Mayweed chamomile (Anthemis cotula) is a troublesome weed in small grain and pulse crops throughout the high rainfall zones of the Inland Pacific Northwest (PNW). It is an annual that can germinate in the fall or spring and that reproduces only by seed. Individual plants can produce as many as 17,000 seeds, and seed remain viable in the soil for many years. Preventing seed production is the key to managing mayweed chamomile! While herbicides are an effective tool for mayweed chamomile control, herbicide-resistant biotypes are an increasing concern. An integrated management approach is required for the sustainable, long-term control of this species.
Mayweed chamomile, also known as dog fennel, mayweed, stinkweed, or stinking chamomile, is a native of the Mediterranean region. It is commonly found in disturbed areas, roadsides, along ditches, and in fields of wheat, barley, and pulse crops throughout the high rainfall zones (greater than 16 inches of average annual precipitation) of the Inland PNW, where it is well adapted to the warm, dry summer weather. The bushy annual, sometimes winter annual, has attractive daisy-like flowers and a strong, unpleasant odor. Mayweed chamomile is a member of the sunflower family (Asteraceae). The species name, cotula, is from a Greek word for “small cup,” which describes the shape of the flower. Contact with mayweed chamomile can result in skin rashes in some people and irritation to the mucous membranes of livestock. It can also impart a strong flavor to the milk of dairy livestock. Mayweed chamomile reduces crop yields through competition for water, nutrients, and light. Yield losses in winter wheat are usually less than 5%, but in spring wheat, mayweed chamomile can reduce grain yields by up to 25%. In pulse crops, particularly lentils, mayweed chamomile infestations can result in total crop loss.
Although mayweed chamomile seed germinates throughout the year, most seeds germinate in the autumn and spring when soil moisture is plentiful. The cotyledons, also known as the seed leaves, are small, stemless, smooth-edged, and oval in shape (Figure 1). The cotyledons often dry up quickly and are frequently not present on plants with more than a few true
Mayweed chamomile has a short, thick taproot. Mature plants grow to be 4 to 24 inches tall, are branched, and contain single flower heads, about 3/4- to 1-inch in diameter, on the ends of branches and unbranched stalks in the leaf axils (the junction of the leaf and stem). The flower heads consist of white ray flowers circling a center of yellow disc flowers (Figure 2). Plants flower from May to October, with the main flowering period occurring June to July (Van Vleet 2006). Stems and remnants of flowers often persist into the winter months.
Similar Weeds Confused with Mayweed Chamomile
Pineapple-weed (Matricaria discoidea) and scentless chamomile (Tripleurospermum maritimum) are two species that can be easily confused with mayweed chamomile (Table 1). Pineapple-weed can be differentiated from mayweed chamomile by the lack of ray flowers in the flower head and by the lack of foul odor when the leaves are crushed.