Identification and Management of Absinth Wormwood

 
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Figure 1. Distribution of absinth wormwood in the United States and Canada (USDA NRCS 2018).

Absinth wormwood (Artemisia absinthium L.) is a perennial broadleaf plant introduced into North America from Europe in 1841. Historically, the plant was cultivated for the sage flavor of the leaves and production of aromatic oil used in preparation of absinth and vermouth. Absinth wormwood naturalized from intentional horticultural plantings and is well established on rangeland, pastureland, and natural areas in the northern half of the United States and southern Canadian provinces (Figure 1). 

The plant causes economic losses by reducing grass production and tainting the flavor of milk from livestock grazing the plant. Absinth wormwood pollen can cause allergies and asthma in susceptible individuals when the plant is flowering in late July and August. 

Absinth wormwood is adapted to a wide range of soil types, from gravely alluvium to loams and clay loams. The plant thrives on disturbed sites, such as fence lines, roadsides, borrow pits, overgrazed pastures, and fields abandoned from cultivation. 

IDENTIFICATION 

Absinth wormwood is an herbaceous plant that initiates growth in spring from a woody basal crown. Plants are commonly three feet tall at maturity but can reach over five feet in height (Figure 2). Leaves are light to olive green in color, two to five inches long, and divided two or three times into deeply lobed leaflets. Leaves and stems are covered with fine silky hairs that give the plant a grayish appearance. Flower stalks appear at each upper leaf node and produce numerous flower heads, 1/8 inch in diameter, which appear in mid-summer. Many small, inconspicuous yellow flowers are produced in each head. Absinth wormwood is a prolific seed producer, and the small seed are scattered easily by wind, water, animals, and transported hay. Seed remain viable in soil for at least four years. Plants may spread by rootstock when plowed. 

Figure 2. Absinth wormwood plant (a), flower (b), and leaf covered with fine gray “hair” (c). 

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Management 

Herbicides

Field trials conducted on rangeland and natural areas showed that absinth wormwood is effectively controlled with Milestone®, GrazonNext® HL, or Transline® specialty herbicides. Milestone at 5 to 7 fluid ounces of product per acre (fl oz/A) or GrazonNext HL at 1.5 to 2 pints of product per acre (pt/A) provide 90 to 100 percent control of absinth wormwood when applied in the spring before plants are 12 inches tall or in fall (Figure 3). Transline at 2/3 pt/A should be applied when absinth wormwood is actively growing and greater than 12 inches tall. For optimum control in the fall, the plant should be mowed in mid- to late-summer to reduce vegetative biomass and stimulate new basal growth. This will improve effectiveness of herbicide applications and increase control. 

Figure 3. Percent control of absinth wormwood 3 and 12 months after application (MAA) with various broadleaf herbicides applied in spring. Control is averaged across multiple field locations in the northcentral U.S.

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Transline provides more selective control than either Milestone or GrazonNext HL and can be applied when non-target trees, shrubs, or forbs are present on the site. GrazonNext HL provides more broad-spectrum control and is best suited to sites with a complex of weed species. 

Cultural Methods

Absinth wormwood invades disturbed areas where there is less competition from other plant species. Management should include proper livestock grazing that maintains desirable competitive vegetation on rangelands and prevents invasion of the plant. Disturbed areas should be seeded with desirable vegetation, to prevent reinvasion and spread of the weed. 

Manual and Mechanical Methods

Individual plants or small patches can be controlled by hand pulling or digging roots when soil is moist. The entire root system must be removed to achieve effective control. Tillage can prevent establishment of the plant in cultivated cropland; however, root fragments may spread and re-sprout without continued tillage. Summer-fallow followed by fall tillage is more effective than spring tillage. 

Mowing alone or mowing with fertilization will not effectively control absinth wormwood infestations. Mowing several times during the growing season may reduce seed production in absinth wormwood but will also reduce desirable grass production. In a South Dakota study, grass shoot biomass in mowed treatments was 32 to 64 percent less than the most effective herbicide treatments. Mowed treatments also had less grass biomass than the non-treated control.

Long-Term Monitoring and Treatment 

Absinth wormwood produces a large amount of seed that can germinate after treatment of the initial infestation.  Controlling newly emerging plants for at least four years and encouraging a desirable plant community will reduce reinvasion potential of absinth wormwood.

References

Carey, Jennifer H. 1994. Artemisia absinthium. In: Fire Effects Information System, [Online].

Dow AgroSciences. Unpublished field data.

Lym, R.G. and A. Travnicek. 2015. Identification and Control of Invasive and Troublesome Weeds in North Dakota—Absinth wormwood. North Dakota State University, Fargo, ND. W1411.

Lym, R.G. 2015. Herbicide mixtures applied in the spring or fall for absinth wormwood control. Res. Prog. Rep. Western Soc. Weed Sci. p.11. 

Lym, R.G. 2010. Aminopyralid applied alone or in combination with metsulfuron or picloram for absinth wormwood control. Res. Prog. Rep. West. Soc. Weed Sci. p. 34-36.

Lym, R.G. and L.W. Samuel. 2006. Control of invasive weeds with aminopyralid in North Dakota. Res. Prog. Rep. Western Soc. Weed Sci. p. 30-35.

Maw, M. G.; Thomas, A. G.; Stahevitch, A. 1985. The biology of Canadian weeds. 66. Artemisia absinthium L. Canadian Journal of Plant Science.65(2): 389-400.

Moechnig, MJ, DL Deneke, JK Alms and DA Vos. 2009. Absinth wormwood control programs that include mowing, fertilization, or herbicides. Res. Prog. Rep. North Central Weed Sci Soc. 64. p.57.

USDA, NRCS. 2018. The PLANTS Database (http://plants.usda.gov, 28 February 2018). National Plant Data Team, Greensboro, NC 27401-4901 USA.

 
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Milestone and GrazonNext HL specialty herbicides are not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state.

Milestone and GrazonNext HL: When treating areas in and around roadside or utility rights-of-way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas or rotating the treated area to sensitive crops. See the product label for details. 

State restrictions on the sale and use of Transline specialty herbicide apply. Consult the label before purchase or use for full details. Always read and follow label directions.