Our continent has a wretched history of invasive species. Ecosystems have succumbed to exotic animals such as foxes, rabbits and carp, while at the same time being beset by strange, virile plants. "Whenever I travel, I'm always amazed by the hold that exotic species have obtained over this land," says Tim Low of the Invasive Species Council. Two centuries ago, when British fleets brought the first bedraggled convicts to Australia, they brought foreign flora as well - including the infamous prickly pear. Shipped across from South America, the cactus slowly became recognised as one of the most invasive weeds ever imported into the country. By the 1920s, the pear was no longer a garden ornamental or a ferocious homestead hedge; it had stretched its spiky pads, grown into dense, tangled structures, and covered 25 million hectares of Queensland and New South Wales.
Last spring, a federally funded weed-awareness campaign was launched to educate people about the plants we purchase, as a third of weeds in Australia are said to originate in our backyards. "When you talk about weed invasion, it is often very pretty plants escaping out of our gardens that cause the problem," says Low. Weeds, we're told, cost the economy around $3.3 billion a year in control efforts and lost production. At the same time, significant resources have been spent determining what constitutes a weed in the first place. In 1999, after recruiting specialists to study and rank Australia's most troublesome plants, commonwealth ministers announced the inaugural list of ‘Weeds of National Significance', with species including pond apple, bridal creeper and lantana labelled especially problematic. On the federal government's weed website, each plant's information is accompanied by a floral mug shot.
The story of the prickly pear in Australia, and the launch of the plant-awareness campaign, is also a story of the slippery criteria we use to define a weed. By the late nineteenth century, the rampant cactus with the baubles of fruit was no longer the economic hope it had been a century earlier. In 1787 Captain Arthur Phillip docked in Rio de Janeiro on his way to New South Wales and collected the prickly pear because it hosted the cochineal fly. The female fly held in its abdomen a dark red fluid that produced a rich carmine pigment: the fabric dye used for English soldiers' red coats. "Phillip hoped the pear would be an export crop back to the textile industry in England," explains Jodi Frawley, a cultural-history researcher at the University of Sydney. However, that species never fully adapted to Sydney, so more varieties continued to arrive and settlers found use for them as animal fodder and garden hedges. The pear was recognised as a problem by 1870, but it took another 16 years for the first Prickly Pear Destruction Act to be passed.
Australia has around 2700 species classified as weeds, but the term is "functional rather than scientific", explains Surrey Jacobs of the National Herbarium of New South Wales. "The decision to declare a plant a weed is one-third economic, a third biological and a third political," he says. The prickly pear threatened native flora for decades but was only declared a weed when it had become so dense and thickly spread that it posed a problem for landholders. Even these days, conservation concerns aren't sufficient to prohibit the cultivation of particular plants. According to the Invasive Species Council, gamba grass is erasing native vegetation in Northern Australia, yet governments haven't declared it a weed because some graziers use it for fodder. At the same time, indigenous species, supposedly protected, can be labelled as troublesome: according to Jacobs, some farmers in western New South Wales are seeking permission to control native shrubs. While definitions of a weed vary, many experts simply describe it, rather ambiguously, as a plant that is out of place and doing harm.
The criteria we use to name plants ‘weeds' tell us something about our attitudes to what - and perhaps by extension, who - has a place in our world. In the late 1800s, settlers described the pear as not belonging; after federation, Jodi Frawley says, the prickly pear was recast as an enemy. There was also a correlation between the military language associated with World War I in Europe and this homeland battle for land. Frawley cites Commissioner Power, a politician discussing settlement in prickly pear land in the 1920s:
Despite the endeavour of landholders, the line of invasion was continually advancing. The efforts of the landholders only temporarily check the line of invasion here and there; each year it gradually advanced as some selectors gave up hope and abandoned their holdings, thus allowing the pear not only to outflank their stronger-hearted neighbours who continued the fight, but in many cases to surround their holdings, leaving them isolated and entirely surrounded by pear.
The plant's dominance was finally overcome by its "natural enemy", the cactoblastis moth, which was imported into Australia in 1925. That summer, soldiers in convoy trucks manually inserted 9 million eggs into patches of prickly pear, and armies of caterpillars slowly began their consumption. The operation became a textbook example of biological control. It was so successful that in 1965 the people of Dalby, Queensland, erected a memorial cairn to record their indebtedness to the tiny moth. (The introduction to Australia in 1935 of 102 cane toads to control the cane beetle has, as yet, failed to inspire similar veneration.)
The new weed-awareness campaign asks us to monitor our backyards and keep certain species in their place. As websites and advertising campaigns circulate pictures of potentially destructive species, we are encouraged to become amateur botanists, categorising and monitoring our agapanthus and lilies. The federal government has recognised 21 Weeds of National Significance, but many others are considered possible threats. "Agapanthus is emerging as a potential weed threat in parts of New South Wales and Victoria because of its hardiness and drought resistance. In the Blue Mountains in New South Wales, the ledges below the Three Sisters lookout are full of agapanthus, in place of the native flora," reports the national weed website.
For us, it is not so much a case of enemy lines advancing as of protecting borders from rogue tendrils penetrating through fences. We are urged to cover compost so that seeds cannot be carried by wind or animals, and to dispose of seeding weeds by placing them in a black plastic bag, sealing it and "baking it" in the sun until destroyed. "Monitor your garden to ensure that plants are not spreading and posing a threat to nearby bushland or pastures," we're told. In this way, it is not the plants themselves that belong or don't belong; it is their activity. Agapanthus and lilies are acceptable if they behave.
In Sydney's Botanic Gardens, within the fortress walls of the succulent section, Opuntia robusta plants - a type of prickly pear - sit silently on display. Their 50-centimetre-long paddles with 5-centimetre spikes make them look like medieval torture instruments. There's now only one species we're allowed to cultivate in Australia, Opuntia ficus-indica, and fruit shops stock the pear. At home, I pull on rubber gloves and peel one. It's not the barbs you can see that are dangerous, but the ones you can't, their fine needles burrowing into unsuspecting palms. I strip the casing, pierce sweetness, and begin to swallow flesh.