A NORTHERN ARIZONA HOMEOWNER'S GUIDE TO IDENTIFYING AND MANAGING INVASIVE PLANTS

HOREHOUND

Common name(s): Horehound, white horehound

Scientific name: Marrubium vulgare

Family: Mint family (Lamiaceae)

Reasons for concern: This plant forms large, dense monocultures, which have a very negative impact on native vegetation, wildlife, and pollinators. Its dense root system makes it very difficult to eradicate.

Classification: Non-native

Botanical description: Can be upright like small shrub, or creeping. Entire plant has pungent/aromatic odor.

Leaves: Oval to circular. Opposite on stem. Grayish-green. Toothed (jagged) edges, located below flower whorl. Lower surface covered with white woolly hairs. Upper surface wrinkled or puckered. About 1 ½ inches long.

Stem(s): Up to 3 feet tall. Usually erect, 4-sided, with a somewhat woolly base.

Flowers: Tiny to ¼ inch long, white, growing in dense round clusters where leaf meets stem. Tubular. Blooms April through September.

Seeds: Each flower produces 4 dark brown nutlets, each containing one seed.

Roots: Taproot, branched, woody, with numerous fibrous lateral roots.

Native to: Europe

Where it grows: Desert, uplands, mountain, riparian. Along roadsides. Disturbed places. Elevation 2,000 to 8,000 feet.

Life cycle: Perennial

Reproduction: From seeds and spreading roots

Weedy characteristics: Can form dense monoculture stands over large areas, reducing native plant diversity. Seeds can remain viable up to 5 years.

Control strategies: Do not let them go to seed. Hand pull or dig out seedlings. Dig out more mature plants with the roots, but be careful not to disturb the soil too much. Step down any loose dirt to prevent seeds from finding a welcome environment to germinate. The roots make it very hard to eradicate. Repeatedly monitor previous infestations for new growth. You may need to consult a professional. Plant desirable native species to outcompete invasives.

Images: Click on an image to enlarge and see the image citation.

References:

  • Marrubium vulgare Learning Center of the American Southwest, Fact Sheets for Invasive Exotic Plants
  • White horehound A WEED REPORT from the book Weed Control in Natural Areas in the Western United States
  • Marrubium vulgare Texas invasive plant and pest council