Heather N. Kolich, ANR Agent, UGA Extension Forsyth County
Weeds are the number one pest for agricultural production. The development of the herbicide 2,4-D in the 1940s simultaneously reduced the need for physical labor and increased crop yields.
Fast-forward to today, and 2,4-D is still a staple for controlling broadleaf weeds, but many other herbicides are available for agricultural and home lawn and landscape use. Herbicides affect plants in different ways, but each mode of action (MOA) requires interaction with the plant. When that interaction doesn’t happen, weeds can survive. Let’s look at some common reasons that herbicide applications fail.
Wrong herbicide for the weed – Proper identification of the weed is crucial to developing an effective weed control strategy. The herbicide that works well on dandelions (2,4-D) has little effect on chamberbitter, chickweed, henbit, white clover, and other common broadleaf weeds, and it doesn’t work at all on grassy weeds. Some types of herbicides work only on grasses. Non-selective herbicides, such as glyphosate, can kill or injure annual and perennial broadleaf, grass, and sedge weeds.
Untimely application – To achieve the desired lethal interaction with a plant, the appropriate herbicide must be applied when the plant is in the growth stage that the herbicide’s MOA targets. Pre-emergence herbicides, such as benefin and pendimethalin, work on seeds to disrupt cell division shortly after germination. Pre-emergence herbicides offer several weeks of soil activity to prevent weed emergence, but they are only effective if they are already in the soil when seeds begin to germinate. Conversely, active plant growth is a key factor in the success of post-emergence herbicides, which rely on uptake by the plant. Post-emergence herbicides lose effectiveness on mature, stressed, and dormant plants that have slowed or halted growth.
Improper application – The application rates and frequencies specified on herbicide labels are maximum legal limits for safety and the minimum rate for effectiveness. Applying herbicides unevenly or at lower than recommended levels allows some weeds to survive and potentially develop herbicide-resistance.
Weather effects – Rain after application can wash away herbicide before it is taken into the plant. Temperature extremes and environmental conditions, such as drought, can also prevent herbicide uptake.
Maybe it is working – Depending on the MOA, symptoms of herbicide damage may take days or weeks to appear. Perennial weeds may require multiple herbicide treatments as regrowth emerges from undamaged roots that were beyond the herbicide’s translocation reach. Animals, people, flowers, and wind continually reintroduce seeds that may sprout quickly or remain dormant for years.
Follow these practices for effective herbicide applications:
· Identify the weed.
· Refer to the Georgia Pest Management Handbook (https://ipm.uga.edu/georgia-pest-management-handbook/) to find products that effectively control identified weeds.
· Read and follow all label instructions.
· Apply herbicides when weeds are young and actively growing.
· Check the weather before applying herbicides.
· Learn how the herbicide affects the plant and when to expect symptoms.
· Reduce introduction of weed seeds with cultural practices, such as frequent mowing to prevent weeds from flowering and cleaning boots and equipment that may carry plant material to your yard.