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Weed-and-feed fertilizers have limitations

Now is a great time to kill weeds but a bad time to feed the lawn.

Clover in bright sunWeed-and-feed fertilizers are popular among gardeners. We can kill weeds and feed our lawn in one step. As the old saying goes, “it’s like killing two birds with one stone.”

Perhaps that proverb of murdering birds is too violent today. Instead of “killing two birds with one stone,” let’s say it is like “feeding two birds with one scone.”

The problem with weed-and-feed fertilizers is the best time to kill weeds is not the best time to feed the lawn.

Late September is a great time to kill weeds, especially after we have received a light frost. The weeds know winter is coming. The nights are longer and colder. Instead of growing new leaves, they are sending their nutrients down into their roots to prepare for winter. If we spray a weed in late September, the weed will take the herbicide along with its nutrients down to the roots. The entire plant—including the root system—dies.

Late September is not a great time to feed the lawn. The grass plants also know winter is coming. They want to stop growing blades and begin preparing themselves for the frigid winter. A fertilizer application prevents this by promoting blade growth and delaying hardening. This makes the lawn more susceptible to winter injury.

It is better to wait until mid to late October to feed the lawn. This is after the grass blades stop growing and after you have stopped mowing. This application is the single most valuable time to fertilize the lawn—any lawn.

You will not see any immediate effects of this dormant application, but the fertilizer will develop a stronger root system this fall and generate new buds for blade growth next spring. Turf roots grow until the ground freezes in mid November.

Weed-and-feed products have other shortcomings. First, they may lack sufficient weed-killing power. Many weed-and-feed fertilizers use 2,4-D as their sole herbicide. This chemical will kill dandelions, but is not effective at controlling many other weeds including clover (see photo), black medic, ground ivy and bindweed. Some weed-and-feed products add mecoprop, which is useful, but still lack dicamba.

Dicamba is a much more powerful herbicide and it is available to homeowners in broadleaf herbicide spray mixes. One should note that dicamba can build up in the soil and damage shrubs, young trees and perennials. Try to limit your use of dicamba to once per year and the best time is in late September.

Another shortcoming is the herbicide granules may roll off the weed leaves before they are absorbed. Most lawn herbicides are ineffective unless they are absorbed by the leaves of the weeds. This is especially a problem with small-leaved weeds such as clover and black medic. The directions of weed-and-feed fertilizers will tell you to water your lawn before you apply the fertilizer so the granules stick onto the weeds, but the granules can still roll off the weeds.

You will be much more successful if you spray the herbicide on the weeds. Add a drop of detergent to the spray mix and that will further help the spray to adhere to the weed leaves.

Health and environmental concerns also need to be considered. Why should we apply toxic herbicides over our entire lawn if we only have a few weeds? A spot treatment is better for our health, better for our environment, and can save us money.

This fall, don’t try to feed two birds with one scone. Instead, feed two birds with two scones (the birds will be healthier and happier). In other words, manage your lawn in two separate operations. Spot spray the weeds in late September and then feed in mid to late October. Your lawn will be healthier and happier.


Written by Tom Kalb, Extension Horticulturist, North Dakota State University. Published in the NDSU Yard & Garden Report, September 22, 2014. The photo was made available under a Creative Commons license specified by the photographer: Steve Corey.

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