Texas FFA Nursery Landscape Practice Exam

Which of the following conditions often necessitates the use of a post-emergent herbicide?

1. Weed seeds have not yet germinated

2. Weeds have already appeared

3. Soil is saturated

4. Newly planted grass

The correct answer is: Weeds have already appeared

The use of a post-emergent herbicide is necessary when weeds have already emerged and are actively growing. Post-emergent herbicides target weeds that are visible and have developed leaves, allowing them to effectively disrupt the growth processes of the plants. These herbicides are designed to be applied after the weeds have surfaced, which is essential for controlling their spread and reducing competition for resources like sunlight, water, and nutrients. In contrast, when weed seeds have not yet germinated, pre-emergent herbicides would be the appropriate choice, as they prevent weeds from sprouting. If the soil is saturated, applying herbicides can be less effective, as the runoff may wash the chemicals away before they can act on the weeds. Finally, using a post-emergent herbicide on newly planted grass may be inappropriate due to the potential for damaging the young plants. Thus, the condition that most clearly necessitates the use of a post-emergent herbicide is the presence of already visible weeds.