University of Illinois Extension


Sharon Yiesla
Unit Educator, Horticulture
Lake Unit

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Understanding Herbicides

Summer is arriving and so are the weeds. You decide you need to take action, so you buy an herbicide and the problem is solved. Or is it? Did you get the results you wanted? Did you get any results? Did more things die than you had intended? Every year gardeners struggle with obtaining good results from herbicides.

The problem stems from the fact that there are so many herbicides and they don�t all do the same thing. Before purchasing an herbicide, it is important to have some information about them.

Herbicides fall into different categories. A pre-emergent herbicide kills weeds as they germinate (sprout) from seeds. Pre-emergent herbicides generally form a barrier against newly germinating seeds. That barrier may be formed by incorporating the product into the soil either manually or by watering it in. Check the label of the product. It will give you the proper directions to follow. Once that barrier is in place, it can be broken when the soil is dug for any reason, such as putting in new plants. If the barrier is broken, control of weeds may be compromised. Pre-emergent herbicides will not control existing weeds.

A post emergent herbicide will control existing weeds, but will do nothing to stop new weeds from germinating from seed. Consult the label for the amount to use and the correct timing for use.

Herbicides can be selective or non-selective. A selective herbicide kills certain plants but not others. Herbicides labeled for control of weeds in lawns, for example, will kill broadleaf weeds like dandelions and thistles, but will not kill grasses. Other herbicides are specifically formulated to kill grassy weeds and so would not be appropriate to use in a lawn area.

Non-selective herbicides kill or damage all plants. These products can be useful for killing weeds growing along fence rows and in cracks in sidewalks and driveways. They can be difficult to use in areas where weeds and desirable plants are growing side by side. Care must be taken to treat only the weed and to not allow any of the product to get onto desirable plants.

Herbicides can also be classified as contact or systemic. Contact herbicides kill only the part of the plant on which they are sprayed. The root system is not killed and the weed may grow back from the roots. Systemic herbicides are absorbed by the plants and taken into the root system, so the whole plant can be killed.

The label on the herbicide package is a valuable tool for understanding the proper use of the herbicide. It is best to read the label prior to buying the product. This insures that the best product for the job is purchased.

The label provides a wealth of useful information. It will list the types of weeds controlled, the correct time to apply the product, and the proper amount to use. The label will also state where the product can be used (lawn, flower garden, vegetable garden.) Don�t assume every herbicide can be used in every site. Read the label and be sure. The label will also give information on any protective clothing you may need to wear during application of the product. Information regarding dangers to pets, birds, bees and other wildlife is also included on the herbicide label.

So, before purchasing an herbicide, take time to understand the product. Using the right product leads to better results.

June/July 2004: Prevent Garlic Mustard from Setting Seeds | Gypsy Moth: Know the Facts | Wetlands and Mosquitoes | Understanding Herbicides | Choosing Home Lawn Care Services

Past Issues

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