T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network


Marrubium vulgare (Horehound)

Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Asterids
Order: Lamiales
Family: Lamiaceae
Genus: Marrubium
Species: M. vulgare
Binomial name: Marrubium vulgare
Common name: White horehound, Common horehound, Horehound

Marrubium vulgare is a semi-erect, perennial herb with stems to 50 cm tall. It is multi-stemmed and can have up to 200 individual stems. It is native to Europe, northern Africa, and south-western and central Asia. It is also widely naturalized in many countries including New Zealand. Its abundant in the dry areas of Canterbury and Otago in the South Island but only occasionally in the North Island. It inhabits dry pastures, farm yards, sheep yards, roadsides, railway lines and wasteland.

Marrubium vulgare somewhat resembling mint in appearance, and grows to >45 centimetres tall. The stalked, heart shaped leaves are in opposite pairs and are > 35 mm long and 45 mm wide. They have a densely crinkled surface which is covered in downy hairs. The leaves underside is very hairy to woolly. The leave’s margins are wavy and serrated with blunt teeth. 
The flowers are white and are borne in clusters on the upper part of the main stem. They appear from November to March. Mature plants can produce in excess of 20 000 seeds per year, although the more numerous and smaller plants produce about half this number. Anecdotal evidence indicates that the seeds can survive in the soil for 7-10 years.

Horehound leaves contain marrubin, a bitter alkaloid that makes it unpalatable to grazing animals. The meat of animals which are forced to eat horehound is tainted by the plant's strong flavour. Horehound burrs are a nuisance as they catch in clothing and socks and in wool, thus reducing fleece values.
It is a foul-smelling invasive weed that is become increasing problematic on dry land farms in recent years where it contaminates wool, taints meat and plays havoc with lucerne crops. It is recognised as one of the worst weeds in lucerne crops.
In 2016 because conventional control methods have proved useless, there is a bid to introduce two moths that are successful bio control agents against the weed in Australia. 

One moth species feeds on the weed's foliage and the other on the roots.
The Horehound plume moth (Pterophorus spilodactylus) is specific to horehound; the caterpillars (larvae) feed on the growing tips of the plants and then work their way down the shoot, progressively defoliating the stem. This weakens the plant and reduces the number of seeds and flowers produced.
The other moth is the Horehound clearwing moth (Chamaesphecia mysiniformis). Its larvae feed within the growing tissue of the root and lower stems. Larval activity affects the flow of water and nutrients through the plant, weakens it, reduces growth and increases the likelihood of the plant dying (especially when water stressed). Clearwing moths primarily attack the young horehound plants, killing them completely and thus reducing the ability of the weed to replace losses of older plants or invade new gaps. The clearwing moth should work well in combination with the plume moth which suppresses larger plants.