Horehound Hunters: Weeding out invasive species at Mt Majura

By u6051787

A Brief History

Mt Majura is a towering feature of Canberra, with its scenic hikes, stunning views, and abundant biodiversity. It’s hard to miss as it stands 890 metres above sea level, making it Canberra’s highest peak. However, the history of the mountain and its surrounding reserve has not always been so picturesque. In the 1830’s the naturally treeless grassy plains attracted sheep farmers to the region. Livestock grazing became the norm and, over 150 years, it drastically altered the environment.

FoMM weeding at Mt Majura ridge. Photo taken by author.

The Weed Invasion

I volunteered with the Friends of Mt Majura (FoMM) group and spent the day destroying Marrubium vulgare L. (horehound) regrowth. Horehound is a woody, perennial weed that has a natural distribution in Europe, the Middle East, and the Mediterranean region. Its burr seed has allowed it to disperse to and over the Mt Majura reserve by clinging to the fur of stock and other animals. Once established the plant is highly unpalatable as it contains a bitter alkaloid. The surrounding vegetation becomes targeted by grazing which leads to the clearing of competing species. This allows horehound to persist and increase in the environment.

Horehound weed. Photo taken by author.

Horehound can only overtake areas of previously disturbed native vegetation and this is what had occurred at a former sheep camp on the reserve. We spent the day clearing the area which, until last year, had been completely overrun with horehound. This land would have been targeted by livestock due to its desirable climate. It sits at a higher altitude and is positioned between Mt Majura and Mt Ainslie making it warmer in winter and breezier in summer. The stock would have congregated here, leading to overgrazing and an increase in soil nutrients. Most native vegetation is adapted to nutrient depleted soils and cannot survive where nutrients are too high. Horehound, however, thrives off it. For years it grew, undisturbed, over the whole reserve. It was destroying any chance of native plant regeneration, until 13 years ago when the community decided to take the land back.

The Retaliation

Thousands of hours of community time has been spent trying to get the land back to something similar to its original box-gum grassy-woodland state. The aim for FoMM  is to remove all traces of horehound from the 5 hectare area of the former sheep camp. I contributed to this large scale project armed with a pickaxe and a trowel. Whilst some weeds can be killed by removing the exposed stem, this is not the case for horehound. It was important to remove the majority of the root to prevent the weed from re-establishing. This was difficult as it was rooted in rock crevasses and hard, compact soil.

Left image: Getting ready to weed on the old sheep camp at Mt Majura.
Right image: Using a trowel to remove horehound.

Luckily these weeds were still young since 6 months ago the FoMM group had cleared this area and laid down woody mulch. The mulch retains soil moisture by creating a barrier that decreases evaporation. It also provides microbes and fungi that decompose the organic matter, which help to take up the nutrients in the soil. The reduction in nutrients creates a better environment for native vegetation and decreases the chance of invasive species taking over. This can already be seen in the reserve as native plants have been popping up more and more frequently.

What’s next?

Whilst the community has worked extremely hard to fend off the onslaught of invasive weeds, more government support is needed. Horehound and many other woody weeds are not recognised as a threat for native woodland. It is important that awareness surrounding the destructive impact of these weeds on biodiversity is improved. Funding should also be increased to support the amazing community work.

About Biodiversity Conservation Blog

I am an Associate Professor at The Australian National University and convene a (very awesome) course called Biodiversity Conservation. Myself and students in the course contribute to this blog.
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