It can be frustrating to see breakthroughs of crabgrass and goosegrass in the spring and early summer after a pre-emergent herbicide was applied earlier in the year.
Here are the top five reasons why applications of pre-emergents sometimes fail to prevent weed growth:
1. Improper Timing
The germination of crabgrass and goosegrass are highly correlated to soil temperature. Conventional models for crabgrass recommend applying pre-emergents when soil temperature at the 1-inch depth is 55°F or above for five consecutive days. Not getting the herbicide down early enough will allow crabgrass and goosegrass to get a head start and make the herbicide ineffective.2. Improper Application Rate
This is mainly a problem of not properly calibrating equipment before application. Before each application, it is important to check the label for recommended rates, then to calibrate sprayers and spreaders to make sure they are applying the proper amount.
3. Not Watering-In the Product
Pre-emergents need to be watered into the turf to activate the herbicide and form a barrier to prevent weed growth. Consult the label to verify how much water needs to be applied and when watering must be done after application.
4. Skips or Misses
This is simply an application error and easy to do when making applications to irregular-shaped areas. Skips and misses are usually visible in strips or triangulated shapes consistent with an application pattern.
5. Weed Resistance
Weed resistance can develop with repeated use of an herbicide from the same chemical family. Several of the pre-emergent herbicides are in the dinitroaniline (DNA) family, including Pendulum, Dimension, and Barricade. There is one that falls outside this group and that is Ronstar, which is in the oxadiazole family. If allowed by the herbicide label and the law, it is recommended to alternate between the use of Ronstar and the other DNA herbicides to avoid weed resistance.
By planning ahead as you prepare to lay down pre-emergents, you will be able to avoid weed growth and set your turf up for success this spring.
For more info about strategies for applying pre-emergents, read Back to Basics: Pre-Emergent Weed Control on our blog.