One of the most difficult chores for landowners and homeowners is controlling unwanted weeds and brush. The constant mowing, trimming, pulling and spraying is an aggravating battle, especially during the summer months.

The frustration only increases when nuisance plants grow back, or other weeds move in and take over. When nothing else will grow, it’s a sure bet the weeds will.

Herbicides are usually the method of choice for effective weed control. However, herbicides are not a “silver bullet,” and it is critical that they are used correctly for maximum effectiveness.

When using herbicides, you are not going to see pesky plants cry out and melt before your eyes like the Wicked Witch in “The Wizard of Oz.”  A little understanding of how herbicides work can help you get the most bang for your buck.  


Herbicides work by hindering or stopping processes in a plant that are essential for life. They might inhibit growth, stop photosynthesis, shut down protein synthesis by blocking enzyme activity, or break down cell membranes and desiccate the cells.

The effect can be slow or fast, and result in different symptoms, depending on the mode of action and the target plant species.

Roundup: For example, glyphosate, commonly sold as Roundup, is absorbed through the leaves and translocated to the plant’s growing points. Because of its movement within plants, it is classified as systemic. It then inhibits the production of certain amino acids by blocking a specific plant enzyme. This results in a slow death as the plant depletes its limited amino acid reserves. The leaves gradually yellow and then brown out completely. Woody stems often die from tips downwards. This process can take anywhere from a week in annual weeds to a few months in woody perennials. It is important that you do not cut or mow plants recently treated with glyphosate. You need to give the herbicide time work before cutting.

2,4-D: In contrast to glyphosate, the very popular broadleaf herbicide 2,4-D kills plants by mimicking a hormone that regulates plant growth. It is also absorbed through the leaves and is systemic, moving throughout the plant. However, it turns the plant on its head, inhibiting new growth at the growing points and stimulating mature cells to try to continue growing. This results in a cascade of visible symptoms including leaf cupping and curling, stem twisting and cracking, and yellowing of the leaves. Some folks have likened this to the weed actually growing itself to death. This mode of action is often more rapid and has visible results faster than glyphosate. Accidentally spray it on your beans or tomatoes, and you will often see those symptoms appear in a day or two.

Diquat and vinegar: Herbicides such as diquat and acetic acid (vinegar) are contact-type materials. They do not translocate, but damage exactly what they are sprayed on. These herbicides break apart the cell membranes and the cells quickly dessicate. Necrotic patches appear on the leaves and the leaves quickly brown out. You can often see the effects start within a few hours, and if you drift some onto nontarget plants, you often see a flecking pattern of damage. Using contact herbicides is as close to melting the Wicked Witch as you can get.


As with most things, timing in everything when it comes to applying herbicides.

For annual weeds, all three herbicide classes above are very effective at killing seedlings and very small plants.

However, as plants get larger and approach maturity, contact herbicides quickly become ineffective. They may cause damage, but larger plants can often recover.

Both glyphosate and 2,4-D can control larger annual weeds, but don’t use this as an excuse to wait too long. Any herbicide treatment is generally wasted if annuals reach flowering and begin to produce seed.

Contact herbicides are also ineffective on herbaceous perennials and woody plants. They will definitely damage them, but with no translocation, perennials will rapidly resprout and recover.

Systemic herbicides often follow the translocation stream in perennial plants that moves sugars to the active growing points. That is why spring season applications on perennials do not work well, as sugars are often moving upwards in the plants from the roots.

With that in mind, late summer and fall is often an excellent time for application of systemic herbicides for herbaceous perennials and woody brush. This coincides with the period of energy storage and strong downward translocation of sugars.

Kudzu, sweet gum, poison ivy and cogongrass and more effectively controlled in the fall. Unfortunately, many folks have had enough by then and just mow things down. That is fine, but it is an opportunity missed to get ahead for next year. Remember, mow one this year, get 10 coming back next spring!

For a list of herbicides or assistance, contact your local county Extension office or visit

Shane Harris is an extension agent for the Alabama Cooperative Extension System.