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Harnett County,
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Cooperative Extension 

Why does my RoundUp work sometimes, but not all the time?


Question Why does my Roundup work sometimes, but not all the time?

Answer Roundup is the trade name of several products that contain glyphosate. Most people refer to glyphosate products as “Roundup.” It is like referring to all facial tissue as Kleenex, copy machines as Xerox or soft drinks as Coke. Glyphosate products are probably the most common herbicides used in the world, certainly in the United States. You can find glyphosate products under many trade names like Glystar, Kleenup, Accord or Rodeo.

Glyphosate herbicides work on the same principle. They inhibit a specific enzyme that plants need. The specific enzyme is called EPSP synthase. Without this enzyme, plants are unable to produce other essential proteins. The enzyme EPSP synthase is found in plants and not animals. This concept of enzyme disruption is the same process we use when taking antibiotics to kill bacteria.

In order for glyphosate to work, it needs to be applied when the plants are actively growing. It also needs time to be absorbed through the leaves. Therefore, it should be applied at least 6 hours before a rainfall or scheduled irrigation.

Stressed plants (too hot, too dry, too wet, diseased or insect ridden) may not be affected by a glyphosate application. Typically, stressed plants are not actively growing. Nurse the targeted weed back to good health, then kill it.

Spreader/stickers also have a big impact on herbicide efficiency. These additives are called surfactants. Some glyphosate formulations have a surfactant already added. Check the label to see if you need to add one.

Timing or season of application also plays a major role with hard to kill plants. For example, English ivy and catbriars die best when sprayed in the spring. Poison ivy and honeysuckle are most vulnerable during the summer. Blackberry and trumpet creeper are best sprayed in the fall. Most plants are not picky. They will die whenever you spray their leaves. Keep in mind that glyphosate products are non-selective herbicides. If it doesn't kill a plant, it's not because it didn't try.

Glyphosate moves inside plants, but accumulates in their actively growing parts. It is typically not a “fast acting” herbicide. Growth inhibition occurs within a couple days, but symptoms (yellowing) may take 7 to 10 days. Death may take a few weeks. Woody plants not initially killed may show injury symptoms on their new growth for 2 or more years.

Ironically, glyphosate products are some of the safest herbicides on the market. They are not taken up by plant roots, and they are broken down very quickly by microorganisms in the soil.

For a guide to glyphosate application, visit If you do not have internet access, call the Cooperative Extension Office at 893-7533 or email me at While some plants are hard to kill with glyphosate, others will quake at the mention of the word “Roundup.” You can be sure that it will kill your prize plants if they are accidentally sprayed. Use a blue dye in your tank mix. That way you can see who has been marked for death.

Gary L. Pierce

Horticulture Extension Agent

Harnett County

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