Imazapyr is a common herbicide used in forestry primarily for pine management. It is a non-selective systemic herbicide that targets grasses, broadleaf weeds, vines, brambles, shrubs, and trees (especially hardwoods). There are several plants that are resistant to imazapyr. They include elms, woody legumes, wax myrtle, croton, blackberry, buckeye, eastern Baccharis, and pine. Hence, we like to use it in pine management.
Imazapyr is in the chemical family Imidazolinone. It is an amino acid synthesis inhibitor. It inhibits the enzyme acetohydroxy acid synthase (AHAS), which is also known as acetolactate synthase (ALS). By inhibiting ALS, the plant does not have the enzymes needed for protein synthesis and cell growth. Plant death is usually slow with ALS inhibitors. Since only plants have ALS, imazapyr has very low toxicity to animals, insects and fish.
When using Imazapyr, one does have to be careful to prevent contact with non-target plants. Imazapyr can be absorbed through foliage and/or roots. Imazapyr can persist in the soil from one to five months. It typically does not leach from the soil and is slowly degraded by microbial metabolism.
While Imazapyr does have soil activity and can act as a pre-emergent, it is typically more effective to use the foliar application when the plants are actively growing, especially in a pine management application. When using the foliar application, a surfactant is recommended to increase the uptake of the herbicide. It is also commonly used in hack and squirt and stump applications for selective control to reduce damage to crop trees. Keep in mind that imazapyr can damage hardwoods if sprayed on the ground where the tree’s roots could absorb the herbicide.
Some common brands that are used in forestry applications include Arsenal AC, Chopper Gen 2, Polaris AC Complete, Polaris SP, Imazapyr 4SL, and Rotary 2SL. Some of these products contain different amounts of imazapyr and are used in different applications, like site preparation or understory treatment. BE SURE TO ALWAYS READ THE LABEL.
Before you use imazapyr, I highly recommend that you speak with your local Extension Agent. Identifying the weeds you want to control, the crop trees you wish to protect, and any sensitive areas that do not need to be sprayed is essential. Since imazapyr is soil active and remains in the soil for months, it is important to use it properly to avoid damage to non-target plants.
Author(s)
Jeff Fellers, Cooperative Extension, Forestry and Wildlife Agent
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