What is a weed?
A weed is a plant growing where it is not wanted. A flowering plant that is welcome in a garden may be considered a weed on the roadside or in a bush reserve.
Of New Zealand’s 26,500 vascular plant species, only 9% (about 2,500) are native. At least 1,800 introduced plant species have become naturalised, which means they grow in the wild – many of them climb over, smother or outgrow native species. They are sometimes called introduced weeds, adventives, exotics or alien plants. Weeds not only threaten native vegetation – if they become dominant they can alter ecosystems and displace animals adapted to living in native bush.
From the time of first settlement, Māori and European farmers and gardeners brought useful or ornamental plants to New Zealand. Over 75% of bush weeds have come from gardens. Called garden escapees, many weeds have entered the bush as wind- or water-borne seeds, bulbs or broken fragments. Others have been spread as dumped waste or by stock, vehicles and birds. Reserves and picnic areas near cities or roads, or places visited by large numbers of tourists, are particularly susceptible to weedy invaders. People dump garden waste in these areas, or may carry seeds on muddy footwear.
In warm northern New Zealand, tropical or subtropical plants can escape from gardens and become aggressive weeds, thriving in frost-free areas and spreading until they reach their southern limit. The north, especially Auckland, is afflicted with many weeds that are rare or unheard of in frostier upland or cooler parts of the country.
Friend or foe
Weeds are plants that people judge to be bad. Whether a plant is a weed can be a matter of opinion. Some people were upset when the native tutu was included in Common weeds of New Zealand (1998). A farmer would consider tutu growing in a field a weed as it can poison stock. But the same plant growing on reserve land is protected as a valuable part of the natural vegetation.
Weeds in the bush
Weeds make little headway into dense, mature bush. But they can thrive in scrubland, at the edges of bush, in clearings, in areas damaged by possums, deer, goats, pigs and farm stock, and in regenerating bush. Weedy grasses, thistles and wind-blown seeds often take root on ground newly exposed by slips, changed river courses, forest fires and road cuttings.
Monitoring and eradicating weeds is difficult and expensive. The Department of Conservation lists about 300 weeds as serious problems in national parks and reserves. The department monitors all its reserves for newly arrived weeds, paying special attention to areas where lots of people walk.
City and regional councils tackle weeds in their reserves, and increasingly, volunteers are clearing small areas. For example, the New Zealand Trust for Conservation Volunteers and Weedbusters rid reserves of bridal creeper, ivy, Chilean flame creeper, sweet briar, pine, holly, hawthorn and sycamore.