Pennyroyal

Mentha pulegium L.

Synonyms - Pulegium vulgare.

Family: Lamiaceae.

Names:

Mentha is the Latin name for mint and comes from the name of the Greek nymph, Minthe, who was changed into the mint plant.
Pulegium is Latin for fleabane because it was thought to suppress fleas.
Pennyroyal is a derivation from the Old French pulyole ryale meaning thyme.

Other names:

European Pennyroyal
Pennyroyal Mint

Summary:

A prostrate, erect or ascending, summer growing, rhizomatous, perennial, hairy herb with a strong minty smell and dense, purple flower clusters from December to May.

Description:

Cotyledons:

Two. Tiny, 1 mm long.

Leaves:

Opposite and paired. No stipules. Grey green. Narrows abruptly into the petiole. Floral leaves normally smaller than others.
Petiole - Short, 1-6 mm long or none.
Blade - Grey green. Oval to almost round. 5-30 mm long x 4-20 mm wide. Spreading or bent downwards. Sparsely to densely hairy with rough and multicellular hairs, on both surfaces usually with more hairs on the lower surface. Edges often folded in and with shallow blunt teeth. Strong minty smell. Oil glands on both surfaces, especially near the edges, with more on the lower surface. Veins arranged in a feather like pattern.
Leaves near the flower head are smaller.

Stems:

100-700 mm long, green to red and may appear grey due to hairs, branched, lie flat on the ground or bend upwards at the ends. Hairy with multicellular hairs. Young stems square. Often forms roots at the nodes. Lower parts woody and rough.
Flowering stems usually erect.

Flower head:

In leaf axils. Densely clustered in a many flowered cylindrical spike 10-150 mm long, 10-18 mm across with floral leaves underneath. Flowers appear to be in rings or whorls of many flowers with a section of bare stem between the clusters.

Flowers:

Purple-pink, many. On short, hairy stalks 0.5-2.5 mm long. Bisexual or female. Strong lavender fragrance.
Ovary - Superior.
Calyx - Tubular, cylindrical, 5 lobed, 10-15 veined, 2-4 mm long, 10 ribbed, marked with clear dots (oil glands). Shortly hairy on the outside with a ring of white hairs closing the throat. Lobes are hairy on the outside and hairless on the inside. Lobes, different lengths, about 1 mm long. Lower two lobes narrow and tapering to a pointed tip. Upper 3 lobes triangular.
Corolla - Lilac pink to purple, 4-7 mm long, tubular, 4 hairy oblong lobes with round tips. Hairy on the outside. Has a large inflation on lower side below the throat.
Stamens - Exserted. Filaments 2.5-3.5 mm long, hairless, attached to corolla tube.
Anthers - Yellow. Less than 0.5 mm long. 2 celled.

Fruit:

Capsule that divides into 4, one seeded, hairless nutlets.

Seeds:

Nutlets, pale brown with many dark sports, smooth, angular, egg shaped to almost spherical, less than 1 mm diameter.

Roots:

Creeping stolons or prostrate stems root at the nodes. Short, woody rootstock with a mass of fibrous roots to 400 mm deep.

Key Characters:

Calyx throat closed by a ring of hairs. Calyx lobes hairless on the inside. False whorls in the axils of floral leaves. Flower clusters dense. Hairy stems. Leaves hairy on both sides.

Biology:

Life cycle:

Perennial. Flowers from December to May. Seeds germinate at any time of year with a flush in autumn and spring and grow very slowly in cool weather. Only those germinating around May have a reasonable chance of developing into perennial plants. Ground hugging stems (stolons), that usually take root at the nodes, develop and continue growth through summer if moisture is available. They tend to flower in their second year. Mature plants form flowering stems in September to October and the interconnecting stolons die, separating the daughter plants from the parent. Flowering may start in November and continue to May often with a peak in flowering following rainfall events. In March most of the flower stems die and new stolons are formed. It is semi dormant in autumn and winter.

Physiology:

Drought tolerant once it is established.

Reproduction:

By seeds, stolons and stem fragments.

Flowering times:

Summer to autumn in western NSW.
December to March in SA.
December to February and occasionally in May in Perth.
Late spring and summer in WA.

Seed Biology and Germination:

Under field conditions light is required for seed germination.
Optimum temperatures for germination are alternating in the range from 10-300C.
Seeds can germinate under water and survive prolonged periods of inundation with water up to 150 mm deep.
Will remain viable and dormant in the soil for many years.

Vegetative Propagules:

Stem fragments.

Hybrids:

Allelopathy:

Population Dynamics and Dispersal:

Spread mainly by seed. The hairy calyx remains attached to the seed and provides buoyancy for dispersal by water flows and attaches to wool, fur and cloth for transport by animals. The seeds and fruit also contaminate hay and produce. The fine seeds are also easily transported in mud cling to machinery, animals and birds. It is claimed to pass through animal guts but feeding tests on small animals failed to confirm this.
Plants are occasionally and intentionally spread as an ornamental garden plant.
At the local level, patches increase in size and density by stolon growth and production of daughter plants as well as seedling recruitment. Stem fragments and seed moved by cultivation also assist spread.
Water flows are a very important mechanism for spreading Pennyroyal.
In pastures it often forms dense patches where little else grows.
Plants from drier areas set more seed and are hairier and smaller.
It set large numbers of seed that remain dormant until they are exposed to light. Trampling that buries seed induces dormancy in pasture situations.

