Ornamental and Turf Pesticide Applicators Practice Exam

When do post-emergence herbicides show their best effectiveness?

1. In cold weather conditions

2. In bright sunlight with high humidity

3. During nighttime

4. With low air temperatures

The correct answer is: In bright sunlight with high humidity

Post-emergence herbicides are designed to kill weeds after they have emerged from the soil. These herbicides generally work best when the environmental conditions favor the uptake and absorption of the chemical by the weeds. Bright sunlight increases the photosynthetic activity of plants, which can enhance the absorption of herbicides. Additionally, high humidity can improve the effectiveness of post-emergence herbicides because it reduces stress on the plants, allowing them to take in the herbicides more effectively. When plants are actively growing and have optimal conditions—such as bright sunlight and high humidity—they are more likely to absorb the herbicides thoroughly, resulting in better weed control. In contrast, colder weather and low air temperatures can slow down the metabolism of plants, reducing their ability to effectively take in and respond to herbicides. Using herbicides during nighttime or in conditions with limited light can also hinder their effectiveness, as these plants may not be actively growing or absorbing the herbicide efficiently. Thus, the best effectiveness of post-emergence herbicides is achieved under sunny conditions with high humidity.