Family: Lamiaceae (Mint Family)
Duration and Habit: Perennial Herb
Horehound has square stems (often woody near the base) densely covered with white hairs with leaves opposite each other. Leaves are hairy above, very hairy to woolly underneath, rounded with a crinkled surface and sharply aromatic when crushed. It has small white flowers in dense clusters above the nodes (where the leaves join the stem) around the upper sections of the stems. Clusters of flowers dry to form brown burrs with small hooked spines. Each burr contains up to 4 small (1-2 mm long) spear-shaped seeds.
Ecological Threat: The spread of horehound in pasture land poses a problem for successful management.
Biology & Spread: Horehound spreads by seed. It is an opportunistic germinator with most seeds germinating in response to autumn rainfall, but germination also occurs throughout winter and spring whenever sufficient water is available. In low rainfall areas, however, there are rarely follow-up rains that allow for recruitment of seedlings. Most seedlings that germinate in spring and summer do not survive the first summer. Horehound, as with most members of the Lamiaceae family, is primarily bee pollinated but there have been no studies, however, to indicate what the seeding potential of horehound would be without bees.
History: Introduced as a plant for gardens.
U.S. Habitat: Open fields, lawns, disturbed areas
U.S. Nativity: Introduced to U.S.
Native Origin: Asia, south. Europe, north. Africa, Canary Is., Azores (Bailey, L.H. and E.Z. Bailey, Hortus Third: A Concise Dictionary of Plants Cultivated in the United States and Canada, MacMillan Publishing Co., Inc., New York , (1977).); NatureServe Explorer
U.S. Present: AK, AL, AR, AZ, CA, CO, CT, GA, HI, IA, ID, IL, IN, KS, KY, MA, MD, ME, MI, MN, MO, MS, MT, NC, NE, NJ, NM, NV, NY, OH, OK, OR, PA, RI, SC, SD, TN, TX, UT, VA, VT, WA, WI, WV, WY
Distribution: Throughout the midwest and western U.S. Found through out west and central Texas, and found in various counties in the panhandle. Also found in Cameron county.
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Prevention: It is important to keep uninfested areas clear of horehound. Identify and treat existing or potential sources of this plant before it invades. Once an infestation is established, prevention of spread into surrounding areas should be a priority. The area may be quarantined to stop movement of seeds and burrs on vehicles and equipment (both management and recreational). Ensure stock are quarantined or clean of burrs prior to entry onto land.
Hand-pulling/grubbing: Manual removal is labor-intensive and needs to be repeated as new plants establish from seedlings. Very small patches are suitable for eradication by this technique, or it could be used as a containment measure to prevent spread from a larger infestation. Care is needed to ensure that hand pulling does not spread seeds to uninfested areas.
Slashing: If repeated at least annually slashing may restrict seed production, limit spread of established plants. Slashing is unlikely to achieve rapid reduction of horehound infestations unless combined with other techniques and seed may be spread to uninfested areas on machinery
Cultivation: Where feasible, deep cultivation will destroy existingplants especially if repeated in summer so that plants dry off. Reseeding pasture species will reduce horehound seedling establishment but controlled grazing and/or herbicide application will probably also be needed. Cultivation is not compatible with biological control agents unless carried out in a number of stages or adjacent to an uncultivated infestation so that some agents can survive in uncultivated areas and recolonize later.
Herbicide: The following active constituents or combination of active constituents are effective in different situations. Ensure that the herbicides are registered for your particular State/Territory: 2,4-D dimethylamine salt, 2,4-D isopropylamine salt, 2,4-D ethyl ester, triclopyr butoxyethyl ester, bromacil, bromacil + trichloracetic acid, bromacil + diuron, dicamba dimethylamine salt, dicamba dimethylamine salt + MCPA dimethylamine salt, diflufenican and MCPA 2-ethyl hexyl ester and metribuzin.
Spot-spraying: Best effects are achieved in autumn when horehound is growing strongly. Small areas along creeks, tracks, fencelines or near rabbit warrens can be treated easily; it is more difficult to deal with widely scattered plants. Follow-up is needed to control seedlings and this technique is not ompatible with biological control agents. Not all of the herbicides listed above are registered for spot spraying.
Boom-spraying, aerial spray or large-scale handgun application: Vehicle access is required for ground application. Spraying is likely to be considered too damaging to indigenous vegetation unless it is already very degraded.
Burning: Burning is an effective means of killing larger plants but the large numbers of seedlings produced require follow-up treatment. The horehound seed bank is greatly reduced after fire due to large numbers of seeds being killed and the large numbers germinating immediately afterward. Nevertheless, it seems unlikely that the seed bank could be decreased to the point where horehound seedlings would not rapidly reappear in suitable gaps, so fire should always be combined with other techniques. Regeneration of indigenous species may be aided by fire if the circumstances are right.
Weiss J., N. Ainsworth, and I. Faithfull. 2000. best practice management guide: Horehound, Marrubium vulgare. Accessed 5 December 2008: http://www.weedscrc.org.au/documents/horehound.pdf.
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