Cool Season Herbicides

Photo Credit: Randy Lemmon
Virginia ButtonweedPhoto Credit: Randy Lemmon
Virginia Buttonweed
Photo Credit: Randy Lemmon

Cool season herbicides are an effective way to control weeds like Virginia Buttonweed and Doveweed.  They are great for knocking out certain weeds once our high temperatures consistently drop below 80 degrees. But here’s a catch: If you want them to work perfectly, you need to pull up as much as you can first. Then, one week after cool temperatures have set in, hit any new growth with the herbicides.
Cool-season herbicides include products like Fertilome’s Weed Free Zone and Bonide’s Weed Beater Ultra.  If you’ve tried Metsulfuron-methyl (MSM) herbicides, let’s put those on hiatus. They work better in warmer temperatures, if they work at all. I’ve had many people tell me they didn’t see any kill with MSM herbicides, but most of the time it’s because they didn’t add a surfactant. More on that in a moment.  But cool-season herbicides don’t work below 40 degrees, either.  So, 78°F – 40°F is the proper temperature range.
Before the advent of cool-season herbicides, broadleaf weed control November through February was pretty much non-existent. That’s because standard broadleaf weed controls (including MSMs) used in the spring and summer can actually kill St. Augustine and Bermuda lawns if applied at the wrong time of year.

Conversely, cool-season herbicides used when it’s too warm can do lots of damage, too. So, along came carfentazone-based Weed Free Zone and Weed Beater Ultra, and the art of weed control in cooler months is now perfected. Still, there are some caveats to be respected for them to work properly.

Because most water along the Gulf Coast is so hard, you must understand the importance of adding surfactants to give herbicides the ability to stick to the weeds. Without a surfactant, such as Bonide’s Turbo Spreader Sticker or Hi Yield’s Speader Sticker, the herbicide will just bead up and roll right off the weed’s leaf surfaces and into the soil.

It really helps to pull up what you can a week or so before you apply the herbicide.  Also important is how a herbicide is applied.  I highly recommend spot treating.  Since most herbicides are sold as concentrates, the best tools for spot treating are pump-up sprayers or hand-held trigger sprayers. I suggest avoiding dial-and-spray or ready-to-spray bottles that you hook to the end of a hose, because you can’t be very precise with them.

If ready-to-spray formulas are the only types you can find, please use them with extreme caution – limit their use to spot treatments only. You’ll need to be quick with the on-off switch because overuse on the entire yard can cause the grass to yellow.

And one final reminder about cool-season herbicides: they are specifically for broadleaf weeds. They will not control “grassy” weeds or sedges like nutgrass. But if you have things like clover, spurge and the much-hated Virginia Buttonweed and Doveweed, you have permission to spot treat with those carfentazone-based herbicides as long as we are in the appropriate temperature range.