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Pennyroyal

PENNYROYAL

Mentha pulegium

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Pennyroyal

DESCRIPTION

Pennyroyal is a perennial weed species with a stolon system (i.e a stem that creeps along the ground, taking root at its tip and reaching out again from there). It originates from Europe and North Africa. 

The plant is most easily recognised when flowering.  The flowers are mauve-purple, and appear in late summer (January to March) in distinctive and attractive (depending on you point of view) clusters.

Leaves are egg-shaped and about 20mm long, appearing as stalked opposite pairs.  The stem leaves are hairy and are densely knotted with glands.  The leaves when crushed between the fingers give off an intense spearmint smell, and this is a very reliable identification test for pennyroyal, especially at times when the distinctive flowers are not present.   Although stock are normally reluctant to eat pennyroyal, if it is present in sufficient numbers it will be eaten, and is believed to cause a mint flavour taint in milk from dairy cows consuming it.

Stems are either short and erect, or long and sprawling along the ground (the aforementioned stolon system).  Because of this horizontal sprawl, the pennyroyal can grow to form a dense mat of vegetation which chokes out pasture species.  The root system takes the form of creeping underground rhizomes.

Pennyroyal prefers damp pastures and turf areas, and is often found in lawns.  It will also do well in lakeside and streamside habitats.  It appears abundantly throughout New Zealand.

MANUAL REMOVAL, MOWING & CULTIVATION

There are no options for manual removal except on the smallest scale, and in any case the rhizome system would ensure regrowth.  Likewise there are no mowing or cultivation options for the control of pennyroyal in pasture.

HERBICIDES TO CONTROL PENNYROYAL

There is no clover-friendly option.  MCPB and other clover-safe herbicides have no effect against pennyroyal.  Best results from spraying are obtained during either spring or autumn, when the plant is actively growing, although not necessarily easy to spot in pastures.  During the highly-visible flowering stage in late summer the plant is not always growing as vigorously as appearances would suggest, and results can be disappointing.

Boom Spray Herbicides to Kill Pennyroyal
  • 2,4-D amine  at 5-6L/Ha will provide about 60-70% control of pennyroyal.  MCPA750  is not as effective and is not recommended. There is no spray option that will give 100% control in one application.   
  • Dicamba 500 SL - 120-140ml plus 595ml 2.4.-D 720 Amine per 100L water per hectare. Grass friendly but will kill clover.  
Spot Spraying Herbicides to Kill Pennyroyal
  • GrassMate  at 6m/L applied anytime, but preferably when growing actively.  No added penetrant is required.  Grass friendly, but will suppress clover.   
  • Dicamba 500 SL - 20ml plus 90ml 2.4.-D 720 Amine per 15L water. Grass friendly but will kill clover.