When Herbicide Goes Bad

Posted by: Tony Koski, Extension turfgrass specialist

Who would have ever guessed that the turf specialist would start looking up and noticing all these trees in our landscapes? What a strange world. But the more I look, the more fascinating those tall leafy plants become. And one thing I’ve noticed this year is an onslaught of herbicide injury occurring. Recently I’ve been on several site visits and a common theme seems to be chemical injury.  (Don’t worry…we still have all those other problems like drought stress, planting depth and chlorosis.)

So this seems like a good time to remind people to be careful about using herbicides near any plants, as they can be damaging and even lethal to woody plants. First of all, apply all pesticides (this includes fungicides, herbicides and insecticides) according to the label. Use proper application equipment, mix at the recommended rate and wear appropriate protective clothing and eyewear. Some chemicals have limitations of how warm it can be when spraying (chemicals have the potential to volatize and turn into gases). Don’t spray on windy days…the lightest breeze can cause chemicals to travel great distances. It’s best to spray early in the morning when it’s still cool outside.

Possible 2,4-D and dicamba injury on redbud.

Herbicides, in particular, tend to affect plant tissue as growth regulators or amino acid inhibitors. Growth regulators (usually from selective herbicides, like Weed-B-Gon or 2,4-D) can cause leaf cupping, coiling and bending. Leaves can also be misshapen. These herbicides may suppress growth, cause iron chlorosis, blacken tissues and cause defoliation.

Misshapen aspen leaves from possible 2,4-D injury.

Amino acid inhibiting herbicides (from non-selective chemicals like Roundup) can cause strap-like leaves, bushy growth, misshapen leaves and injury may be delayed into the next growing season or longer. This can occur when glyphosate is sprayed on suckers at the base of trees; green, exposed roots on the soil surface; and green bark at the base of trees or on the trunk. 

Glyphosate injury on ash.

Depending on the dose of what was sprayed on/near the tree, herbicides can cause long-lasting damage—and may even kill the tree. Remember the herbicide doesn’t know the difference between a weed in your lawn and your beautiful, blooming linden. But you do. Be careful out there.

An ash severely damaged by glyphosate.