Buddleia or butterfly tree (Buddleja davidii)
This many-stemmed shrub or small tree has sweetly-scented, tapering heads of mauve flowers which attract butterflies, particularly red admirals. Originally from Japan and China, the plant arrived in New Zealand via Europe. It was first noticed growing wild in the 1940s. It spreads to disturbed sites and up shingle river beds into remote, mountainous places such as the Urewera and Remutaka ranges, where it supplants native shrubs. It grows abundantly in the North Island and in the northern half of the South Island.
Radiata pine (Pinus radiata)
Radiata pine trees were first noticed growing wild in New Zealand in 1904, their wind-blown seeds having escaped from plantations. Today, wilding (uncultivated) pines grow above the bushline and in disturbed areas such as on slips and burnt sites, where they overgrow and displace native species. Wilding pines disfigure landscapes in many parts of the country, but are a serious nuisance in Urewera Forest Park, on the east coast north of Napier, in the Marlborough Sounds and in Central Otago. By 2016 wilding pines were thought to covered a similar land area to plantation forests. Controlling them is labour-intensive. Saplings must be pulled out by hand, or small trees felled at the base and the stump pasted with herbicide.
Contorta pine (Pinus contorta)
Contorta pine is a serious problem on the Volcanic Plateau, and is controlled by the Department of Conservation and the New Zealand army.
Tree privet (Ligustrum lucidum)
Tree privet grows up to 14 metres high and can survive for 100 years. Originally from China and Korea, it was first noticed growing wild in New Zealand in 1958. It is a problem in the Auckland, Waikato and coastal Bay of Plenty regions, where it has displaced many native canopy trees. In the north, some areas of native bush have been reduced to tree privet forest, with its own seedlings providing the only new growth. It has spread as far south as the West Coast. Its pollen can induce severe asthma.
Woolly nightshade (Solanum mauritianum)
This tree has large woolly leaves and grows up to 10 metres high. It becomes stag-headed after 25 years. Originally from Brazil, woolly nightshade escaped from New Zealand gardens in the 1880s and is now a common weed in the country’s north, but less so further south. It rapidly invades and out-competes native plants at forest margins and in damaged forest. Rainwater dripping off their leaves poisons anything growing beneath them.
Each year a single gorse bush will produce 500–1,000 seeds (and occasionally up to 20,000) per square metre of ground. Seeds stay viable in the soil for over 100 years. As early as 1859, laws were passed to prevent its spread or sale. But by 1893, gorse was common throughout the country, and in 1900 it was declared a noxious weed.
Gorse (Ulex europaeus)
Missionaries brought gorse, a prickly, yellow-flowered shrub, to Northland in the 1830s for farm hedging. Today, gorse is troublesome and expensive to control on farmland, scrubland and in damaged forest. It often thrives where possums or farm stock have opened up the forest canopy and let light in. On the positive side, gorse adds nitrogen to the soil, stabilises erosion-prone slopes, and is a good nurse plant for emerging native trees. Around Wellington it takes native trees 30–40 years to overgrow gorse, but around Dunedin it takes up to 60 years.
Himalayan honeysuckle (Leycesteria formosa)
This hollow-stemmed shrub grows up to 3 metres high. Birds and water help spread its seeds, and it grows at the margins of bush, in open river beds and forest clearings. Himalayan honeysuckle stops native plants from regenerating and replaces native pioneer plants.