For Professionals: 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.
2. Consider alternating active ingredients every two to three years to further reduce the possibility of forming resistant populations.
3. With the sheer number of liquid formulations and fertilizer+herbicide 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

   	Low	Medium	High
   	———-lbs ai/Acre———
   Dithiopyr WP, EC, or EW	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. For areas south of Indianapolis, 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 (PDF) and vice versa (PDF).

1. 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.  You can use the quick reference table 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.
2. 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 192 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 15, please mark your calendar! In the mean time, please let us know if we can help.