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How Do Systemic Weed Killers Work?

Systemic weed killers are weed killers that are applied directly to the foliage of the plant. Once these weed killers are placed on the foliage, the plant absorbs them. The weed killer is then transported throughout the plant and works against the plant’s internal structures. A systemic weed killer may be sprayed onto the plant’s leaves or painted on with a brush. Systemic weed killers are selective poisons. For this reason they are used in lawn care. The plant may be killed without poisoning the soil or plants that are neighbors to the weed.


Systemic weed killers are herbicides. The word herbicide literally means "plant death." Systemic weed killers work hormonally by preventing production of chlorophyll and protein in plants. In plants, chlorophyll is a key component in photosynthesis, which is an essential process for all plants. A plant that is not able to undergo photosynthesis will die. Typically, a weed that has been treated with a systemic weed killer will die in seven days to two weeks. Systemics are also carried through the sap of a plant to the root structure. Once in the roots, systemics prevent a plant from absorbing minerals from the soil.


According to the government of South Australia, grasses, broad-leaf plants and most annual weeds are the best types of plants to use systemic weed killers against. These types of plants have soft tissues and quickly absorb systemic chemicals. Plants that grow from bulbs, plants with woody stems and perennial plants are harder to control with systemic plants. One example of this second kind of plant is the wild blackberry. With plants such as these, optimal timing for the application of the systemic may be required to kill the plant, or even a second application of the systemic. For example, most varieties of wild blackberry require systemic application in the summer months when the plant is undergoing photosynthesis. Surviving plants will usually die when they are mowed back.


There are several types of systemics on the market. The effectiveness of the systemic and the application varies depending on which type you use. The most common systemic is glyphosate. Glyphosate is sold commercially as Roundup. In most cases, glyphosate needs direct application on leaves during an optimal time period in the plant’s life cycle, and is more effective on soft annuals. Another common systemic is Triclopyr. More commonly known as Brush B. Gone, it is a systemic that is more effective against woody or brushy plants.

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