Horehound (Marrubium vulgare) is a perennial weed from Europe. It is common in disturbed, low-elevation areas throughout California. There are several populations in the foothills of Sequoia National Park, with the greatest concentrations being in and near campgrounds. This species is a source of concern for park resource managers because it aggressively invades disturbed areas and it has the potential to invade neighboring undisturbed areas.
Horehound is a velvety gray-green perennial that rarely exceeds 5 decimeters (1.5 feet) in height. The leaves arise in pairs along the stems. They are round to oval, white-velvety, minutely toothed around the margins, and about 4 centimeters (1.6 inches) long. The flowers are small and pale and are born in regularly spaced clusters along the flowering stems. Seeds are tiny and are enclosed in small burrs that shamelessly colonize human socks in vast numbers.
Hedge nettle (Stachys albens) is a native mint that might be confused with horehound. Hedge nettle is much larger than horehound; mature hedge nettles are generally 5 to 25 decimeters (1.5-8 feet) tall. They are restricted to wet areas, they are very white-woolly, and their leaves are generally much larger than those of horehound.