Herbicide treated mulch

Asked April 10, 2017, 3:50 PM EDT

Are herbicide treated mulches effective? Do they kill the weeds under the mulch? Are there drawbacks to using them?
Thank you.

Oakland County Michigan

1 Response

Are you speaking of the mulches which have the active ingredient trifluralin in them? These mulches stop new weeds from becoming established from seed. They are effective at doing that when used as directed.

They do not stop existing weeds or runners from a weed root establishing new plants. However, mulch when applied deep enough can smother some existing weeds. It is best to remove weeds that extend themselves by runners, such as creeping charlie, or perennial weeds, such as dandelion, first. Then apply your mulch.

One popular brand is 'Preen Mulch'. There may be others. Preen brand makes weed killers, organic products, inorganic products, mulches, and more. You must carefully select a product that is safe for use in the area desired.

The active ingredient in some mulches is trifluralin. It works by affecting seedling root growth. It doesn't stop germination but does keep the seedling from growing and getting established. Therefore, it does not kill existing weeds. Another active ingredient in some mulches is isoxaben, another pre-emergent chemical which stops seeds from getting established.

These products may or may not be safe around edible plants, and may be toxic to fish. Always read the label and follow all precautions.

Here is an informative article from the North Carolina Extension service:

"Preen herbicide has long been used in home gardens for preemergence control of many annual weeds. The active ingredient, trifluralin, is labeled for use around many woody and herbaceous ornamental plants as well as many vegetable and fruit crops. Although, not the most efficacious herbicide, Preen controlled many of our most common landscape weeds including henbit, chickweed, oxalis, crabgrass, and annual bluegrass, without injuring landscape plantings. This product is still widely available in garden centers throughout the country. However, look closely at the label – the Preen you purchase today may not be the same product you previously used.

Today the Preen name is used to identify a diverse product line that includes preemergence and postemergence herbicides. Several Preen products contain 2,4-D for broadleaf weed control in lawns; these products should not be used in landscape beds. The active ingredient in Preen Weed Preventer for Southern Gardens is dithiopyr – the same ingredient found in the herbicide Dimension. This is safe on most ornamentals but should not be used around any food crops. There is Preen Mulch Plus Premium that contains isoxaben plus trifluralin (think Snapshot TG); not to be confused with Preen Plus Mulch Midnight Black that contains trifluralin (but no isoxaben). The isoxaben-containing mulch will damage pansies but the trifluralin-containing product will not. Confused yet? There is also Preen Brush Weed Killer that contains 2,4-DP + 2,4-D + dicamba; Preen Weed and Grass Killer that contains glyphosate (same ingredient as Roundup); and an organic product for vegetable gardens, Preen Vegetable Garden Weed Preventer, that contains corn gluten meal. Many different active ingredients, all sold under the Preen name can lead to confusion.

So, if you go to the garden center to purchase Preen for weed control in your garden or lawn – READ THE LABEL. Make sure you purchase the right product for the right job. Labels and material safety data sheets for the Preen products are available from the manufacturer’s web site at http://www.preen.com/msds

Written by: Joseph C. Neal, Professor and Extension Specialist -- Weed Science"
(End of extension article)

As you can see, companies well known for one type product make many others, so you must make your choice carefully and read the whole label. There are new products and brands being introduced all the time. If you would like to write us back, specifying one or two products by their full name( for example, Preen Mulch Plus) we will be glad to help you sort them out.
Extension does not recommend or endorse any retail product, we mention them by brand just as an example. Thank you.