Origin and History:

Europe, Western Asia, Mediterranean.
Introduced as an ornamental and for the oil industry.
First recorded in a garden in Adelaide in 1841.

Distribution:

ACT, NSW, SA, TAS, VIC, WA.

Courtesy Australia's Virtual Herbarium.

Habitats:

Usually found in patches or colonies.

Climate:

Warm temperate regions. Mediterranean.
Tends to be a greater problem in areas with an annual rainfall of more than 1000 mm.

Soil:

Most abundant on wet sandy soils, summer moist areas and watercourses and other poorly drained areas.

Plant Associations:

Significance:

Beneficial:

It is used in a number of herbal remedies but its toxicity makes it of dubious value.
It is also used as an ornamental, insect repellent, honey plant and lawn.
It is used as a culinary herb and a tea, but this use should be discouraged because of possible adverse effects on health.
Overseas it has been used to bind sandy soils.
Sailors used to use Pennyroyal to purify water.
Extracted oil is used for perfumes.

Detrimental:

Weed of pastures, wet areas, streams, swamps and roadsides.
Its strong odour makes it unattractive to stock.
It taints milk and meat, but the taint soon disappears so it is not of economic concern.
Unpalatable.
Its main effect is to replace more desirable pasture species.

Toxicity:

Ingestion has been associated with poorly defined respiratory problems (coughing) in cattle.
The oil is toxic and a number of human deaths have been attributed to it.
It causes abortion when ingested in near fatal doses.
The oil damages lung and liver tissues when ingested.
Rubbing oil on dogs as a flea treatment has resulted in the death of dogs.

Legislation:

Noxious weed of WA.

Management and Control:

Stolon (winter stem) fragments will re-establish after cultivation. It is often most prolific in flooded areas because it can germinate and survive under shallow water where most other plants die. Drainage is the most important step in a control program for these areas. Sowing adapted vigorous pasture species and applying fertiliser reduces reinfestation. It produces large amounts of seed that won't germinate when buried and lasts in the soil for many years. Cultivation in spring and repeated over summer provides some control. Slashing, mowing and occasional cultivation are usually ineffective. Single plants can be removed mechanically if all rooting material is collected. Sulfonyl urea herbicides are the most effective followed by triclopyr, glyphosate and the hormone herbicides. Useful control in legume pastures can be achieved with an annual 'spray graze' using 2,4-D amine.

Thresholds:

Eradication strategies:

Improve drainage, spray with 40 g/ha metsulfuron before flowering, establish a vigorous perennial pasture such as kikuyu then spray graze annually in early winter with 750 mL/ha 2,4-D amine.
In industrial areas spray each year with 1 L of Grazon plus 250 mL Pulse® in 100 L water when the plants are actively growing in summer before flowering.
In bushland areas spray each year with 10 g chlorsulfuron plus 250 mL Pulse® per 100 L water when the plants are actively growing in summer before flowering.

Herbicide resistance:

None reported.

Biological Control:

Related plants:

Apple Mint (Mentha X rotundifolia)
Eau-de-Cologne Mint (Mentha X piperita var. citrata)
Native Pennyroyal (Mentha satureioides)
Peppermint (Mentha X piperita)
Pennyroyal (Mentha pulegium)
River Mint (Mentha australis)
Slender Mint (Mentha diemenica)
Spearmint (Mentha spicata)
Mentha aquatica
Mentha suaveolens

Plants of similar appearance:

References:

Auld, B.A. and Medd R.W. (1992). Weeds. An illustrated botanical guide to the weeds of Australia. (Inkata Press, Melbourne). P180. Photo.

Black, J.M. (1965). Flora of South Australia. (Government Printer, Adelaide, South Australia). P731.

Burbidge, N.T. and Gray, M. (1970). Flora of the Australian Capital Territory. (Australian National University Press, Canberra). P312.

Cunningham, G.M., Mulham, W.E., Milthorpe, P.L. and Leigh, J.H. (1992). Plants of Western New South Wales. (Inkata Press, Melbourne). P574. Photo.

Hussey, B.M.J., Keighery, G.J., Cousens, R.D., Dodd, J. and Lloyd, S.G. (1997). Western Weeds. A guide to the weeds of Western Australia. (Plant Protection Society of Western Australia, Perth, Western Australia). P170. Photo.

Lamp, C. and Collet, F. (1990). A Field Guide to Weeds in Australia. (Inkata Press, Melbourne).

Lazarides, M. and Hince, B. (1993). CSIRO handbook of economic plants of Australia. (CSIRO, Melbourne). #815.6.

Marchant, N.G., Wheeler, J.R., Rye, B.L., Bennett, E.M., Lander, N.S. and Macfarlane, T.D. (1987). Flora of the Perth Region. (Western Australian Herbarium, Department of Agriculture, Western Australia). P563.

Parsons, W.T. and Cuthbertson, E.G. (1992). Noxious weeds of Australia. (Inkata Press, Melbourne). P500-502. Photos.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.