Scientific name: Marrubium vulgare
- Erect, much branched, downy, perennial about 50 cm tall
- Flowers are small with white petals and grow in dense clusters at regular intervals around the stem
- Dried flower parts adhere to wool and clothing. Sepals (tiny leaf-like structures surrounding the flowers) have hooked teeth and can be prickly to touch when dry
- Leaves are in opposite pairs, wrinkled and densely covered in soft downy hairs underneath
- Very common in drier areas, especially in Canterbury and Otago; less common north of the Volcanic Plateau
- Plants have a bitter taste and are largely avoided by animals. Especially a problem weed in lucerne but also in open, dry pastures.
- Native to Europe, parts of Asia, and North Africa, horehound is now found in many places, including North and South America, South Africa, Australia and New Zealand
- First recorded in New Zealand in 1867.
- Horehound seeds germinate in autumn to early spring, under cool, moist conditions
- Most seeds are normally dormant, with the dormancy broken by a few weeks of cool, moist conditions
- Plants may be long-lived although many die during the first few years after establishment
- One plant can produce up to 74,000 seeds a year
- Seeds are contained in a hooked calyx (ring of sepals just below the petals) which may remain attached to the stems of plants for several months, but easily break off when brushed by animals, livestock, or socks.
- Horehound has been long valued as a medicinal herb and was probably introduced to New Zealand and Australia for that purpose
- Extracts have been used to expel internal parasites from livestock (and humans)
- Extracts of the plant have long been recommended as herbal remedies, especially as an expectorant and cough medicine
- Flowers provide a source of nectar for bees.
- Horehound tends to invade heavily stocked areas and is only found in disturbed places, especially on calcareous soils
- Horehound seedlings do not compete well with other plant species
- In New Zealand it is very common in drier areas, especially in Canterbury and Otago
- It is also very common in many Australian States, especially South Australia.
Impacts on animal products
- Horehound can taint meat and dry seed heads contaminate wool
- Leaves have a bitter taste and are hairy, both of which discourage livestock from grazing them
- Dried horehound flowers can reduce the value of wool. In Australia in 1988 it was estimated to cost the wool industry A$680,000 a year.
Impacts on pasture
- Horehound is rarely a problem in well-managed pasture with adequate nutrients and appropriate grazing
- Grows on poor soils and where there is little competition in autumn.
- Care should be taken that sheep are not moved from infested paddocks to clean paddocks
- Small new infestations of horehound on a property or in a paddock should be destroyed when they first appear by pulling them up
- Sowing improved pastures with appropriate pasture species, adequate fertiliser and well-managed grazing can prevent horehound from establishing
- In arable land seedlings are easily controlled by cultivation, but established plants may need blade ploughing, discing or repeated cultivation
- Topping of horehound will remove the flower stems and improve stock access to pasture.
- Several herbicides can be used to kill individual plants or very small patches but most of these also kill more desirable plant species. Among such herbicides are glyphosate, chlorsulfuron, hexazinone, metsulfuron-methyl, triclopyr and fluroxypyr. Some of these can persist in the soil for many months and could damage subsequent crops or pasture species
- Winter treatment of lucerne with herbicide is probably the most effective treatment for control of horehound. Recent research in Central Otago supported earlier work showing that glyphosate and atrazine can be used successfully as a winter herbicide on lucerne when the crop is dormant (July or August) and lucerne cover below 100 kg DM/ha.
- Two biological control agents for horehound have been released in Australia. These are the horehound plume moth (Wheeleria spilodactylus) and horehound clearwing moth (Chamaesphecia mysiniformis)
- The horehound plume moth was first released in 1994 and is now established at over 100 localities throughout south-eastern Australia. The caterpillars feed on the plantâ€™s growing tips and then progressively defoliate the stem
- The horehound clearwing moth was first released in March 1997. Larvae feed on the growing tissue of the root and lower stems. The insect mostly affects young horehound plants. This moth is unlikely to spread as quickly as the horehound plume moth as it has only one generation a year
- The brightly coloured horehound bug (Agononscelis rutile) is often found in large numbers on horehound plants in Australia but does not seem to have any significant effect on the plant.
- Roux M, Leask SK, Moot DJ, 2014. Yield and composition of lucerne stands in Central Otago after different winter grazing and weed control treatments. Proceedings of the New Zealand Grassland Association, 76: 89-96. (accessed 7 December 2016).
- Popay, AI (2013). Marrubium vulgare (horehound). CABI Invasive Species Compendium. (accessed 7 December 2016).
- Popay I, Champion P, James T 2010. An illustrated guide to common weeds of New Zealand. New Zealand Plant Protection Society, Christchurch, New Zealand. 416 p.
- Sloane, Cook & King Pty Ltd. 1988. The economic impact of pasture weeds, pests and diseases on the Australian wool industry (Aust. Wool corp. Melb.) Cited in: Carter, R. J. (1990). Biology and control of horehound, Marrubium vulgare. Proceedings of the 9th Australian Weeds Conference. Adelaide, South Australia. August 6-10 1990: 382-386. http://caws.org.au/awc/1990/awc199013821.pdf