Pennyroyal (Mentha pulegium)
Pennyroyal is a creeping, mat-forming perennial weed that thrives in damp or poorly drained soils. It’s commonly found in lawns, golf courses, and pastures across New Zealand, particularly in regions with consistent rainfall or heavy soils that stay wet through winter.
While it can tolerate brief dry periods, pennyroyal flourishes in moisture-retentive ground and often becomes most visible during summer when it bursts into clusters of purple-blue flowers. The crushed foliage releases a strong mint aroma — a signature trait of the Mentha genus, which includes common mint.
Although some people deliberately grow pennyroyal for its fragrance, it’s considered a nuisance in lawns, where it forms dense mats that crowd out desirable grass. Livestock tend to avoid it, and in dairy systems, ingestion can lead to milk tainting.
Identification
Pennyroyal spreads via creeping stolons, rooting at nodes along the stems to create thick mats close to the ground. The leaves are small, oval, and slightly hairy, arranged in opposite pairs along the stems.
The plant’s most distinctive feature is its strong mint smell when crushed. During summer, clusters of purplish-mauve flowers appear in the leaf axils (where leaves join the stem).
When not in flower, pennyroyal can be confused with turf speedwell, which has similar growth habits but lacks the mint scent. It’s also sometimes mistaken for selfheal, but selfheal produces blue-purple flower spikes at the ends of stems rather than around the leaves.
Why It’s a Problem
Pennyroyal forms dense, low mats that smother turfgrass and reduce lawn quality. It thrives in wet, compacted soils where grass struggles to grow, spreading rapidly in poorly drained or shaded areas.
In pastures, livestock avoid grazing it, allowing it to dominate over time. In dairy herds, consumption can taint milk and lower product quality. Because pennyroyal is resilient and perennial, it can persist year after year if left untreated.
Management and Prevention
Cultural control methods:
- Improve drainage: Pennyroyal thrives in waterlogged soil. Regular aeration, topdressing with sand, and removing surface depressions can help reduce favourable conditions.
- Encourage dense turf: Maintain healthy lawn growth with proper fertilisation and mowing to prevent the weed from spreading.
- Mow regularly: Frequent mowing helps limit flowering and seed spread, though the plant spreads mainly by stolons.
- Hand removal: Small infestations can be lifted and removed by hand, ensuring all stolon fragments are collected to prevent regrowth.
Chemical Control:
In lawns and fine turf, selective herbicide control can be achieved using combinations of broadleaf herbicides.
Effective options include:
- 2,4-D or MCPA: Apply during spring or autumn when the weed is actively growing.
- Dicamba combinations: Mixing dicamba with 2,4-D or MCPA significantly improves control in turf settings.
- Triclopyr/picloram mixtures (NZLA Gold): Suitable for tougher infestations in established turf.
- In pastures, 2,4-D is typically more effective than MCPA but should be used carefully to avoid clover damage. Avoid spraying in midsummer when plants are flowering but not actively growing.