Herbicide resistance in the nursery and landscape maintenance industries

Despite substantial reliance on herbicides for weed management, herbicide-resistant weeds have generally been perceived to be less of an issue in the nursery crop and landscape maintenance industries compared with larger-acreage agronomic cropping systems. Annual blue• grass, common groundsel ( Senecio vulgaris), horseweed

( Conyza canadensis), and fringed (northern) willowherb (Epilobium ciliatum) have been reported to be herbicide resistant in nursery production in at least one country. A majority of the reports are weeds developing resistance to the Photosystem II (PS II) inhibitors, such as simazine (Princep). Although simazine is commonly used in field nursery production, it is not used in maintenance of landscape ornamentals.

One weed that has developed resistance to glyphosate in Virginia and other states is horseweed, often called marestail. Resistance developed primarily due to the extensive use of glyphosate in agronomic crop production. Horseweed seed can blow a considerable distance in the wind, so it can spread from crop fields to nursery production areas, landscape beds, and noncrop areas.

Generally combinations of preemergence herbicides are used in nursery production and in maintenance of landscape ornamentals to broaden the spectrum of weed control. The herbicides being combined usually differ in their mode of action so applying such combinations helps to reduce the potential for resistance development. A common combination as an example would be combining Gallery (isoxaben), more effective on annual broadleaf weeds, with Barricade (prodiamine), more effective on annual grasses. These two herbicides differ in their mode of action. Besides combinations of sprayable herbicides, there are quite a few granular formulations that contain two active ingredients. An example would be Snapshot, a combination of isoxaben and trifluralin.

If herbicide resistant weeds are present in nursery production areas, these biotypes could be spread to landscape beds through the planting of infested trees or shrubs. However, we do not have good information on the spread of herbicide resistant weed populations from nurseries to landscape beds.


Preventing herbicide resistance

What can we do to prevent the development of herbicide resistant weeds? One should rotate herbicide modes of action of look at tank mixes of herbicides with different modes of action. One does not need to do this every year, but maybe every three of four years. For example, one could alternate use of a dinitroniline herbicide like prodiamine with Spectide (imdaziflam) or oxadiazon in bermudagrass turf on a golf course. Simazine could be alternated with flumioxazin at a tree nursery. Halosulfuron could be alternated with sulfentrazone (Dismiss) for yellow nutsedge control in woody landscape beds and in turf areas. Alternating between FreeHand and Snapshot would be an option in ornamental beds. Utilize both preemergence and postemergence herbicides where possible as these two groups generally have different modes of action. To use this strategy, one needs to learn how each available herbicide controls weeds (their mode of action). Use nonchemical means of weed control in your control program. Hand weed plants that survive an herbicide application, thus preventing seed production. Clean equipment when going to other locations to prevent spread of weed seed. By following these suggestions, there is less potential for herbicide resistance to appear on properties you maintain.