Because preemergence herbicides form the foundation of any early season weed control program, this article will concentrate on the fundamentals of their effective use.
The preemergence herbicides commonly used in lawn and landscape turfgrass kill weeds by stunting their growth. Barricade (prodiamine), Pre-M, Pendulum, Scotts Weedgrass Preventer (pendimethalin), Surflan (oryzalin), Balan (benefin), Treflan (trifluralin) and Dimension (dithiopyr) are primarily inhibitors of root growth. As the roots of germinating weed seeds come in contact with these herbicides, cell division in the root tips does not proceed normally. The result is thickened, club-shaped primary roots and an absence of secondary roots. This inhibition of root growth is also why these products should only be used on established turfgrasses. These herbicides are most effective on annual grasses and will control some small, seeded broadleaf weeds.
It is sometimes incorrectly assumed that these herbicides prevent weeds from germinating. They do not. Weed seeds germinate and emerging roots and shoots absorb the herbicide. Once absorbed, these herbicides interfere with cell division in the new roots and shoots, which slows growth adn produces stunted or dead plants. Common visual symptoms on surviving plants are the aforementioned club-shaped primary roots and absence of secondary roots.
While timing is the key to preemergence annual grass control, lawn care operators should not ignore basics such as herbicide selection, calibration and uniform application. Use the research data and recommendations developed by the land grant university in your state to help in selecting a herbicide. If you do not feel comfortable in interpreting the data, call the turfgrass specialist at the university for advice on herbicide selection. Because herbicide performance varies with climate, soil type and weed spectrum, recommendations must be tailored to local conditions.
Attention to calibration tends to vary widely among lawn care operators. I have never seen anyone spend too much time on calibration. There is no sub-stitute for accurate application. In our work, we calibrate our equipment every time we apply. Research data shows that the error range can be from 10 percent to as high as 50 percent for commercial applicators that think they are right on target. I have investigated hundreds of weed control failures with herbicides and they are all too frequently related to inaccurate application.
The other common pitfall in herbicide application is lack of uniformity. A contractor may have applied one pound of material per 1,000 square feet but it may not be uniformly distributed across that area. Lack of uniformity leads to weed breakthroughs in the light spots and the potential for turfgrass damage in the heavy spots.
Work hard to avoid skips and overlaps in an application. Overlaps result in a double rate of herbicide while skips lead to an unsightly strip of weeds. Applicators using granular products with drop spreaders need to be especially aware of the potential for skips and overlaps.
WHAT TIME IS IT? Discussing application timing for preemergence crabgrass herbicides in a national publication is difficult due to the wide range of climate variations across the country.
Crabgrass germinates from February to May when the soil temperature reaches 53°F to 58°F. Before discussing application dates, it’s important to stress that having the herbicide on the soil surface before germination occurs is not enough to guarantee control. To maximize control, the herbicide must be watered in before crabgrass germination begins with ½-inch of rainfall or irrigation. If irrigation is not available, it is a good idea to apply preemergence herbicides well in advance of the time that they need to be active so that there will be adequate opportunity for rainfall to occur. Ideally, preemergence herbicides should be applied just before weed seed germination begins. However, a good deal of research indicates that successful preemergence summer annual grass control applications may be made as early as January.
In the Deep South, preemergence crabgrass herbicides should have been applied and watered in by February 15th. In the central part of the country, March 1st is a good target date, while in mountains and northern states crabgrass germination typically occurs from March 15th to the 30th.
Timing preemergence herbicide applications is another area where a land grant university can be of help. The turfgrass professionals located there can offer target application dates for the climate zones in your state. In addition, some university research stations record soil temperatures, which can be helpful in pinpointing application timing.
The length of time between application and watering in seems to be more critical with sprayable formulations than with granular products. This is probably due to the fact that granules are less susceptible to volatilization and photodecomposition than sprayables. The important point to remember is that the sooner the herbicide is watered in, the better. The data in Table 1 (on page 61, from The Ohio State University, Columbus) shows the benefit of immediately watering in the wettable powder (WP) formulation of pendimethalin. See Table 1.
| Effect of Irrigation Timing and Formulation | ||||
| FORMULATION | RATE LB AI/AC |
IRRIGATION DELAY |
CRABGRASS COUNTS | |
| 1990 | 1991 | |||
| Granular | 1.5 lb | 0 days | 6 | 15 |
| Granular | 1.5 lb | 7 days | 8 | 10 |
| WP | 1.5 lb | 0 days | 18 | 9 |
| WP | 1.5 lb | 7 days | 68 | 16 |
| Table 1. The Effect of Irrigation Timing and Formulation on Pendimethalin Performance. Credit: The Ohio State University | ||||
Whatever your location, remember that it is better to be early than late with preemergence crabgrass herbicides. While some preemergence herbicides have a modest amount of postemergence activity on very small crabgrass, contractors shouldn’t expect these applications to be acceptable substitutes for a well-defined postemergence plan. The philosophy that weed researchers have promoted over the years is that preemergence herbicides should go out just before the target weed begins to germinate.
The reason for just-in-time application is to avoid dissipation of the herbicide level in the soil during the time between application and weed seed germination. Several processes including volatilization, breakdown by sunlight and soil microbial activity decrease the amount of herbicide in the soil.
After herbicides are watered in, microbial degradation is probably the most important of these mechanisms. Low soil temperatures reduce soil microbial activity so there is little loss when crabgrass herbicides are applied in late winter. For example, lawn care operators in central Arkansas begin preemergence applications in mid to late January although the typical germination period for crabgrass in this area is the first week in March.
