Applications of pre-emergent herbicide are used to prevent grassy weeds and crabgrass is one of the most common warm season grassy weeds.
Crabgrass arrives early. In fact, it is one of the first warm season grassy weeds, and this is due to its ability to germinate at a lower soil temperature (around 58 degrees Fahrenheit). Crabgrass's existence is a symptom of soil, fertility, and watering problems. Fix these problems and the pre-emergent herbicide will not need to be applied.
As early as mid-April, crabgrass may begin to appear in the middle to lower transition zone. It will first appear along sidewalks, driveways and parking lots, where it warms up quick. Do not worry if crabgrass is showing up and you have yet to put control down. Early, germinating crabgrass is often killed by late season frost. However, this early germinating crabgrass is your call to action. Now is the time to put a pre-emergent herbicide down.
How do pre-emergent herbicides work?
Apply pre-emergent herbicide before a seed germinates. This is when the pre-emergent is most effective. The first week of April should be a target date, especially, if the first week of April is not unusually cold or warm.
Stay as near to that first week of April as best as possible when applying pre-emergents. Try not to put them down any earlier. Why? The herbicide barrier will start to degrade (quickly in the beginning, and then slower as time passes) after the application. Weeds will start to break through as the herbicide brakes down to a minimum level in the soil. Put down an application too early and the barrier will have more time to degrade.
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. Consistent speed of application is also of utmost importance: +/- 0.5 mph is equivalent to a 16.7% change in rate.
