In the previous posting, we dealt with the subject of preemergence weed control of species like crabgrass. This posting will consider postemergence control of broadleaf weeds like dandelion and clover. For postemergence control, we apply the herbicide to the weed after it has emerged and become established in the turf. This means that we need herbicides that will selectively kill the weed without damaging the turf we are trying to protect. Fortunately, we have a number of these herbicides to choose from. As with preemergence control, you need to know the characteristics of the turf you are trying to manage and the biology of the weed you are trying to kill. Some of the materials available to us are effective on certain types of weeds and not others. They may also differ in their level of safety for use on certain turf species. Always read the label carefully whenever using any pesticide.

There are many perennial broadleaves that infest turf. Dandelion (Figure 1) is one of the best known because of is attractive yellow flower that almost everyone picked as a child. At maturity, the flower produces a puff ball of seed that blow in the wind and land on neighboring lawns where they readily germinate and form a new plant. It is this efficient means of seed disbursement that makes dandelions such a universal problem in turf. Fortunately, dandelions are easily controlled by several common lawn herbicides.

White clover (Figure 2) is another very common weed in turf. Particularly in wet weather, clover can nearly take over the lawn if something is not done about it. Clover is surprisingly hard to control with postemergence herbicides and you will need to know which material to apply if clover is a problem,

The plantains are also a problem through much of the US. There are two types and it makes a big difference which one you have. Common plantain (Figure 3) has a wide leaf and a narrow seed stalk covered with seed. Buckhorn plantain (Figure 4) has a narrow leaf and a seed stalk with the seed found just at the tip. Common plantain is easy to control like dandelion and most common broadleaf herbicides will kill it. Buckhorn is very difficult to control and if you have this one, you will need to choose the herbicide carefully and probably treat more than once during the season to control it.

There are many other broadleaf weeds that can become a problem depending on where you are located. We will not have space to cover them all here. I direct you to Fundamentals of Turfgrass Management or the Scotts guide (see “books” below) for more information.

The broadleaf weeds are dicotyledonous and the grasses are monocotyledonous. This refers to the structure of their seeds. Without going into more details on the two, it is important to know that there are herbicides that will selectively kill dicots in monocots. In other words, if you know what you’re doing, you can kill dandelions in Kentucky bluegrass without damaging the bluegrass.

These herbicides are known collectively as the “auxin-like” herbicides. Auxins are naturally occurring plant hormones, The herbicides are synthetic imitators of these naturally occurring materials, It turns out that the dicots like dandelion are unable to deal with these synthetic hormones, whereas the monocots can. The result is that with the proper herbicide at the right rate, we can selectively kill broadleaves in grasses selectively and postemergently. Before the development of these herbicides, weeds like dandelion were a huge problem and every year lawns would turn yellow with their flowers. These herbicides became available shortly after World War II and they have totally changed the way we manage turf.

Table 1 contains a list of the major auxin-like herbicides available to the homeowner for broadleaf weed control. They are divided into 3 distinct types. These are the phenoxy herbicides, the benzoic acids, and the pyridine compounds. The phenoxies are probably the one most readily recognized by the public. They include materials like 2,4-D, MCPP, and 2,4-DP. There is just one benzoic acid and that is Dicamba, which is a common material used in agricultural weed control in the US. The pyridines are a little less well known, but you find them in many commercial weed control products on the shelf in your area. These materials include clopyralid, triclopyr, and fluroxypyr.

The 2,4-D is the best known of this group. It was the first one released in the US and has proven to be a reliable broadleaf weed control in turf for nearly 80 years. It is an excellent dandelion and plantain control, but it is a little weak on clover and buckhorn plantain as well as some of the other hard to kill broadleaves. MCPP, dicamba, and triclopyr are excellent clover controls, but each have their strengths and weaknesses with other weed species. As a result, most of the broadleaf weed control products found on the shelf are a combination of these basic herbicides. A popular material over the years has been Trimec, which contains 2,4-D, MCPP, and dicamba. This has been an excellent combination for the control of a variety of broadleaf weeds in turf.  There is no limit to the number of combination products that have been released in the last few years. Most of them will include some type of combination of the phenoxy, benzoic acid, and pyridine compounds. They may also include other additives to enhance activity against certain hard to kill weeds. There is no right or wrong among these combination products and any of them can be quite effective. You will want to check with your local dealers to see which products are most widely used in your area for the combination of broadleaf weeds that you are confronted with.

We apply preemergence herbicides in the spring before seed germinates. The postemergence herbicides are best applied in the fall. The reason for fall application is that these herbicides can kill any other dicot in the landscape, including flowers, trees and shrubs if you are not careful. In the spring, these sensitive species are vulnerable.  In the fall, these species are often finished for the season or have lost their leaves for the year and are less susceptible to damage. Fall is also a better time to achieve translocation of these herbicides down to the roots of target weeds. Broadleaf herbicides can generally be applied well after the first frost in the fall.

Table 1. The auxin-like herbicides used for postermergence weed control in turf.

CLASS	CHEMICAL NAME	COMMERCIAL NAME*
PHENOXY
	2,4-D 2,4-DP MCPP MCPA	MANY COMMERCIAL NAMES MECOPROP MCPA-4-AMINE
BENZOIC ACID
	DICAMBA	BANVEL
PYRIDINES
	CLOPYRALID TRICLOPYR FLUROXYPYR	LONTREL TURFLON SPOTLIGHT

*MOST OF THESE HAVE MANY COMMERCIAL NAMES

  

Figure 1. Dandelion in full bloom and seed dispersion phase.

Figure 2. White clover in lawn.

Figure 3. Broadleaf plantain.

 

Figure 4. Buckhorn plantain in left picture. Second picture shows the difference between seed stalks of broadleaf plantain left and buckhorn plantain right.

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. 5^th ed  http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html

Web site:  https://www.hort.iastate.edu/directory/nick-christians/

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