Motherwort (Leonurus cardiaca) is another of the many introduced plants that has escaped domestic cultivation and has become a weed throughout the Midwest and most of upper North America. Motherwort originated in Eurasia and came to North America with our immigrants. Over time, motherwort has been known by several names including cowthwort, lion's ear, and lion's tail.
Motherwort can be found in a variety of habitats but thrives in partial shade with moist, rich soils. It is often found around older farmsteads and was planted as a medicinal plant. Young leaves were used as a steamed green. This plant may be prevalent in flood plains, riverbanks, roadsides, woodlands, fence lines, and the urban setting.
Motherwort is a perennial plant in the mint family and, like other plants related to mint, has a square stem. The plant can grow to approximately 3 to 5 feet in height. The leaves originate directly from the main stem and its limited branching makes this a very narrow, upright plant. As with many plants related to mint, crushing the leaves releases a distinct smell. The pink to light purple flowers develop from May through September and arise in the leaf axil, where the leaf is attached to the stem. Flowers are grouped in clusters of 6 to 15 flowers with prickly sepals located at the base of each flower. This gives the plant an irritating characteristic when skin contact is made.
Propagation results from both seeds and rhizomes. Each flower produces up to 4 small, triangular nutlets with hairs at one end. These small hairs help spread the seed. As animals or people brush against the plant, the hairs on the seed act as hooks and attach to the animal's coat or people's clothes and are moved to new locations. New plants can arise from the underground rhizomes, creating a dense stand if left unattended. The rhizomes spread out from the crown, which can support several stems.
Control motherwort with both cultural practices and herbicides. Regular mowing reduces the overall vigor of the plant and with time will reduce populations. Cultivation will help in control through disrupting the growth of the plant as well as the loss of soil moisture. Motherwort prefers moist, somewhat shady conditions and the loss of soil moisture reduces overall vigor. Changing the growing environment can also reduce populations. Reducing shade and increasing soil drainage can modify the growing environment to the point where the conditions are less favorable and reduced vigor results. In areas with smaller populations, hand pulling can effectively remove the population.
Herbicides can be used on larger stands. Motherwort is a very resilient plant when it is growing in its preferred environment. Both selective and non-selective herbicides can be used. Selective herbicides that contain the combination of 2,4-D and dicamba have shown good results. Non-selective herbicides such as glyphosate can be used, but additional caution must be taken to reduce drift to non-target vegetation. Control through the use of herbicides may require multiple applications. As always, read the herbicide label carefully before purchase and use.