Cold nights trigger growth of winter weeds


By Ted Manzer

Friday, November 10, 2017

I looked out at my lawn the other day and noticed how green it was getting again. At first I thought the cold nights we’ve experienced had only slowed the growth of my grass a little. When I looked closer I saw the truth.

All the cool-season weeds had begun to make their appearance. Some were obvious, like chickweed and henbit. They are broadleaf weeds.

Grassy weeds like annual bluegrass and grass-like ones like wild garlic were beginning to show themselves too. One has to look a little closer to notice them. They look a lot like the rest of the grass.

I shrugged my shoulders. None of this really bothers me. All it means is I might have to mow my lawn one or two more times. Winter annual weeds will melt out once things warm up in spring.

For some folks it is a big deal, and for them I’ll discuss how to tackle the problem. The weather forecast for the next few weeks is mild. This means these weeds will grow like crazy. It also means that they’re very susceptible to being killed by herbicides.

Weed killers work best when plants are actively growing. In cold weather plant growth slows down and the weeds don’t absorb as much chemical. Consequently, herbicides aren’t as effective.

During favorable weather most broadleaf chemicals will wipe out clover, dandelions, chickweed, henbit, dead nettle wild geranium and other cool-season weeds in short order. The grassy and grass-like weeds are a little tougher.

Annual bluegrass is that grass that sends out prolific seed heads at low cutting heights. Most turf professionals consider it the most troublesome cool-season weed in turf.

Dimension is a pre-emergent granular herbicide that’s effective against annual bluegrass. Unfortunately it’s too late for it to work. Annual bluegrass has already begun to grow. Pre-emergent chemicals are great because they prevent seeds from sprouting. They’re worthless once plants begin to grow.

Pendimethalin is another common pre-emergent chemical. It is the one found in ‘Halts’ and can be bought in most garden centers. Again, if this chemical was not applied to the soil a couple months ago you’re out of luck.

Several chemicals can kill annual bluegrass in warm-season turf, but most of them are restricted use only. That means one must obtain a pesticide license to apply those products. If you can tolerate the weed for another month or so, Round-up is an effective control in dormant warm-season turf. Pick a warm winter day to apply it and spot treat as much as possible.

Image is a chemical that will kill wild onion and garlic. However, it should not be used on fescue lawns. In fact it should not be used in warm-season turf in spring or early summer.

If infestations of these pungent perennial weeds are light, the best control method is to dig them out. That’s probably the only safe method for fescue lawns.

As for me, wild onions and garlic aren’t a problem. When the grass turns brown, I go out and harvest them. They’re great on baked potatoes.

Ted Manzer teaches agriculture at Northeastern High School.