What type of herbicides is most likely ineffective against perennial weeds?

Preemergence herbicides are primarily designed to prevent the germination of weed seeds by creating a barrier in the soil. They are effective against annual weeds that grow from seeds, but they do not affect established perennial weeds, which propagate through their extensive root systems or vegetative structures. These perennials can survive and regrow even in the presence of preemergence herbicides because the active ingredients are not designed to target existing plants.

In contrast, postemergence herbicides are formulated to target and kill actively growing weeds, including perennial species, making them a more appropriate choice for managing those types of weeds. Systemic pesticides are absorbed by the plant and can kill both roots and shoots, which helps in the control of perennials. Contact pesticides, while effective at killing the parts of plants they come into contact with, will generally have more limited efficacy against deeply rooted perennial weeds unless they are applied under ideal conditions, but still typically do not match the effectiveness of systemic approaches.

Thus, while preemergence herbicides can effectively manage annual weeds, they do not address the issue of established perennial weeds, making them less effective in that context.