The heat and humidity may slow us and some of our plants down, but those very successful plants we call “weeds” continue to do very well.

In that on-going battle against the weeds, glyphosate — commonly known as Roundup — was a game-changer when it debuted in the 1970s. Suddenly, we had a product that would control just about anything that was green and growing.

And, despite what you may have heard and read, it’s safe for critters, humans and the environment because it attacks a protein synthesis chain that animals don’t have and breaks down quickly once it reaches the soil.

The product is a great weed control tool, but that doesn’t mean that we haven’t injured some plants along the way with it. We’ve also learned that — especially on some hard-to-control weeds — certain times of the year are better than others.

Glyphosate is considered a non-selective herbicide which means that it controls anything that is actively growing at the time of application. It also has the ability to move around in the plant system, so it is called a “systemic.” That ability really helps when you’re trying to control plants with extensive root systems like Bermudagrass. That same characteristic will bite you in the posterior if you apply Roundup to a plant you didn’t want to injure.

Just like everything else in your life, timing is everything. If you get a bit of a glyphosate solution on a young tree with thin bark such as a maple or crape myrtle in early spring, you probably won’t see much injury. Hit that same tree this time of year and you may see distorted growth and even death next year. This has to do with what’s going on with plants once the days begin to shorten. Deciduous plants (the ones that drop their leaves) shift their focus from lots of new growth to storing energy for the coming winter. Systemic products like glyphosate may be absorbed through those young stems and get into the vascular tissue of the plant to wreak its own brand of havoc. Older trees with thick, corky bark don’t usually have a problem.

We’ve also learned over the years that Roundup works better on certain weeds at particular times of the year. Early to mid-summer is a great time to attack poison ivy and honeysuckle. Late summer into early fall is the best time to apply glyphosate to trumpetcreeper and Virginia creeper. From now until the end of summer is just right for controlling Chinese wisterias that are taking over the world. If smilax or greenbrier is your nemesis, you won’t get much relief now. Early spring when the leaves first appear is best.

Even though glyphosate is a pretty safe product, you still need to read the label and follow the directions carefully. Keep your sprayer is good shape. Make sure the nozzles are doing what they were designed to do. A slight mist emerging from one side of a damaged nozzle will really make a mess when you’re spraying shrub bed edges. (yes, I did that) Avoid immediately walking through an area wet with spray solution and then traipsing across your lawn. That’s a good way to have dead shoeprints across the turf.

Help with gardening problems is just a mouse click or phone call away. Check out our website where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: If you live in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610. You can also find great local information at and on Facebook. Just search for “New Hanover County Arboretum. Or, stop by the Plant Clinic at the Arboretum between 10 am and 4 pm Monday through Friday.


Al Hight is the extension director for the North Carolina Cooperative Extension in New Hanover County. Contact him at 910-798-7666.