Understanding the dynamics of weed dormancy and the effectiveness of herbicides in cold weather is crucial for effective weed management. Dormancy, a temporary state where weed seeds do not germinate due to certain factors, acts as a survival mechanism, preventing germination when survival conditions are suboptimal. Typically, cold temperatures in fall and winter break this dormancy, usually by mid-winter, allowing seeds to germinate in spring.
However, the effectiveness of burndown herbicides can be notably reduced in cold weather, potentially leading to sub-optimum performance. This is particularly true for herbicides applied in late weeks during cold conditions. For optimal efficacy, the first warm day—characterized by daytime temperatures in the high 50s to low 60s and nighttime temperatures above 40 degrees Fahrenheit—is preferred. It’s worth considering that weed scientists note the ideal temperature for applying most post-emergent herbicides is between 65°F and 85°F.
When temperatures dip below 60°F, weed growth slows, resulting in slower herbicide uptake and translocation. This not only increases the required rainfast period but also delays the onset of symptoms and herbicide efficacy. Moreover, temperatures below 40°F are particularly challenging, especially for glyphosate-based herbicides. A severe frost forecast can further complicate matters.
Additionally, cold temperatures may influence crop safety and weed control from herbicides, as plant metabolism, responsible for herbicide degradation, slows during cool or cold conditions. This extends the duration required for herbicides to degrade in plants.
Penn State Extension further explains that during late fall, many ponds exhibit a full flush of plants, including duckweed, which is often mistaken for algae overgrowth. However, these weeds are typically killed off by cold temperatures, offering temporary relief until spring.
Lastly, the active ingredient in pre-emergent herbicides like Preen, trifluralin, prevents seeds from germinating. It’s crucial to apply these after direct-sown vegetables like beans or lettuce have sprouted and become established, as they inhibit both weed seeds and desirable seeds.
While the evidence suggests that cold weather can impact herbicide performance and weed dormancy, there seems to be a need for further study to fully understand the complexities of this interaction.