Pennyroyal

Scientific name: Mentha pulegium

Pennyroyal in pasture

Pennyroyal flower

Pennyroyal seedling

Pennyroyal pasture infestation
  • Key characteristics
  • Biology
  • Impacts
  • Control
  • Further information

  • Summer-growing perennial herb with distinctive blue flowers, and pungent minty-smelling leaves when crushed
  • Small mauve flowers are produced in dense clusters in the leaf axils along each flowering stem, between November and May
  • Stems are often square, covered with hairs and can vary in length (usually < 50 cm high). Some stems creep along the ground while others are more erect
  • Leaves are stalked, oblong to rounded, up to 3 cm long and 1 cm across, and are in opposite pairs. Leaf margins may have small, blunt teeth
  • Stems that creep along the ground (sometimes called stolons) can root at the nodes to produce new plants.

Origin and habitat

  • The English name ‘pennyroyal’ was derived from an early Latin name for the plant, which could be translated as ‘a sovereign killer of fleas’!
  • Pennyroyal is in the mint family. (‘Mentha’ is Latin for ‘mint’ and ‘pulegium’ is Latin for ‘fleabane’)
  • Native to Europe, the Mediterranean and western Asia
  • Can heavily infest summer-moist pastures where annual rainfall exceeds 1000 mm but can also survive in drier areas
  • Forms dense mats in damp pastures, where it crowds out other more desirable species.

Life-cycle

  • Flowers are produced on hairy stems up to 50 cm tall, from November until May
  • Seed production can be high in pastures, with densities of 55,000 – 176,000 seeds/m2 having been recorded (see references listed under ‘further information’)
  • Seeds require light for germination and will not germinate when buried. Given sufficient moisture, they can germinate at any time of year
  • Seeds can germinate and seedlings survive even when flooded to a depth of 10 cm for long periods.
  • Seeds that germinate in late spring are unlikely to produce flowers in the first summer. Plants remain vegetative and produce flowers in their second spring/summer
  • Seeds that germinate in autumn flower in their first summer
  • As the seedling establishes, stolons or low-growing stems grow in all directions, producing roots at the nodes
  • The parent plant becomes dormant during winter, and growth begins from roots in the spring
  • Seeds buried by stock trampling may remain dormant for 12 months or more
  • The contribution of buried seed from previous years may be more important than current seed production in determining seedbank size
  • When flowering stems are produced, existing and interconnecting stolons die. The daughter plants become separated from the parent plant and from each other
  • New stolons are produced after flowering has finished
  • Seeds are spread by running water, machinery, livestock, contaminated hay and pasture seed.

Impact on pasture

  • Avoided by sheep and cattle. This reduces available feed
  • Replaces more desirable species.

Impact on livestock

  • Pennyroyal can taint the milk of dairy cattle and also sheep meat
  • Plants produce a monoterpene called pulegone which is toxic and can cause lung and liver damage.

Benefits

  • Pennyroyal was used as an additive to food and for medicinal purposes in early Greek and Roman civilisations. However, it is no longer used as a culinary herb
  • Still used for making herbal teas, for which some benefits are claimed
  • Herbalists use fresh or dried leaves as remedies for several medicinal problems, and also as an insect repellent
  • NOTE that pennyroyal oil is extremely poisonous, even in small amounts

  • Single plants or small infestations can be grubbed out and the site checked for any subsequent growth
  • The large seed bank and presence of dormant seed make pennyroyal eradication unlikely and control by herbicides difficult. Full cultivation during flowering can help to control it
  • Draining areas prone to water-logging, to increase the vigour and competitive ability of desirable pasture species, will help suppress pennyroyal
  • Adding lime to improve soil structure and drainage can also help promote the growth of desirable species and suppress pennyroyal
  • It is difficult to find pasture grasses or legumes which will compete well with pennyroyal when it is actively growing over summer. Kikuyu will suppress it
  • Chemical control is costly and often only short-term because of regrowth from the roots and reinvasion from the seed bank. The following herbicides can be used in pastures:

ALWAYS READ PRODUCT LABELS BEFORE APPLYING

Active ingredient When to apply Residual effect Grass damage Clover damage
MCPA Post-emergence at any stage Moderate No Slight
2,4-D Post-emergence at any stage Moderate No Slight
dicamba Post-emergence at any stage – spot or wiper Severe No Severe

Consult your farm consultant, industry rep or the New Zealand Agrichemical Manual for more information about chemical control.

  • Dean KR 1990. Pennyroyal – a weed of summer-moist pastures in Western Australia. Proceedings of the Ninth Australian Weeds Conference, August 6 -10, Adelaide, South Australia, Heap JW ed. Crop Science Society of South Australia Inc. Pp. 447-450.
  • Panetta FD 1985. Population studies on Pennyroyal Mint (Mentha pulegium L.) I. Germination and seedling establishment. Weed Research 25: 301-309.
  • Panetta FD 1985. Population studies on Pennyroyal Mint (Mentha pulegium L.) II. Seed banks. Weed Research 25: 311-315.
  • Parsons WT, Cuthbertson EG 2001. Noxious weeds of Australia. CSIRO Publishing, Collingwood, Melbourne, Australia. 698 p.
  • 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.
  • Young S 2013. New Zealand Novachem agrichemical manual. Agrimedia Ltd., Christchurch, New Zealand. 767 p.