Turf Tips
02/23/2010

Choosing a Preemergence Herbicide

There are many, many formulations and combinations of preemergence herbicides available for the professional. Following are some suggestions for purchasing and using preemergence herbicides.

  1. Control from every preemergence herbicide can vary from year to year, or location to location. Therefore, you can’t judge the performance of a product or a specific rate of that product on one year’s experience. The three best performing preemergence active ingredients are dithiopyr, pendimethalin, and prodiamine and in any given year, they’ll all provide equivalent control at the correct use rate.

  2. Consider alternating active ingredients every two to three years to further reduce the possibility of forming resistant populations.

  3. These products are available in a wide range of formulations and loaded on a wide range of fertilizers., including generic and proprietary products.  With the sheer number of formulations, it is difficult to compare of all the product and rate combinations. Therefore, we prefer to use lbs ai/A for comparisons:

    Ranges of labeled rates of preemergence herbicides for the Indiana and north (rates may differ be slightly higher for the transition zone.

     

    Low

    Medium

    High

     

    ----------lbs ai/Acre---------

    Dithiopyr 2 EW (liquid formulations)

    0.25

    0.38

    0.5

    Dithiopyr on fertilizer

    0.125

    0.18

    0.25

    Pendimethalin on fertilizer or sprayable

    1.5

    2.0

    3.0

    Prodiamine on fertilizer or sprayable

    0.38

    0.5

    0.65

     

  4. It is usually not worth trying to lower the rate to save money. Control will be sacrificed requiring more expensive rescue treatments and extra labor. However, on very thick lawns or lawns that have received many years of preemergence herbicides, rates can often be reduced or the preemergence herbicide possibly eliminated. Also consider using only spot applications in hot spots next to sidewalks, etc.

  5. From Indianapolis south as well as any other area with history of crabgrass, split applications should be considered. Check the label for specifics, but an application at the medium rate plus a second application at the low or medium rate is typical.

  6. Depending on the label, the recommended rate is usually provided in lbs product/Acre or in lbs ai/Acre and conversion of those rates can be done as follows:
    ** Be sure to convert % to decimal before calculating; 0.07%=.0007, 0.10%=.001

                                        lbs ai/A recommended
    lbs product/Acre   =                                                                            
                                                    % active ingredient (as a decimal**)

    lbs ai/Acre  =  pounds product/A recommended X % active ingredient (as a decimal**)

    Or you can refer to the quick reference table for converting pounds active ingredient to lbs ai and vice versa [PDF]

  7. Limit the amount of nitrogen applied. Lawns receiving urea last November may not need any nitrogen with the preemergence. No lawns will require more than 0.75 lbs N/1000 with the preemergence application.  However, you cannot simply lower the rate of a fertilizer/herbicide combination because you’ll also lower the herbicide rate. Therefore purchasing the correct product is critical.  You can use the quick reference table [PDF] to determine how much N/1000 you will apply, given your product and rate. Just find the %N of your fertilizer+herbicide combination, move right to the approximate amount of product applied/acre, and the move vertically to the pounds N applied/1000 sq ft or /Acre. For example: if you have a fertilizer+herbicide combo that contains 7% N and you need to apply 500 lbs product per acre for the correct preemergence rate, you will apply about 0.8 lbs N/1000 ft2.
    Compare products on a cost per acre basis:

                                       cost              
Cost /Acre   =                                                               X          lbs product/acre           
                                                Unit (lbs)

For example 20-4-10 with 0.13% Dimension is applied at 189 lbs/A  or 0.25 lbs ai/A and costs $31/50 lb bag

                                       $31               
Cost /Acre   =                                                               X          192 lbs/A          = $117.48/Acre
                                                50 lbs

We’ll be showing many of the crabgrass control products at our Field Day on July 20, please mark your calendar! In the mean time, please let us know if we can help.

Zac Reicher, Professor/Turfgrass Extension Specialist


Send corrections, suggestions, and comments to biehlj@purdue.edu