Application timing critical for controlling crabgrass
There are close to thirty-five species of crabgrass. The two most common species are smooth crabgrass (Digiteria ischaemum) and hairy crabgrass (Digiteria sanguinalis). Upon germination, crabgrass establishes well in warm, sunny and dry locations.
Figure 1. The above photograph is a picture of smooth crabgrass one of the most common crabgrass species. Although there are nearly 35 species of crabgrass species they germinate to similar soil temperature thresholds and plant phenotypic events.
In temperate regions the most important and likely the first weed control practice of the year is the proper timing of a pre-emergent herbicide application for crabgrass. Ideally, the pre-emergent herbicide should be applied just before crabgrass germination. If done properly the application will prevent germination and give control longer into the season.
Besides the traditional calendar date for application, soil temperatures and flowering ornamentals can be used to predict crabgrass germination. Below are some of those keys. It should be noted factors like turf cover, and soil moisture for example can influence crabgrass germination and emergence.
Soil Temperature Monitoring
Soil Temperature (pre-emergent herbicide applications should be made prior to these occurrences)
1) Minimum soil temperatures of ~ 55 to 59 at the 1 inch depth at daybreak for 4-5 consecutive days.
2) For significant crabgrass emergence and appearance soil temperatures greater than 73 F are required
At this website you can sign up for crabgrass and goosegrass pest alert. Fore example, if soil temperatures reach 55 F for 3 consecutive day, a message alert will be sent to you letting you know the threshold for crabgrass germination has been met. You can sign up for this alert by going to the above resource tab and look under Agronomic alerts.
Plant Phenotypic Monitoring
* Phenotypic Keys for crabgrass germination (pre-emergent herbicide applications should be made 14-days prior to these events:
* Forsythia bloom withering (more applicable in northern temperate regions)
* Daffodil (Narcissus spp.) bloom withering
*Dogwood (Cornus spp.) bloom withering (more applicable in warmer regions)
Pre-emergent herbicides work within a finite period of time. The interval can range from weeks to several months. Given the finite control period, you could risk crabcrass control failure later in the year if you apply the pre-emergent too early. As a general guide, the warmer the temperature and the higher the rainfall, the faster the product will degrade. Increasing the rate is one option to account for a rapid herbicide barrier breakdown.
When using a pre-emergent herbicide product, is important that it has a long-lasting active ingredient like prodiamine (Barricade® 4FL or Barricade 65WG ). Barricade and a few other long lasting pre-emergent herbicides applied within recommended rates provide 90% crabgrass control for up to 16 weeks. Pre-emergent herbicides work within a finite period of time. The interval can range from weeks to several months. Given the finite control period, you could risk crabcrass control failure later in the year if you apply the pre-emergent too early. As a general guide, the warmer the temperature and the higher the rainfall, the faster the product will degrade. Increasing the rate is one option to account for a rapid herbicide barrier breakdown.
About the author
Dr. Karl Danneberger is a professor of Turfgrass Science at The Ohio State University. Dr. Danneberger's contact information can be found here. You may also follow Dr. Danneberger on Twitter: