A perennial weed, horehound (Marrubium vulgare) has aggressive, invasive characteristics. Horehound invaded the United States from its native home in Southern Europe, Northern Africa and parts of Asia, when it was first introduced into the country for cultivation. The plant was widely grown and harvested for use in candies and cough drops but escaped into the surrounding areas and began to rapidly spread. The plant grows in U.S. Department of Agriculture Hardiness Zones 3 to 10 with ease. It tolerates a wide range of soils. Horehound often grows in wetlands and dry arid regions with ease.
Horehound stands between 1-1/2 to 3 feet in height. The foliage is grayish-green in color and lightly furred to the touch with fine white hairs. Each leaf measures 1-1/2 inches in length and arises in pairs up and down the plant's stems. Leaves are either oval or round in appearance and crinkled. The plant is a member of the mint family and its leaves are highly aromatic if crushed, according to the Texas Invasives website.
Horehound stems are square in appearance. At the base near the soil level, the stems are woody in texture. The entire length of the stem is covered in fine, woolly, white hairs. The plant produces numerous stems that give the horehound a shrubby look.
White flowers are produced along the stems of the plant at the axil of the leaves. Flower production begins in May and is normally maintained into July. The flowers form dense clusters toward the upper portion of the stems and leaves of the plant. Bees pollinate the flowers.
Burrs and Seeds
After the flowers appear, they begin drying and forming burrs. Each dried burr contains four seeds. The burr has a protective appearance around the seeds with tiny hook-like structures that protrude outward. The seeds are small, spear shaped and spread by the wind or animals. The small burrs that hold the seeds easily stick to fur, clothing and shoes. One plant can produce thousands of seeds, according to the National Park Service.
Once the seeds spread, they await rainfall before germination begins. If there is adequate rainfall in the autumn, the seeds immediately begin germination upon dispersal. If the rains are delayed, the seeds may begin to germinate in the winter or spring months with tiny seedlings appearing. When germination occurs in the late spring or summer months, the seedlings rarely live due to lack of water and the hot summer conditions. Once established, the plant requires very little water to thrive.
The horehound plant was widely cultivated for use in cough expectorants. It was used by ancient Egyptians for its believed medicinal properties. In 1989, the U.S. Food and Drug Administration banned the use of the plant in cough medicine in the United States because it could find no evidence that it actually worked, according to Medline Plus. Horehound is still widely used in Europe in cough medications.