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

Mentha pulegium (Pennyroyal)

Kingdom: Plantae
(Unranked): Angiosperms
(Unranked) : Eudicots
(Unranked): Asterids
Order: Lamiales
Family: Lamiaceae
Genus: Mentha
Species: M. pulegium
Binomial name: Mentha pulegium
Common name: Pennyroyal

Pennyroyal is a plant in the mint genus, within the family Lamiaceae and is mainly a weed of moist pastures and turf. It generally only dominates pastures in areas with reliable rainfall for much of the year or where the soil is poorly rained. A noticeable thing about pennyroyal is the strong smell of mint given off from the foliage when it is crushed or cut.
Pennyroyal is a perennial weed with a stolon system. Thus it has creeping stems with roots along them to hold it tightly to the ground, apart from the ends of flowering stems which are more upright. It forms dense mats of vegetation within pastures and lawns. In pastures, livestock is not keen to eat it, so it can begin to dominate the pasture if soil conditions are moist enough to allow it to do so. If lactating cows do eat this weed, milk tainting can occur.
The flowers are also quite distinctive, with purplish-blue-pink flowers clustered around the base of leaves. 

Pennyroyal is a traditional culinary herb, folk remedy, and bortifacient. The essential oil of pennyroyal is used in aromatherapy and is also high in pulegone, a highly toxic volatile organic compound affecting liver and uterine function.

Mentha pulegium growing on the side of a road. 

The leaves.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/