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Mānuka: the sweetest weedkiller?

The honey from mānuka kills bacteria, but does this plant also kill weeds? Schoolkids around Aotearoa are finding out by looking at the chemistry of local mānuka.

Hearing the word mānuka usually makes us think only of the famous honey, but scientists have discovered another chemical in this plant – not found in the honey – that could give us a new natural weedkiller.

Primary and intermediate school students are exploring whether their local mānuka plants contain enough of this chemical – called grandiflorone – to kill weeds, and whether the grandiflorone levels differ in mānuka growing in different parts of New Zealand.

Student at Musselburgh School measuring the seedlings

Tori, 10, from Musselburgh School in Dunedin says, “This is important ‘cause we're building an online database where we're adding information about the different types of mānuka, so that we can find out which makes the best weed killer.”

Scientists Elaine Burgess from Plant and Food Research and Dave Warren from the University of Otago are leading the project.

They say that the database is hosted online at NatureWatchNZ so that schools can compare local mānuka with others from around the country, and look for overall patterns that emerge as more information is added.

“This is really exciting for us too, as we’ve not done this before either!” Dave says.

Dave Warren with students smelling the extract samples

At the heart of the research is an experiment that tests whether extracts from different mānuka plants stop lettuce seeds (the 'weed') from growing.

At Musselburgh School, Dave, Elaine and the students started the experiment by grinding up leaves from local mānuka and adding acetone (a chemical found in nail polish remover) to draw out the mānuka chemicals into a liquid extract.

“I really liked mixing together the mānuka leaves with the acetone and the sand because it was interesting seeing how to get the extract out,” says Hannah, 11.

Next, the students took some of the local mānuka extract they had just made and diluted it, to test whether a weaker version is also good at killing the lettuce seeds.

The students then took extract from an Australian mānuka plant, called Copper Glow. This is known to act as a weed killer because it is high in grandiflorone, so will help them to see if their local mānuka is as effective as Copper Glow. They also diluted this to match the local mānuka’s weak version.

Measuring lettuce seedlings

All the seeds were watered and left to grow in discs containing each of the extracts and their diluted versions. The students needed to have normal lettuce seedlings to compare the treated ones against, so they also had some grow on a plain disc with nothing added except water.

Similarly, the students put some seeds on an acetone-only disc, to check that the acetone in the extracts is not interfering with any effects from the mānuka.

A week later, the students checked how well the lettuce had grown by measuring the roots, stems and leaves of the seedlings, as well as noting the colour and shape.

“I liked measuring the seeds because it was really interesting finding out what chemicals made them grow smaller,” says 11-year-old Juandre from Musselburgh School.

Students working together to measure and record lettuce seedling lengths

During the measuring session, Elaine and Dave wanted the students to experience just how different the ‘same’ plant can be. They brought in extracts and charts from different mānuka around the country for students to smell, and then looked at the charts to link each extract’s scent with its dominant chemicals, which are shown as spikes in its chart.

All the extracts smelt completely different from each other. The students said that while some were sweet or floral, others smelt unpleasant like stale urine. None smelt like how mānuka honey tastes.

“It’s weird and surprising how different they all smell even though they are all mānuka,” says Ariana, 11.

Girls smelling different extractsClose-up of discs containing an extract with its chart

The Musselburgh students’ work have helped Dave and Elaine discover that there are quite big chemical differences between local mānuka. This means that not all the mānuka in one place will have the same levels of grandiflorone.

The team have also found that, so far, New Zealand’s mānuka plants aren’t as good as Copper Glow at killing the lettuce seedlings – and that they contain much less grandiflorone. However, they point out that these findings could change once they get information from other schools doing the same experiment.

Elaine and Dave will soon send out kits and instructions to other schools, such as those on the East Cape, so that they can also take part.

“Once we’ve fine-tuned how we’re going to do the research, with help from these students here in Otago, then we’ll be going to other regions,” Elaine says. “The East Cape especially has a lot of mānuka, which they use already in the honey industry there.”

Mānuka plant samples

About the project

University Otago logoPlant and Food Research logoThis project is run by the University of Otago and Plant and Food Research with support from the Unlocking Curious Minds contestable fund.


Unlocking Curious Minds

Unlocking Curious Minds supports projects that excite and engage New Zealanders who have fewer opportunities to experience and connect with science and technology.

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