PREEMERGENCE WEED CONTROL IN TURF
, 7 min reading time
Weed infestation of turf areas is a continuous problem. The species of weeds will vary somewhat by location, but they generally fall into two major categories. These are annual weeds that complete their lifecycle in a 12-month period (not necessarily a calendar year), and perennials that come back year after year and don’t have to rely on germination from seed each season. To control these weeds, there are a variety of selective herbicides available that will kill the weed and allow the turf to survive. These herbicides are generally classed as either preemergence or postemergence depending on their activity.
Preemergence herbicides kill the germinating seed before it can become established, while postemergence herbicides will kill a mature weed actively growing in the turf. Knowing the characteristics of the weed you are trying to kill and the activity of the herbicide that you plan to use is critical to achieving satisfactory weed control. In this post, we will consider preemergence weed control.
Preemergence herbicides are generally used to control annual weeds. Many of them do have activity on the seed of perennials as well, but perennial weeds do not require germination from seed to persist once they have become established. Perennials are generally controlled by postemergence materials. We will deal with perennials in the next posting.
The annual weeds that infest turf areas are usually either grasses or broadleaves. The annual grasses are generally our target with preemergence herbicides because we can control broadleaves effectively with postemergence materials. There are many annual grassy weeds that infest turf and they may vary somewhat by location. The one that gives us the biggest problem both in cool- and warm-season turf areas throughout the US is crabgrass (Figure 1). It is easily the most hated turf weed in the country.
There are many different types of crabgrass around the world. The two most common types here in the US are large hairy crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum). In the Midwest we generally see both types. Its Latin name Digitaria refers to “digits” or fingers. It is called by this name because its seedhead looks like fingers sticking up out of the turf. It is a summer annual meaning that it completes its lifecycle during calendar year. It germinates in the spring, lives through the summer and produces a vast number of seeds in the fall. The plant then dies and it must come back from seed in the spring. It is a warm-season grass like bermudagrass and buffalograss, meaning that it thrives in the heat of midsummer. This is a big problem in cool-season turf like Kentucky bluegrass because the cool-season turf slows down in the summer and the crabgrass takes over the lawn. The fact that it is a true annual and must come back from seed each spring gives us an edge in control. If we properly time a preemergence herbicide that kills the germinating seed of crabgrass but does not damage the perennial turf that we are trying to grow, we can stop it. This means that you must apply the herbicide when you don’t see the weed. You must know that if you do not apply the material, the crabgrass will show up later. This requires discipline and good timing.
Crabgrass seed will begin to germinate at the time that the soil reaches approximately 55 degrees F. in the spring. This will vary by location. In Des Moines, IA that is around May 1. In Kansas City that can be two weeks earlier and in Minneapolis that can be a week later. If you are not sure when crabgrass germinates in your area, contact your local extension service. The TV weather person may also announce when soil temperatures are reaching the critical temperature. Timing is important because the herbicide will only kill the seed if it is applied before germination. Once the seedling has emerged it is too late. You can also put the material down too early and it may lose some of its effectiveness by the time that crabgrass germinates. Try to put the herbicide down about two weeks before germination in your area. In Des Moines, IA and Omaha, NB, that is around the 2nd week of April.
Another similar weed that is almost as big a problem is goosegrass (Elusine indica) (Figure 2). It also has a seed head that resembles fingers, but it is not a crabgrass. The seeds of goosegrass are larger and are arranged like a zipper on the seed stalk (Figure 3). It often has a silvery stem base and is sometimes given the mistaken name of silver crabgrass. Goosegrass does not extend as far north as crabgrass. You will not find much of it north of Omaha. But as you go south into the transition zone and into the southern US, it becomes a big problem. Like crabgrass, it is a warm-season, summer-annual and thrives in mid-summer. Goosegrass germinates a couple of weeks later than crabgrass. It also shows a level of resistance to some of the common herbicides that do well against crabgrass. If you have goosegrass, you will want to get some additional information from the extension service or a reliable garden store in your area on which herbicide to use and when to apply it.
There are several other annual grass species that infest turf, including foxtail, barnyardgrass, and sandbur. We do not have time to cover these here. I would direct you to one of my books listed at the end of this posting for more detailed information on these species.
There are several herbicides on the market to control annuals like crabgrass. They include materials with chemical names like benifin, pendimthalin, prodiamine dithiopyr, and oxadiazon. These basic active ingredients are sold under many different common names. They will also be found combined with fertilizer in spring weed-and-feed products. As is the case with any pesticide, read the label carefully. Timing is critical with these materials. They must be applied before the seed germinates in the spring. If you are late, they will not work. I generally like to use the preemergence and fertilizer combinations in the spring about two weeks before weed seed germination. They generally will provide approximately 0.75 lbs. N/1000 sq. ft. This will take the place of one of your spring fertilizer applications. They not only kill the germinating annual weed but thicken the turf to help prevent future weed infestations.
The oxadiazon mentioned above is the one that works best for goosegrass. The others are not as effective on that species. Note that all of these will kill the germinating seed of all the turfgrasses. So, if you plan to seed, you will not want to use them. You can usually use them in the spring and seed grasses like Kentucky bluegrass in September or later, but a spring seeding will not work.
Preemergence herbicides may differ if you are growing warm-season grasses like bermudagrass. If that is the case, I would direct you to Fundamentals of Turfgrass Management for more detailed information.
Figure 1. Crabgrass, notice the finger like seedhead.
Figure 2. Goosegrass also has finger like seedhead.
Figure 3. Crabgrass left and goosegrass right. Notice larger seed and zipper-like appearance of goosegrass.
Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University
books: http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0
turf blog: http://www.iaturf.blogspot.com/
Fundamentals of Turfgrass Mgt. 5th ed http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html
Web site: https://www.hort.iastate.edu/directory/nick-christians/