Herbicide Carry-over Damage by Mary Small

Damage on Ash leaves at the Jeffco Fairgrounds

When I worked in an extension office in another state, a few gardeners reported that their vegetables and flowers started crashing quickly. They also had distorted, stunted growth.  As it turned out, the gardeners applied manure to their gardens earlier in the year.  Unfortunately it was contaminated with an agricultural herbicide. The product was used on pastures where the cattle later grazed. It passed right through the animals’ digestive tract into the manure. Because of the particular product used, gardeners would not be able to grow plants for a while in the areas where the manure was applied.

I was reminded of this incident while attending a class on herbicides (weed killers) last week, because herbicide carry-over and contamination are still happening.

Glyphosate is the active ingredient in many non-selective weed control products commonly used around homes.  Roundup is one of the more popular brands.  It kills both grassy and broadleaf weeds. Once applied to a plant, glyphosate moves to actively growing areas and kills by interfering with amino acids production, the building blocks of protein. It should not be applied to any green plant tissue you don’t intend to kill.

But accidents happen. Often in spring we see a branch of a tree or shrub with narrow, strap-like, yellowed leaves bunched together tightly.  Typically, weeds in the area were sprayed in the fall. The spray drifted (or was accidentally sprayed on) green tissue of the desirable tree or shrub.  Plants store it over winter in bark, roots and buds. When new growth began in the spring, it appeared stunted and malformed.  Plant survival and health ultimately depend on the health of the plant before it was damaged and the dose received.

Broadleaf weed killers used on lawns can “carry over” in grass clippings. That means there is some residue present for a while following weed treatment.  In general, it’s best to return grass clippings to the lawn for at least a month following herbicide applications, rather than apply them to gardens. If not, enough residues may be present to cause injury to plants.  Broadleaf weed killers don’t know the difference between weeds and desirable plants.

How can you protect your plants from damage? Read the label of the product you’re using and be careful around desirable plants.  Before purchasing manure from a farm or ranch, ask about what products were used to treat the pastures the animals fed upon and how long the manure has composted.