Though the holiday season has just ended, it’s time to start thinking about weed control tactics for the coming spring. Your first line of defense should be pre-emergence herbicides, especially for control of summer annual weeds such as crabgrass and goosegrass. While these herbicides are your best bet for control, know you won’t always get the level of control that you want. A pre-emergence herbicide’s success requires application precision, accurate timing and a little help from Mother Nature to maximize effectiveness. However, here are a few guidelines that can help increase your odds for success.
Germination cycles are key
While there are many elements that influence the effectiveness of your pre-emergence herbicide, the No. 1 cause for a season of faulty control is most likely timing. If you don’t time the application correctly, the herbicide won’t provide active control at the time it’s needed. More often than not, landscapers don’t get pre-emergents on the turf soon enough. For these chemicals to perform optimally, you must apply them before the onset of the weed germination cycle.
So how can you tell when a weed is about to germinate? It’s critical to have some knowledge of your target weed and its life cycle. That’s why weed identification is so important. Before you can fight it, you have to be certain what it is. So use your resources.
If, for example, the weed you want to control is crabgrass, the best way to get your timing down is to use soil temperature as a gauge. Monitor the soil temperature on a daily basis in the top 1/2-inch of soil. To get the most accurate reading, check the soil’s temperature in the morning, before the site is exposed to direct sunlight. When soil reaches more than 50 degrees Fahrenheit for three consecutive days, it’s time to apply your pre-emergence herbicide. For goosegrass, your pre-emergent application will be a little later, as it germinates about three weeks after crabgrass.
Irrigation and weather play a role, too
Now that you’ve got your timing down, consider other outside factors that may be working against you – namely the weather.
To work correctly, pre-emergence herbicides must be “activated” with water. Water concentrates the chemical in the surface layer of the soil. Ideally, you should irrigate immediately after your chemical application. Your irrigation should equal about 1/2-inch of rainfall. This is especially important if you spray on your pre-emergence herbicides because water is needed to move the herbicide from the foliage to the soil. For non-irrigated turfgrass you’ll probably want to bump up your pre-emergence herbicide application a little earlier so that there is more time for rainfall to ensure that the herbicide moves into the soil. If you can’t irrigate after your pre-emergence herbicide application and it doesn’t rain, know that you’ll lose much of the chemical to photodegradation.
However, too much water will cause the “active” pre-emergent to quickly fade away. If it rains after watering-in the chemical, the additional moisture will promote leaching and surface runoff. The result: The pre-emergent will control your crabgrass for only a portion of the season.
Splitting up applications can maximize control in warmer climates
With timing and weather limitations, your best bet often is to split pre-emergence herbicide applications. Doing so not only takes some of the pressure off getting it right in one application, but can result in better control of crabgrass. Some researchers also report that a split application promotes healthier turfgrass rooting and allows it to better withstand seasonal weather fluctuations that may affect weed germination.
If you live in warmer regions where soil temperatures are likely to reach that 50-degree mark earlier in the season, a split application is almost always more effective. Depending on your location, your crabgrass germination period could be as long as five months, requiring at least two applications just to get you through the season. Goosegrass also has a long germination period, making it difficult to control with a single application.
If you’re splitting applications, know that you should make your first application a little earlier in the season that you normally would. The standard is to make the follow-up application about six to eight weeks later, using half the labeled rate. For example, if the label calls for 3 pounds active ingredient per acre, then you’ll want to use 1.5 pounds for the first application and 1.5 pounds again for the second.
Finally, remember that healthy turf is more tolerant turf. If you follow cultural practices to optimize turf health, it will be able to out-compete weeds and better fend-off disease. To optimize turf’s chances against weeds, make sure your cultural practices result in a dense turf canopy. Without a lot of light, summer annuals like crabgrass will find it much more difficult to grow. Make sure soil is not compacted by performing fall aeration, and fertilize turf in the fall, as well. Doing so will help boost turf vigor and thickness next spring.