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Horehound
Mint Family
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.
Identification
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.
Look-Alikes
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.
Natural History
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Horehound flower clusters are pollinated by a honeybee.
Photo by Brother Alfred Brousseau,
St. Mary's College.
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An individual horehound plant can produce thousands of seeds. These
seeds, in their tiny burrs, can be dispersed to great distances by
animals. Horehound colonies can become very dense in disturbed areas,
and are resistant to trampling. This aromatic member of the mint family
was once cultivated in great quantities for use in making candies
and cough drops.
Management
Horehound has been treated in previous years by park maintenance
personnel, who estimated having removed 60,000 plants from the foothill
campgrounds of Sequoia National Park. The combined efforts of park
maintenance personnel and weed crews should lead to ultimate eradication
of this species from the parks. Plants will be removed by hand pulling
or severing of roots well below ground level. An approved glyphosate
herbicide may be used in a sparing and targeted manner where necessary.
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