The data in Table 2 from our studies in central Arkansas illustrate that applying preemergence herbicides as early as January did not affect performance compared to a March application.
| Application Timing and Herbicide Performance | ||||
| HERBICIDE | RATE LB AI/AC |
DATE APPLIED |
% CRABGRASS CONTROL (JULY) | |
| 1993 | 1994 | |||
| Dimension | 0.5 | Jan. 7 | 90 | 85 |
| Dimension | 0.5 | Feb. 7 | 90 | 83 |
| Dimension | 0.5 | March 7 | 85 | 86 |
| Surflan | 2.0 | Jan. 7 | 89 | 85 |
| Surflan | 2.0 | Feb. 7 | 93 | 83 |
| Surflan | 2.0 | March 7 | 91 | 86 |
| Table 2. Application Timing and Preemergence Herbicide Performance. Credit: University of Arkansas | ||||
Another approach to early timing of crabgrass control is to be really early with the preemergence herbicide by applying the previous fall. We have done quite a bit of work with Barricade applied in the fall and it has worked quite well on crabgrass the following spring and summer.
It is important to delay the application until soil temperatures drop and soil microbial activity lessons. Table 3 shows the performance of fall and spring applied Barricade.
One disadvantage of this approach is that delaying an application until soil temperatures cool will result in missing control of annual bluegrass and other winter annuals. In addition, applying crabgrass control products early enough to get winter weed control (Sept. 1) may reduce the effectiveness of spring and summer crabgrass control (See Table 3).
| Summer Crabgrass Control | |||
| HERBICIDE | RATE LB AI/AC |
DATE APPLIED |
% CRABGRASS CONTROL (JULY, 1997) |
| Barricade | 0.75 | Oct. 14, 1996 | 82 |
| Barricade | 0.75 | March 4, 1997 | 95 |
| Barricade | 1.0 | Oct. 14, 1996 | 95 |
| Barricade | 1.0 | March 4, 1997 | 93 |
| Table 3. Summer Crabgrass Control with Barricade Applied the Previous Fall. | |||
As we have been discussing, preemergence herbicides do not last forever. Residual control ranges from six to 16 weeks depending on the product and the environmental conditions, including rainfall, weed pressure and the density of the turfgrass stand. In most cases, a repeat application is needed for full season control.
Contractors should time the second treatment to occur about 60 days after the onset of crabgrass germination. If you operate in an area where fall overseeding of cool-season grasses such as tall fescue is routine, a second application may create problems. Consult the herbicide label for the required interval between herbicide application and establishing turfgrasses from seed.
WINNING COMBINATIONS. Will tank mixing preemergence crabgrass herbicides provide any benefits? Obviously, manufacturers think so because there several premixed preemergence products on the market.
Bear in mind that, with the exception of Ronstar (oxadiazon), the commonly used preemergence crabgrass herbicides all have pretty much the same mode of action (root growth inhibition). But, do not be lulled into thinking that because a group of herbicides has the same mode of action that performance will be identical.
We have looked at number of combinations in our research trials and have found that the two areas in which tank mixing enhances performance are goosegrass and broadleaf weed control. However, goosegrass is not typically a lawn weed. Goosegrass is more likely to be found on golf courses and sports fields where traffic, compaction and moist soil conditions come together.
That leaves broadleaf weed control as the area in which lawn care operators can make the biggest gains in weed control through tank mixing. Applied alone, crabgrass herbicides will control winter annual broadleaf weeds such as common chickweed, henbit and corn speedwell but do not do as good of a job on lawn burweed and parsley piert. This may be too much hair splitting for most contractors but fine-tuning may be the future of turfgrass weed control.
With the appearance of new products dwindling, the future may be one of very specific prescriptive weed control programs. Mixing isoxaben with crabgrass herbicides will significantly improve broadleaf weed control. Isoxaben is convenient because it may be used on warm- and cool-season turfgrasses.
In tolerant warm-season grasses, combining preemergence crabgrass herbicides with Princep or Aatrex (atrazine) will enhance pre and postemergence broadleaf control and provide some postemergence control of annual bluegrass. Princep seems to be more popular in the transition zone while atrazine is used more in the Deep South on St. Augustinegrass and centipedegrass lawns.
HAVE A PLAN. Too often, weed control measures are a reaction to a crisis rather than part of a well-planned effort. Turfgrass professionals should spend at least as much time learning the conditions that lead to weed infestation as they do studying control strategies after weeds become established.
The value of planning ahead can’t be underestimated. The old cliche, “failure to plan ahead on your part does not constitute an emergency on my part,” applies here.
A big part of having a plan is scouting and mapping the weeds. As you travel to the sites that you maintain, collect information that will allow you to be ready with the correct herbicides and plan of attack come treatment time. Late summer or early fall is a good time to make weed surveys. Follow the fall survey with a spring assessment to observe spring germinating weeds. Put your survey data on paper.
The ability to conduct a useful weed survey is dependent on a lawn care professional’s ability to identify weeds. Skill in weed identification is important from more than a control standpoint. The first question a client is going to ask will usually be, “What is that weed?” So, weed identification is important in establishing your credentials as a professional.
It is also very easy for contractors to fall into a pattern of devising elaborate herbicide strategies, often of less value, instead of concentrating on the fundamentals.
A recent training program for county extension agents focused on weed identification and how herbicides work and never mentioned specific herbicide recommendations. The feedback was tremendous. The message is clear – for maximum control, master the fundamentals.
The author is a weed scientist at the University of Arkansas, Little Rock.
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