Any plant has the potential to become a pest when introduced to a new environment. In many cases they do cause problems in the environment and therefore need to be controlled. Many different plants have been introduced to New Zealand as innocent ornamentals but have become pests and the focus of large control programmes. Gorse, Blackberry, Lantana and Woolly Nightshade are major pests of the environment that affect native bush as well as farmland.
Problems with plants are numerous. Some plants are difficult to control once they have established due to large seed banks in the soil like gorse. Plants like ragwort and woolly nightshade can be toxic to stock. Quick growing like Wandering Jew and Woolly Nightshade can rapidly become invasive, out-completing natives. Some affect land use by establishing in pastures and excluding stock and humans by forming large prickly thickets, which is what gorse and blackberry do. Some can choke waterways and affect water quality like water hyacinth. All of the above mentioned plants have been introduced to New Zealand and found the ideal climate to thrive and cause problems in all areas of the environment.
Both manual and chemical methods can be used to control pest plants. These methods can be used on an individual basis or on mass. The most labour intensive but possibly most effective method is to pull out individual plants ensuring the removal of all the root system. Control of large plants tends to be by stump cutting, direct drilling or scarfing the tree first, then using chemicals to poison the tree and stop any further growth from the tree or stump. For large areas of pest plants they can sometimes be controlled by spraying and then slashing after the plant has died.
Plant Biosecurity is partially about preventing the spread both locally and nationally of pest plants. Plants of national significance are listed on the National Pest Plant Accord which is administered by the Ministry of Agriculture and Forestry. These plants are prohibited from sale and propagation. Some pest plants are listed as controlled with enforced actions. This is mainly looked after by the Regional and Local councils who create pest management strategies to give direction in how to control pest plants.
Gorse causes many problems in the environment and costs millions to control. While it is a nitrogen fixer which is beneficial, the adverse effects on farm pasture outweigh the benefits. It is quick growing and due to its spiny nature, forms thickets that are impenetrable to stock and humans. It produces large numbers of seeds in seed pods which explode open, enhancing seed distribution. Seed can last decades in the soil waiting for conditions to become suitable for germination. While burning can get rid of some plants this stimulates gorse seed germination and regrowth!
Blackberry causes a few problems in the environment. By it’s nature it is a fast growing very prickly, rambling vine. Blackberry has an extensive rhizome system which makes it difficult to control. The whole plant must be removed; otherwise the problem still exists if the rhizomes are left in the ground. Blackberry can form dense thickets of up to 2 metres tall can prevent stock and human access. This also provides an ideal habitat for other pests like possums and rabbits.
The problem with Woolly Nightshade is that it is highly invasive. It is a quick growing plant that can reach in excess of 5 metres tall. The trees form dense stands that prevent the growth of other plants. They fruit prolifically and birds can widely spread the seed effectively and efficiently. Without management it can quickly take over areas of forest margin, scrub, farm land, hill country, waterway margins and reserves. The plant is thought to be toxic to stock and can affect humans.