Which herbicide is least likely to be affected by soil moisture levels?

Contact herbicides are designed to affect only the parts of the plant that they come into direct contact with. Since they do not enter the plant's system or translocate throughout the plant, their effectiveness is largely independent of conditions such as soil moisture. Contact herbicides typically act quickly on the foliage and require sufficient coverage for maximum efficacy, but they are not influenced by how much moisture is in the soil since they do not rely on the plant's uptake of water and nutrients.

In contrast, systematic herbicides must be absorbed by the plant, translocated through its vascular system, and are thus more dependent on the plant's health, which can be influenced by soil moisture levels. If the soil is too dry, the plant may not take up the herbicide effectively, reducing its overall effectiveness. The selectivity and action of non-selective herbalides, which kill all plant material, can also be impacted by moisture, as it can affect how the herbicide is absorbed and utilized by plants.