Question of the Week: January 17, 2019
“Can I use Roundup on my dormant lawn to kill weeds right now?”
I’m asked this several times a week this time of year. I mentioned that it makes me really nervous to answer it, and here are the reasons.
• I never know which Roundup product people have in mind. Roundup was originally a glyphosate herbicide used to kill grasses. It was really good at that task, and it hit remarkable sales. Then the manufacturer broadened the name to include other products containing other herbicides with other properties intended to kill other weeds.
So instead of asking about one specific product, people were asking about a collection of products, in effect, a brand. That’s why I still want to know that they’re talking about “the original Roundup with glyphosate being the only active ingredient.” And, for the record, there are other brands of glyphosate products from other companies. Roundup is no longer the only one.
• It’s hard to know for sure that bermudagrass, and to a lesser degree, St. Augustine, are totally dormant. At this point, before this weekend’s cold, they definitely are not. Runners are green, and were you to spray glyphosate on them, you would do harm to your lawn that wouldn’t need to be done. The only time I would ever consider agreeing to using a glyphosate herbicide on turf in winter is soon after a really cold spell (low 20s or colder). But if you’re willing to take a chance and spot-treat only the weeds, the risk is all yours.
• What weeds are you trying to kill, anyway? If it’s big, ugly broadleafed weeds, there are much more effective products to kill them. You need to be applying a broadleafed herbicide containing 2,4-D. In fact, just mowing them down will often eliminate them. Hoeing them out is often easier than spraying and a whole lot quicker.
If you’re talking about grassy winter weeds like annual bluegrass and rescuegrass, leave those alone. As your lawn greens up in a few weeks you won’t even notice them. Mark your calendar to apply pre-emergent granules the last week of August or first week of September to prevent them. That would be Dimension, Halts or Balan. That’s the only good remedy.
The only weed I’d ever see myself spraying with glyphosate at this time is…
Dallisgrass. This is a perennial weed for which we have no good control. Pre-emergents don’t help because it comes back from its roots. And we have no post-emergent product that won’t also kill our permanent lawngrass.
If you can catch dallisgrass greening up just a week or 10 days before your permanent lawngrass does (see photo above), that would be your window to treat it with a glyphosate-only spray. Use a pump sprayer and apply it only to the dallisgrass blades. Hopefully there will be enough green growth on the weed to kill out its clumps. You’ll probably have follow-up treatments to make during the growing season.