ATTRA Question of the Week

L.R.
Colorado

Answer: Historically, research in absinth wormwood (Artemisia absinthium) control has predominantly been oriented toward chemical control. However, using an integrated, non-chemical approach including mechanical and biological controls is gaining precedence in the western states. Mechanical controls include the use of fire and mowing, and biological controls include multi-species grazing and targeted beneficial insect release. I will address each of these approaches in turn and will conclude by offering some resources for further reading.

Any integrated weed control program should be oriented toward the facilitation of vigorous perennial grass growth in order to out-compete the targeted weed population. This is particularly important with absinth wormwood control, as a vibrant perennial grass stand can out-compete it (1). For detailed information on grass management see the ATTRA publication Pasture, Rangeland, and Grazing Management and the publication Revegetation Guidelines for Western Montana: Considering Invasive Weeds.

There is not much research on grazing absinth wormwood, although sheep and goats are known to eat it. Research on other plant species such as knapweed and leafy spurge suggest that targeted grazing can be a useful tool for control if the grazing animal’s diet is sufficiently diverse. I recommend the book (available online) Targeted Grazing: A Natural Approach to Vegetation Management and Landscape Enhancement.

Beneficial insects are those that are known to target a particular plant species, and for some, can provide significant control. Research has shown that this method is useful if the target weed population is large enough to allow the insect population to reach a critical mass and begin to defoliate leaves, mine the stems, or populate the seed heads or flowers. This method usually requires several years to allow the insect population to reach critical mass. The pyralid moth Euzophera cinerosella is the only species I know of that targets wormwood; however the insect could also target other Artemisia species, making its usefulness problematic. Contact your county weed board for information on insectaries and release programs in your area.

For cultural control, a spring burn followed by periodic mowing has shown some success. Mowing in the spring can help prevent seed production, but burning alone has not shown effective for wormwood control (3). For a mowing regime, periodic mowing during the vegetative stage of growth through flowering can significantly impact seed production and weaken the plant, enabling perennial grasses to establish and take over the stand.

References and Resources

1. Absinth Wormwood: Options for Control. Lincoln County, WA Noxious Weed Control Board.

2. American Sheep Industry Association. 2006. Targeted Grazing: A Natural Approach to Vegetation Management and Landscape Enhancement. A handbook on grazing as a new ecological service. Karen Launchbaugh, Editor.

This handbook represents a compilation of the latest research on harnessing livestock to graze targeted vegetation in ways that improve the function and appearance of a wide variety of landscapes.

3. Evans, J.E. and Nancy Eckardt. 1987. Element Stewardship Abstract for Artemisia absinthium. The Nature Conservancy.

4. Owsley, Cindy. 1999. Biological Weed Control – Grazing. Boulder County Parks and Open Space.

Livestock can be a valuable tool within an Integrated Weed Management system.