By Tom Kalb, Horticulturist

NDSU Extension

Killing weeds with poisonous herbicides requires us to balance the benefits and risks.

Everybody wants an attractive and weed-free lawn, but nobody wants to expose themselves to toxic chemicals.

Start by having realistic expectations about your lawn. It is not natural and practically impossible to have a lawn without any weeds.  

Next, use poisonous herbicides only when they are necessary and effective.

That said, now is the most effective time to kill weeds in lawns.

Timing is critical. The secret to killing perennial weeds such as dandelions and thistles is to get the herbicide down into their roots.

The root systems of some weeds are extensive. One Canadian thistle plant, for example, can grow 300 feet of roots in one summer!

It is a challenge to get herbicide into a vast network of roots, and that is why now is the best time.

The nights are getting longer and the weeds in your lawn know winter is coming. In response, these weeds are beginning to channel their sugars down into their roots to store them over winter.

That is perfect for us. If we spray a weed with a systemic herbicide in fall, the weed will accidentally channel it along with its sugars down into its roots. Gotcha!

A spray done once per year—now in fall—can control most broadleaf weeds and minimize our exposure to poisonous chemicals.

For most situations this seems like a good balance of benefits and risks.

Another sensible strategy is to spot-spray in weedy areas rather than spraying the entire yard. This will reduce the exposure of us and our landscape plants to herbicides.

Trimec formulations are commonly available and work well. These products contain a blend of 2,4-D; mecoprop (MCPP) and dicamba. Some of these formulations contain quinclorac as well.

Follow the instructions on the label. Mix at the proper rate. Avoid using higher than recommended rates because this may harm nearby landscape plants.

Wear protective clothing when spraying poisonous chemicals. Protect yourself by wearing a long-sleeved shirt, long pants, socks, and non-absorbent gloves and shoes. Additional recommendations may be listed on the label.

To prevent herbicide drift, do not spray when the temperature is in the high 80s or higher. Spray when winds are calm and use heavy droplets rather than fine mists.

Keep children and pets off the lawn at least until the herbicide dries. The label may list longer waiting times.

Other cultural practices can reduce our need for herbicides. Mow your turf tall, let grass clippings fall to the soil and fertilize your lawn. These practices can create a thick turf that smothers emerging weeds.

If you are going to spray herbicides to kill weeds in your lawn, make it count.

Do it safely. Do it now.

For more information about gardening, contact your local NDSU Extension agent. Find the Extension office for your county at https://www.ag.ndsu.edu/extension/directory/counties.

NDSU Agriculture Communication – Sept. 28, 2021

Source: Tom Kalb, 701-328-9722, tom.kalb@ndsu.edu

Editor: Kelli Anderson, 701-231-6136, kelli.c.anderson@ndsu.edu