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A weed by any other name

One habitat’s sweet meadow flower is another’s worst nightmare. So what makes a plant a weed and how do they get about?

With its orange, daisy-like flowers dancing beneath the summer sun in a European meadow, Hierarcium aurantiacum is a delight.

But stalking across the Victorian high country with a will to obliterate all other plant species in its path? Not so much.

A European native, the three species of Orange hawkweed that are problematic in northern Victoria are on an environmental watch list of 23 plants that have snuck in Australia’s back door, threaten biodiversity and primary production, and have the potential to seriously degrade our ecosystems.

Although Orange hawkweed was introduced to Tasmania as a garden plant in the early 20th century, it was identified on the mainland more recently, discovered on Victoria’s Bogong High Plains.

Volunteers on the hunt for Orange hawkweed in the Victorian high country. Picture: Roger Cousens

University of Melbourne researcher Dr Linden Gillbank is an historian of Australian botany who says it’s been suggested that another species, the yellow-daisied King Devil Hawkweed, may have come to the high plains via skiers or ski tow equipment from New Zealand, where it’s an entrenched problem.

Each summer Dr Gillbank joins other fluoro-vested volunteer weed warriors, systematically searching for the three species of hawkweed which have invaded the Bogong High Plains above Falls Creek.

But hawkweeds are not restricted to alpine areas. They can thrive in various habitats and compete with cultivated as well as indigenous species.

“Many of our precious landscapes are found nowhere else in the world, and it’s really important we preserve these unique eco-systems. Other species are deeply interconnected with them, and depend on them for survival.

It’s also a cultural obligation, in my view. It would be a criminal act if we didn’t preserve our native biodiversity for future generations.

The danger of hawkweed is largely due to its reproductive habits. It produces runners (like a strawberry plant) to invade nearby areas and wind-borne thistle-down seed to invade distant vegetation.

“And hawkweed uses chemical warfare: it secretes chemicals in the soil to deter non-hawkweed neighbours.

“If left unattended to, a Hawkweed infestation would eventually squeeze native plants out altogether and create a monoculture,” Dr Gillbank says.

Clearly, one habitat’s sweet meadow flower is another’s worst nightmare. So what makes a plant a weed and how do they generally get about?

“Weeds are masters of seed and root dispersal and creep about from neighbouring areas on animals, and in wind, water and machinery,” says Professor Roger Cousens, from the School of BioSciences, a plant population ecologist who was also involved in the Orange hawkweed eradication project. He now focusses on the many unchecked weeds infesting our coastlines.

“If you were to stand on one of Australia’s beautiful iconic beaches and look inland, you’d see vegetation made up almost entirely of introduced species. The cost to native birds and animals and entire ecosystems is impossible to calculate.”

Slow progress: water hyacinth clogging waters in Kerala, India. Picture: The Hindu Times

Professor Cousens defines weeds as “plants perceived to be out of place”, as not all introduced plants are pests, and some native species can have a weed status too.

“Whether plants are perceived as weeds or not depends on who you are and what your interests are, where you are and at what point in time.

“For instance the now globally ubiquitous Australian eucalyptus tree is a useful tree for wood in some places, but out of control in others.”

An interesting case study is the water hyacinth, a plant native to South America. It was shown in New Orleans in the late 1800s and people liked it so much it was quickly spread to gardens in North America, and then overseas.

“I saw a picture of a river in Bangladesh where people couldn’t use their boats for transport because it was so clogged with water hyacinth, so they tied their boats together and made a pontoon bridge,” Professor Cousens says.

Water hyacinth is now clogging up the waterways of just about every tropical or sub-tropical country.

So can we win the war on weeds? Professor Cousens is quietly confident some battles can be won, but says the science and protocols have to be rigorous.

Cane toad = Introduced species fail. Picture: Pete Hill (Flickr)

“We’ve won the war on some species. A good example was the prickly pear, which was driving people off their land in NSW and Queensland, but the timely introduction of a moth from Central South America did the trick. It was a successful natural biological control agent. You still see prickly pear but not to past levels.”

“All is not doom and gloom with the water hyacinth, either,” he says. “Australia leads the world in biological control because we’ve been able to manage this and other pests with introduced insects and diseases.”

A quick response is critical though.

“Eradication rarely succeeds unless you find a solution early, and you do need to be careful what you wish for: many people worry that introductions for biological control will become the new cane toads, but our national protocols are now such that this is extraordinarily unlikely.”

Banner image: Hawkweed on Prince Edward Island, Canada. Picture: Martin Cathrae/Flickr

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