Mayweed Leaf.
Common Weeds of the US, USDA

When weeds with fernlike leaf structure show up in my office for identification, I usually smell them first to catch their aroma, and then I sneeze very loudly. Some of these "feather duster" looking weeds pack a wallop with odor or their pollen triggers hay fever. However, that odor or taste serves a purpose such as warding off cattle from browsing, attracting insects for pollination, serving as a plant hors d’oeuvre, or making people doing weed identification sneeze.

Minnesota has numerous plants that have fernlike or dissected leaf structure. Some of these plants also carry a disagreeable or pleasant odor.

Pineapple-weed (Matricaria matricariodes), carries a pleasant odor as the name implies. Some examples of plants carrying a disagreeable odor and taste that are found in Minnesota are briefly described below. All of these have the fernlike or heavily dissected leaf structure.

Pineapple-weed
Oregon State
University

Common Yarrow (Achillea millefolium).

A native species to the US and is generally unpalatable, but sheep, deer, and pronghorn will occasionally eat the flowers. Rarely will cattle and horses browse on this plant. It is a perennial.

Mayweed (Anthemis cotula)

Mayweed is an introduced annual from Europe that also goes by other names, Dog Fennel or Stinkweed. It has a daisy-like flower. The plant contains acrid substances that are distasteful, irritates mucous membranes of livestock, and causes blistering of muzzles. The blistering causes animals to reduce grazing which results in reduced weight loss and milk production.

Wormwood species (Artemisia)

Wormwood species carry the characteristic sage odor. Absinth wormwood (Artemisia absinthium) is found in pastures, roadsides, wastelands, and is commonly found in dry soils. If grazed by cattle it can result in flavoring the milk. Absinth Wormwood is found in pasturelands in central, west central and northwest Minnesota. It is a perennial. Biennial wormwood (Artemisia biennis), ever expanding its range and irritation amongst soybean producers, is less aromatic than Absinth wormwood. Both of these are native to the US. (To learn more about Biennial wormwood and its control look under Weed Management at the following web page www.agro.agri.umn.edu/).

Tansy (Tanacetum vulgare)

Common Tansy is a perennial and a native species of Europe. This plant is found along roadsides, parks, pastures, waste areas. It has an unpleasant odor and cattle tend to avoid it. This plant can grow up to six feet tall. Its yellow, button-like flower clusters are very conspicuous. This plant is more confined to eastern Minnesota.

Two Brassicaceae (Mustard Family) plant species that are very similar in just about all their plant characteristics are Tansy Mustard and Flixweed. Both species are found in Minnesota crop production land. Tansy Mustard is more likely to be found in western part of Minnesota along the South and North Dakota borders. Flixweed is more common throughout Minnesota. It is very difficult to separate these when they are in their vegetative stage. Two characteristics can be helpful in separating the two plants. Tansy mustard will have a blue-green look to it whereas Flixweed will be greener in color. The seed pods of Flixweed are longer, have a longer pod stalk, and contain more seeds than Tansy Mustard. Since both of these are winter annuals, their flowering period has already begun and will run through June. The table below compares the very similar plant characteristics of these two species.

Comparison of Tansey Mustard and Flixweed