To avoid injury to nearby trees, when should persistent nonselective herbicides be applied?

The application of persistent nonselective herbicides can pose a risk to nearby trees due to potential herbicide drift or root uptake. Applying herbicides outside an area equal to twice the height of the desirable trees is a more conservative and effective measure to reduce the risk of injury.

This practice accounts for both the height of the trees and any lateral root growth, ensuring that the herbicide does not negatively affect the tree's health. Trees typically have root systems that extend well beyond their canopy, so a buffer that is twice the height provides a safer distance. This distance helps to ensure that any potential exposure to herbicide is minimized, protecting the desirable trees from potential harm.

Other options may not offer sufficient protection; for example, applying herbicides only in the fall depends on various factors like weather conditions and tree dormancy, which could still pose risks. Simply staying outside the drip line doesn’t take into account the root system’s extent, similarly applying from a fixed distance like 10 feet may not be adequate depending on tree size and species. Thus, opting for a distance that is proportionate to the tree's height is the most reliable approach.