Postemergence herbicide are also affected by application issues, species impacts, and environmental conditions.


1. Applications under drought conditions. Weeds do not readily absorb and translocate herbicides when they are under drought stress. Either irrigate several days before an intended application or wait until a few days after a rainfall to ensure that the targeted weed species is actively growing under good soil moisture.

2. Rain soon after application. If a shower kicks up 15 minutes after a glyphosate application, for example, much of the herbicide could run off the weed foliage, resulting in poor weed control. Hold off spraying if there is a good chance of rain that day.

3. Applications under cold conditions. Applications when temperatures are cold, especially below 50°F can result in erratic weed control. Again, weeds do not absorb and translocate herbicide when under stress, including cold temperature stress. Ideal spraying conditions would be temperatures above 60 F with good soil moisture. We have applied certain postemergence herbicides at air temperatures in the 40’s, with acceptable weed control but control takes longer and control can be erratic.

4. Wrong herbicide. For example, applying a postemergence grass herbicide like fluazifop (Ornamec, Fusilade II) will not control sedges. So use the term nutsedge when dealing with this sedge species. Using the term nutgrass is confusing and could result in someone picking the wrong product to spray. In general, grass herbicides do not control sedges, and sedge herbicides often do not control grasses. Identify the target weed species and then choose the appropriate product.

5. Forgetting to add a surfactant. Certain products require an adjuvant for effect weed control. Reward/Diquat is a contact herbicide and a nonionic surfactant must be added to spread the spray droplets evenly across the leaf surface of weeds. Check the herbicide label to see if a surfactant, crop oil, or MSO (methylated seed oil) needs to be added. When spraying overtop ornamentals, especially during summer, we recommend use of a nonionic surfactant as oils can heat up an herbicide, possibly resulting in some contact burn.

6. Antagonism. Sometimes when certain pesticides are mixed together, antagonism can result, reducing weed control through effects on absorption and translocation. For example, we have seen reduced grass control when fenoxaprop (Acclaim Extra) is mixed with postemergence broadleaf herbicides containing 2,4-D like Trimec Classic (2,4-D + MCPP + dicamba). In this case, these two products should be applied separately, first applying Acclaim Extra and then applying the Trimec Classic a week later. Read the label when considering mixing two or more herbicides together. A small jar test can be done to test for spray tank incompatibility.

7. Incorrect spray volume. A systemic herbicide like glyphosate (Roundup, others) is best applied in low spray volumes, such as 10 to 20 gallons per acre. Thorough coverage of the leaf surface is not required for a systemic herbicide like glyphosate and uptake seems to improve with more concentrated spray droplets. For a contact herbicide like diquat, however, higher spray volumes are necessary, especially for taller vegetation, where a 100 gallons per acre spray volume may be required for thorough coverage of taller weeds.

8. Water quality issues. Muddy water with suspended clay particles can tie up herbicides like diquat, rendering the chemical ineffective. A high calcium content is a concern when applying glyphosate as it can lead to insoluble salts that are not readily absorbed. Addition of ammonium sulfate can help as it will tie up the calcium. A very high pH or a very low pH in the spray can adversely affect herbicide stability.

9. Herbicide resistance. As with preemergence herbicides, weed species can also develop resistance to postemergence We have documented annual bluegrass biotypes resistant to the sulfonylurea class of herbicides, which includes Monument, Kata- na, Certainty, Revolver, and Tranxit. Another example in Virginia is the occurrence of horseweed resistant to glyphosate. To help prevent the development of herbicide resistance, rotate herbicides with different modes of action, combine herbicides with different modes of action, utilize both preemergence and postemergence herbicides, and use nonchemical measures where practical.

10. Wrong timing. Dimension (dithiopyr) will control one or two leaf crabgrass but will not provide acceptable control of well-tillered crabgrass. Drive (quinclorac) is more effective on young crabgrass plants in spring compared to multi-tillered plants in

11. Wrong rate. We learned this with our evaluation of pinoxaden (Manuscript), where the spot treatment rates were much more effective than the broadcast rates. Read the label to determine the optimum rate for the target weed.


Identify the major weed species at each site you maintain and develop a plan to address those problems. Scout in the spring to identify winter weeds when they are in flower and thus easier to identify and scout in late summer to identify summer weeds when they are in bloom. These two time periods may not be the opti- mum time for control but will guide future management plans. Consider soil and weather conditions when scheduling herbicide treatments. If herbicide resistance is expected, let us know as we like to confirm resistance.