3/27/2018 - By Harrell's

Most turfgrass weeds can be controlled effectively with the proper use of herbicides.  However, it is important for turfgrass managers to understand the biology of the weed (growth habit), why that particular weed is a problem, where is the weed a problem, and which herbicide(s) would offer the best control under these given conditions.

Of course, the foundation to a successful weed management program is “sound” turfgrass management utilizing the proper cultural practices (mowing, fertilization, irrigation, and aerification).  Your main defense against weed establishment is a healthy, vigorously growing turfgrass.  Once the weed becomes established, it competes with the turfgrass for light, nutrients, space, water, and certain plant essential gases.  Incorrect turfgrass management often encourages weed infestations.  Some of the most common improper management practices include improper mowing height, poor fertilization and liming practices, and the selection of poorly adapted turfgrass species.  However, some particular weed species can exist and thrive even where “sound” turfgrass management practices have been followed.  A good example of such weeds are crabgrass and goosegrass. 

In warm-season turfgrasses, these two weeds are capable of germinating prior to the “green-up” of the warm-season turfgrasses in the spring.  Therefore, these weeds have a competitive advantage over the dormant warm-season turfgrasses.  Crabgrass can germinate when soil temperatures near the soil surface average 52 to 55 F over several consecutive days.  In many areas of the southeast, this can be as early as February through April.  Goosegrass germinates when soil temperatures are approximately 60 F, which is usually a minimum of two to three weeks later than crabgrass.  Depending on the area, many warm-season turfgrass species may not reach the maximum growth potential until late April until mid-June. 

Where crabgrass and/or goosegrass problems exist, the use of the appropriate herbicide(s) are generally required.  Herbicides that are used as a preventative measure are considered Preemergence applied herbicides.  Preemergence applied herbicides are used to prevent weeds from becoming established.  Preemergence applied herbicides are most useful in areas where a certain weed species is known to be a problem.  For maximum control of crabgrass and/or goosegrass with preemergence applied herbicides, they must be applied before any weed seed germinate.  Probably the most common cause of poor control with preemergence applied herbicides is they are applied after crabgrass germination.  Not only do these preemergence applied herbicides need to be applied before weed seed germination for maximum control; they must be also watered in to “activate” the herbicide by creating a herbicide layer along the soil surface.  For proper application and maximum control, it is helpful to understand how these preemergence applied herbicides work. 

If is a fairly common misconception that preemergence applied herbicides prevent weed seed from germinating.  They DO NOT prevent weed seed germination!  The germinating weed seedlings die as they grow through the herbicide layer or herbicide treated zone.  Some preemergence applied herbicides are absorbed by the roots of these germinating seedlings and some are absorbed via the shoots (leaves). 

The dinitroanaline (DNA) herbicides, such as pendimethalin (Pendulum, Pre-M), Balan, Barricade, Surflan, Team, and XL are root absorbed and work by inhibiting cell division in germinating weed seedlings’ roots.  Dimension, although not a DNA, has a similar mechanism of action in that it too is absorbed via the roots of germinating weed seedlings and inhibits cell division.  Ronstar (Restricted Use Pesticide) is a preemergence applied herbicide that is shoot absorbed, but is not registered for use in lawns and landscapes. 

A common question that I always get asked is “Should I apply my preemergence applied herbicides in split or sequential applications?”  The answer to this question depends on where you are located geographically.  In the southeastern US, I would answer this question with a yes.  As a general rule, the longer the frost-free season (growing season), the more advantage there will be to applying the preemergence herbicide in split or multiple applications.  The first application should be applied during the normal “preemergence” window with the second application applied 4 to 6 weeks after the initial application.  For goosegrass control, we normally see improved control with split applications due to the fact that goosegrass germinates about 2 to 4 weeks later than crabgrass and with split applications, the more concentrated second application seems to aid in goosegrass control. 

Most failures with preemergence applied herbicides is due to either improper application timing and/or misapplication (skips).  If the targeted weed(s) have already germinated prior to the preemergence application, then control will be significantly reduced to non-existent.  In addition, if any skips are made during the preemergence herbicide application, then weeds are certain to become established in these “non-treated” areas.  Proper calibration of application equipment and the use of a spray indicator are recommended to alleviate complaints and problems. 

Another consideration when applying preemergence herbicides is the location of the herbicide within the turfgrass profile.  These herbicides can be retained by either the foliage of the turfgrass (leaves), soil, or thatch.  Preemergence applied herbicides that are retained by the turfgrass are not in contact with the weed seeds, and therefore the overall efficacy is reduced.  On the other hand, if the herbicide is retained on the thatch, it may be in contact with some weed seeds, but its effectiveness may be reduced due to losses from photodegradation, hydrolysis, microbial degradation, or volatility.  Therefore, it is recommended to water these herbicides in immediately after their application to create the herbicide barrier or layer.  In addition, many of these preemergence applied herbicides are applied on a fertilizer carrier.  If this is the case, it is important to not only water to dissolve the herbicide off of the fertilizer carrier, but to water enough to move the herbicide down to the soil surface.

There are also many misconceptions about preemergence applied herbicides and their performance.  For instance, it was widely thought that preemergence applied herbicides create a layer or blanket of chemical in the soil and that any soil cultivation practice (aerification) would disrupt or break this chemical barrier.  Research has shown that soil cultivation practices either before or after applying preemergence herbicides did not affect weed control. 

Preemergence applied herbicides are a valuable tool for turfgrass managers, but should be used only as part of an overall weed management program.  By understanding some of the pertinent biology of the weed that is to be controlled, and by understanding how preemergence applied herbicides work, turfgrass managers can utilize these products to their full potential with excellent results.