What do contact herbicides do?

Contact herbicides are designed to kill or injure any plant tissue they come into direct contact with. This mechanism of action makes them effective against targeted weeds because they disrupt cellular functions upon contact and do not need to be absorbed systemically by the plant to be effective.

These herbicides work by causing damage to the plant's exterior, which can lead to wilting, browning, or complete death of the foliage, as they primarily affect the leaf and stem tissues. This is different from systemic herbicides, which are absorbed and translocated throughout the plant, affecting growth from within and potentially damaging the root system as well.

The other choices reflect different characteristics that do not accurately represent the function of contact herbicides. For instance, killing or injuring specific types of weeds suggests selectivity that is not a defining characteristic of contact herbicides, which typically affect any plant they touch indiscriminately. Additionally, affecting only the roots of plants does not apply here, as contact herbicides mainly target the visible parts of the plant that come into contact with the chemical. Promoting healthy plant growth contradicts the purpose of herbicides, which is to control or eliminate unwanted vegetation.