Which condition would make a selective herbicide ineffective?

A selective herbicide is designed to target specific types of plants while causing minimal harm to others. When applied to all plant types, it loses its selectivity and can potentially damage the very plants it is intended to protect. This scenario creates an environment where the herbicide cannot function as intended, thus rendering it ineffective for its primary purpose.

Using a selective herbicide during the growing season is generally appropriate as it allows the herbicide to impact actively growing weeds without significantly affecting desirable plants, provided the latter are not the target species. Similarly, utilizing irrigation can aid in the effectiveness of the herbicide by ensuring proper uptake and absorption, while compatibility with varying soil types can enhance the herbicide's performance by ensuring that formulations remain effective across different environments. Each of these conditions contributes positively to the intended function of the herbicide, emphasizing the critical role of selectivity in determining its effectiveness.