Professionals Only: Premergence herbicide strategies

As spring is closing in, we are fielding a number of questions; four major questions right now:

1. Which preemergnce herbide to use?
   That one is pretty easy. We have only three active ingredients relatively easily available and they are dithiopyr, pendimethalin, and prodiamine. Our research shows that as long as they are applied at reasonable rates, these three tend to perform similarly for season-long crabgrass control.  In any given year, one active ingredient may out-perform the other two or vice versa, but overall these three are effective.

2. What rate should I use?
   Much tougher question and depends on your location, the density and age of turf, if it’s a “hot spot” or not, if split applications will be used, do you want to make early spring applications in round one or do you hold off until April or May for your first applications, and on and on. The Turf Tips on 1/28/2009 discuss many aspects of application rates and my rule of thumb is always to error on the high end of the rate range.  Regardless of the active ingredient, you may get by with a low rate on any given year in a relatively cool summer. However, the “cool” summers are far and few between in Indiana and only the high rates of these active ingredients perform consistently in difficult summers. This is shown in our data from the 1990’s in Figure 1, as the most consistently effective control is seen at the high rate of either Barricade or Dimension EC.  Do the low rates of the herbicides work? Sure, but only in one or two of the three years.  Figure 2 shows more recent data with the newer formulation of Dimension and the same trends hold true. So if you’re willing to play poor odds and bet on a cool summer, go with the lower rates. But if you want consistent and effective control almost regardless of the weather, use the higher rates.

3. What about split applications?
   Split applications should be used throughout the southern ½ of Indiana with the first application made anytime early spring and the second application made in mid-May to June. This is an effective method to extend the duration of control. Always use the same active ingredient in both applications and depending on the product, applying ½ to 2/3 the total rate in spring plus ½ to 1/3 in the sequential is typical. This is also a very good strategy in the northern ½ of Indiana for “hot spots” next to sidewalks, drives, etc., where you apply preemergence herbicide over the entire lawn in the early spring and then only apply to the bordering hotspots in mid-May to June.

4. Can I cut down on the overall amount of herbicide that I use?
   Though I would avoid reducing the application rates of typical early spring preemergence applications, you might consider dropping the preemergence herbicide out of the early spring application entirely in favor of a pre/post product in second round. This would replace the split application strategy in point 3 above. Dithiopyr (Dimension) would be the smartest choice for this strategy because it would be far less expensive than combining quinclorac (Drive) plus a preemergenceherbicide. We consistently find good season-long control from dithiopyr-containing products applied early postemergence on crabgrass.

Much of our crabgrass control research will be highlighted at Field Day on July 21, but please contact us if we can help with questions.