Mānuka – a natural weedkiller?

Mānuka is the Māori name for shrubs or small trees of the species Leptospermum scoparium in New Zealand. Products derived from mānuka – both honey from flowers and essential oil from leaves – are valued for their unique chemical compounds. Now primary and intermediate schools across New Zealand are testing mānuka for another chemical compound – grandiflorone.

Leptospermum scoparium is native to both New Zealand and Australia. Scientists know that some shrubs in the Leptospermum genus, like the Australian copper glow (Leptospermum morrisonii), produce high amounts of grandiflorone. Grandiflorone is a natural herbicide, and in species like copper glow, it can be extracted with good yield and purity.

Kiwi students and scientists are investigating grandiflorone in our native Leptospermum scoparium. They want to find out if grandiflorone levels differ in mānuka trees growing in various parts of the country and if the levels are potent enough to act as a natural weedkiller. Scientists Elaine Burgess from Plant & Food Research and Dr Dave Warren from the University of Otago are leading the project, working with mānuka chemistry expert Professor Nigel Perry. They received funding from the New Zealand Government’s Curious Minds initiative. As of July 2018, Elaine, Dave and Nigel have collaborated with 30 schools. They hope that the project can continue to reach more schools and regions.

Allelopathy – a biochemical defence

Allelopathy is the term used when a plant releases a toxin to suppress the growth of nearby plants. Mānuka leaf litter releases potentially allelopathic grandiflorone, and this may be one reason why mānuka is so successful as a colonising species across New Zealand.

The investigative approach

Each school follows the same approach. First, students collect foliage from local mānuka plants. They prepare a sample voucher – a dried and pressed plant sample, kept for future reference to identify the plant botanically. An image of the sample voucher is uploaded to a database on iNaturalistNZ. Students can access the database to compare mānuka plants and look for patterns.

Next, students grind up leaves from their local mānuka plants and add acetone to create liquid extracts. Then, they put the extracts to work as herbicides, drying them onto paper discs, which are then wet with water for seeds to germinate and grow on. Students use lettuce seeds as target ‘weeds’.

To conduct the experiment, they:

• grow seedlings on discs with the dried mānuka extract and a dilution
• grow seedlings on discs with dried copper glow extract and a dilution
• grow seedlings on discs with dried acetone only
• grow seedlings on discs without extracts or any acetone.

Growing seedlings on wet discs without extract or acetone provides a control group to help students view and measure normal seed germination and growth. Growing seedlings with dried acetone only also helps students determine if any acetone or impurities were left to interfere with normal growth. The copper glow is a positive control, known to inhibit seed germination and growth at the concentrations used. These are standard scientific protocols to check that an experiment is working properly.

Students make observations about lettuce seed germination and seedling growth and record the details. The process takes about 2 weeks. Mānuka samples are also sent to the Plant & Food Research lab at Dunedin’s University of Otago for chemical analysis.

We want students to not only gain new skills from conducting the experiments themselves but also to learn about the nature of science, of testing, of researching and of coming to robust scientific conclusions.

Dr Dave Warren

Surprising results

Elaine and Dave have been surprised by some of the early results. Their work with schools has shown that mānuka plants differ quite a bit when it comes to grandiflorone levels. Even mānuka plants from the same area have different chemical traits. So far, the scientists and young citizen scientists have yet to find local mānuka plants with the same weedkilling potential as the copper glow species. They point out that it is still early in their investigation. There are many other regions in New Zealand still to be tested.

Mānuka – a valuable native tree

Māori have known about and used the medicinal properties of mānuka for centuries. Modern science is revealing the chemical compounds responsible for the effectiveness of mānuka in traditional rongoā and beyond. The mānuka flower produces nectar with unique compounds that result in a honey with strong natural antibacterial properties. Essential oil distilled from mānuka leaves has antimicrobial properties due to different compounds. Now, school students may prove that an additional mānuka compound is valuable as a natural product!