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— Kelly Feehan Extension Educator
COLUMBUS — Weeds have begun to grow. If herbicides are a weed control method used and you tend to apply the same herbicide each year, this could lead to herbicide resistant weeds.
Herbicide resistant weeds are plants able to survive labeled application rates of an herbicide that previously killed the weed. While there are not a large number of resistant turf weeds yet, there is a good likelihood of more weeds becoming resistant if specific management and herbicide application practices continue to be used.
Turfgrass weeds now known to have resistance include annual bluegrass, smooth and large crabgrass, goose grass, spotted spurge and buckthorn plantain. This is according to the International Herbicide Resistant Weed Database.
Weed management practices that lead to herbicide resistance include using herbicides as the only means of weed control. Applying only one type or group of herbicides and making frequent and repeated applications over an extended period of time, e.g. multiple seasons. And not following recommended label rates, such as using less than the recommended rate to save money.
A weed develops resistance when a few weeds survive after an herbicide is applied. When the surviving weeds complete their life cycle and develop seed, resistance can then be transferred to the next generation. If the same herbicide or a different herbicide with the same mode of action is applied, additional weeds survive until the resistant population becomes dominant.
Mode of action (MoA) is the effect an herbicide has on a biological process of a plant that leads to its death. To help reduce resistance, herbicides are categorized into numbered groups based on MoA. Group 1 herbicides inhibit lipid synthesis. Group 2 inhibit amino acid synthesis. Group 4 are growth regulators and Groups 5, 6 and 7 inhibit photosynthesis. The group number is listed on the label.
To help reduce herbicide resistance in weeds, rotate herbicides with different MoAs or group numbers to avoid frequent and repeat application of herbicides having the same MoA.
Even if herbicides have different active ingredients (the chemical that controls the weed), they can still have the same MoA. Rotate herbicide groups from year to year or within the season.
Herbicide rotation also reduces the chance of already resistant weeds surviving. If a weed survives an application with one MoA, the weed could be controlled if treated with an herbicide having a different MoA it is not resistant to. This works best if herbicide group numbers are rotated within the season, such as when repeat applications are needed.
On weed management, do not use herbicides as the only means of control. Manage turfgrass and turfgrass soils to maintain a healthy, dense turf that competes with weeds.
Core aerate to relieve soil compaction, leave clippings when mowing, provide sufficient fertilizer, avoid overwatering, and mow turf at a three-inch height.
A taller height has been shown to provide almost as much control of annual weeds as a preemergence herbicide like a crabgrass preventer. And don’t overlook the very helpful method of hand-digging and removing weeds before they bloom and disperse seed.
