Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Herb Invasion in Hawke’s Bay

Lesser calamint growing wild and unwanted in Hawke’s Bay. Image - Darin Underhill

Lesser calamint growing wild and unwanted in Hawke’s Bay. Image - Darin Underhill

Lesser calamint (Calamintha nepeta) is now a target on Landcare Research’s weed radar

Overseas, this aromatic plant is grown for its medicinal properties and as a culinary herb. A member of the mint family (Lamiaceae) and native to Europe, North Africa, Western Asia and the Caucasus, lesser calamint has become naturalised in the United States and New Zealand. It was first recorded here in the early 1900s and is known to be present in Gisborne, Whanganui, Levin and Nelson, but is primarily a problem so far only in Hawke’s Bay, where there are serious infestations on over 100 farms.

Lesser calamint produces fine, upright stems that are covered with small, shiny, green oregano-like leaves, forming a compact mound 30 to 50 cm tall and twice as wide. In late summer, a cloud of minute pale lavender flowers are produced that bloom for up to 6 weeks and produce many seeds. As the days become cooler, the colour of the blossoms deepens. The plant can become dormant in the winter months, and then blossom again in spring. Lesser calamint smells like a cross between mint and oregano and attracts honeybees and butterflies.

Darin Underhill from the Hawke’s Bay Regional Council reports that lesser calamint is difficult to control with herbicides and is not palatable to stock so it can quickly outcompete pasture plants. “I have been observing this plant spreading for the past 7 or 8 years but because it is small, doesn’t have prickles and isn’t poisonous, it hasn’t come to the attention of many people,” Darin added. However, the plant is already having an economic impact on badly infested properties.

The Hawke’s Bay Regional Council recently asked Landcare Research to explore the feasibility of biocontrol for this plant. “Biocontrol is probably the only remaining option for this plant,” said Ronny Groenteman, who led the study. “Lesser calamint has the potential to become a much more serious weed in New Zealand because of its tolerance range,” explained Ronny. In its native range lesser calamint is valued for its ability to grow in dry, disturbed, low-fertility soils with low organic matter. It prefers well-drained, dry to moist, neutral to alkaline soil, and a warm sunny site, but can withstand temperatures down to -15°C, as well as drought, and full sun to part-shade.

The feasibility study predicts that, in terms of likelihood of biocontrol success, lesser calamint would be an intermediate target. The lack of close relatives to lesser calamint in the New Zealand flora increases the likelihood of finding agents that are sufficiently host specific. New Zealand has five indigenous plant species in the Lamiaceae family, one (which is endemic) is considered threatened, two (one endemic and one non-endemic) are declining, and the other two (both non-endemic) species are not threatened. A biocontrol programme would also have to make sure that widely used culinary herbs in the mint family are not harmed by potential agents; however, these are sufficiently distantly related to lesser calamint that it would be expected that agents could be found that would not harm them.

No detailed surveys of the natural enemies of lesser calamint have been undertaken, and would be needed if a biocontrol project was to proceed. Six species of arthropods and three species of pathogens have been recorded from lesser calamint. Information about the host specificity and damage caused by these natural enemies is lacking, but most of them appear to warrant further study.

The feasibility study recommended that the next step should be to conduct a cost-benefit analysis to see if investment in a biocontrol programme would be warranted for a weed of such limited current distribution, but which potentially has much more serious impacts. This would consider future weed spread scenarios and the economic impact on the small number of businesses growing the plant for its medicinal properties. No other countries have attempted biocontrol of this plant and so a project would have to start from scratch, which adds significantly to the development costs, likely to be around the $2M mark.

Ronny was pleased to be involved with such a proactive project. “For years we have advocated the benefits of controlling plants that are in the early stages of invasion (sleeper weeds) rather than leaving them until they become widespread.” Darin agrees and said that he wouldn’t be surprised if other regions with similar climate and terrain to Hawke’s Bay are also quietly being invaded by lesser calamint but just haven’t noticed it yet.

This project was funded by an Envirolink grant to the Hawke’s Bay Regional Council (1538-HBRC205).