

  • Use a herbicide with a long-lasting, active ingredient: prodiamine (Barricade), dithiopyr (Dimension), or Oxadiazon (Ronstar). These products provide control for up to 16 weeks. Dithiopyr also provides post-emergent control of crabgrass to the first tiller (fifth leaf) stage of growth.
  • Put down your application at the correct time. The first week of April can be used as a target date, but not that the date may be sooner or later based on the weather conditions.
  • Apply a sufficient amount of herbicide: duration of control is directly related to the application rate. The more product that is applied, the longer the control will last. (Always stay within label limits.)
  • Split applications are a way to apply higher rates, while increasing application safety/ minimizing applicator error. Putting down the first application at half-rate, and then following it 4-6 weeks later with a repeat application can be an effective method.
  • Established rate recommendations for common pre-emergent herbicides:
    • Prodiamine (Barricade) 0.65 – 0.75 lbs ai / Acre
    • Dithiopyr (Dimension) 0.25 – 0.50 lbs ai / Acre
    • Oxadiazon (Ronstar) 3.0 – 6.0 lbs ai / Acre (generally only used on Bermuda)
  • Accurate, uniform application is critical for optimum control. Set rotary spreaders at the right setting and ensure proper spacing/overlap for uniform coverage. The consistent speed of application is also of utmost importance: +/- 0.5 mph is equivalent to a 16.7% change in rate.