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Herbicides: How they work

If you have a bad weed problem, you might consider using an herbicide. In order to choose the right one for the situation, understanding how herbicides work is very important. Herbicides are classified in three ways: range of activity, mode of action, and targeted stage of weed growth.

Range of activity is defined by how the herbicide acts on certain plants. Selective herbicides target a specific range of species. Examples include Sedgehammer (halosulfuron) for control of nut sedge, 2,4-D for control of broadleaf weeds (many applications in lawns), and Poast (sethoxydim) controls grasses. Non-selective herbicides target a wide range of species. An example of this is Roundup (glyphosate).

Mode of action is the overall manner in which an herbicide affects a plant at the tissue or cellular level. Herbicides with the same mode of action will have the same translocation (movement) pattern and produce similar injury symptoms. Mode of action is broadly separated into two categories, contact and systemic herbicides.

Contact herbicides kill a plant on contact. They are not absorbed or translocated in the plant or the soil. This class of herbicides leads to very rapid plant death and are dependent of temperature for activity. They are very effective on annual weeds but have poor control of perennials because they are not translocated through the plant. They will burn the foliage down but perennials will come back from the root system. Contact herbicides are usually non-selective, meaning they will kill the foliage of anything they touch. An example of a contact herbicide is Gramoxone (paraquat), a restricted use herbicide, meaning you have to have a pesticide applicators license to apply it.

Systemic herbicides are absorbed by external tissues and translocated to sites of activity. They are temperature dependent and have variable soil activity. They are effective on both annuals and perennials. There are both selective and non-selective forms. Roundup is an example of non-selective systemic herbicide.

Targeted stage of growth is the last herbicide classification. Pre-emergent herbicides inhibit seed germination and are usually selective. There is variable length of activity depending on the herbicide and the weather. Most are commonly used to control annual weeds. Preen (trifluralin) is an example of a pre-emergent herbicide commonly used in flower beds and vegetable gardens. Post-emergent herbicides kill plants once they have commenced vegetative growth. There are both selective and non-selective types. An example is Roundup.

The number one rule of effective herbicide use is that the plant must be actively growing. Being able to identify the weed and why might be a problem are also part of a weed management plan. Always be sure to read the label of any pesticide and use it according to the directions. Use the right solution for the right problems.

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