Horehound (Marrubium vulgare L.) is a common weed throughout southern Australia and most of North America. The woody perennial plant is native to Europe, the Middle East, North Africa and parts of the Mediterranean. Horehound is aggressive and will easily invade disturbed and undisturbed areas. It thrives especially in pastureland and crop fields.
Appearance and Growth
Horehound grows approximately 1 1/2 feet in height and is silvery gray. Flowers appear in the spring in small white clusters. After flowering, the plant produces tiny seeds encased in a burr that is exceptionally sticky and readily attaches to clothing, fur and shoes for widespread dispersal. One plant can easily produce thousands of seeds. Even seeds consumed by livestock can pass through the digest tract and emerge viable.
Horehound produces a very bitter alkaloid substance that is not very palatable or nutritious to livestock. Despite its unpleasant flavor, horehound is accidentally consumed by many grazing animals. The content of the weed taints the meat of livestock that eat it. Animals must consume clean pasture for more than seven days prior to being butchered to remove the flavor of the weed from the meat. Horehound seeds also attach to the wool of sheep and can cause severe matting.
Horehound weeds are a member of the mint family. They produce a very strong minty smell when crushed or trampled. The plant was once widely cultivated for use in cough lozenges and even candy production.
Manual and Chemical Control
Hand pulling prior to seed production has been used as a horehound weed control method. However, because it is so invasive, there are often too many plants to make this a viable option. Glyphosate herbicides have also been shown to kill horehound with ease but can also damage other plants. Burning has also been used as a control method in areas where a controlled burn can be utilized safely.
In 1994 the horehound plume moth, Pterophorus spilodactylus, began to be used in Australia as a biological control. The larvae of the moth can easily defoliate the horehound and render it incapable of surviving, and therefore reproducing. The clearwing moth, Chamaesphecia mysiniformis, has also been used around Victoria with great success since 1997. Currently, biological control is only being utilized on large, wide-scale infestations.