Cool and cloudy weather may reduce the effectiveness of _____ herbicides.

The effectiveness of foliar herbicides can be significantly reduced in cool and cloudy weather conditions. Foliar herbicides are designed to be absorbed through the leaves of plants, and their absorption and translocation can be hindered when temperatures are lower and sunlight is limited. Under these conditions, the physiological processes of plants slow down, which may result in decreased uptake and less effective herbicide action.

Cool and cloudy weather can affect the rate of evaporation and can also influence the plant's metabolic processes essential for herbicide action. This means that the herbicide may not be as effectively delivered to its target or may degrade more slowly due to reduced plant activity and environmental conditions.

In contrast, systemic herbicides are absorbed through the plant's vascular system, granular herbicides are typically applied to the soil and are less influenced by atmospheric conditions, and pre-emergent herbicides work by preventing seed germination rather than by acting on existing foliage. Hence, their effectiveness is not as directly impacted by temperature and light conditions.