Plant Control

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.

Manual and chemical methods for controlling pest plants can be done on an individual basis or on large areas of vegetation. The most effective but labour intensive method is to pull out the individual plants ensuring removal of the entire root system. For the control of large trees a combination of manual control (stump cutting, direct drilling or scarfing the tree first), then use chemicals to stop any further growth from the tree or stump can be used. Large areas of pest plants they can sometimes be controlled by spraying and then slashing.

Plant Biosecurity is partly about preventing the spread both locally and nationally of pest plants. This is important within areas of high conservation importance. There are different levels of significance for pest plants. Some are listed as controlled with enforced actions. In New Zealand this is mainly looked after by the Regional and Local councils that create pest management strategies to give direction in how to control pest plants. Those of national significance are listed on the National Pest Plant Accord which is administered by the Ministry of Agriculture and Forestry.

Gorse is the source of several problems in the environment. The main problem is the mainly adverse affect the plant has on farm pasture. On a positive note the root system of the gorse plant does fix nitrogen which eventually is released back in to the soil and this is beneficial. It is quick growing and due to its spiny nature, it 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 may last 100 years in the soil before conditions become suitable for germination. Burning stimulates regrowth.

Blackberry is a very prickly, rambling vine. The problem with blackberry is that it has an extensive rhizome system which can be difficult to get rid of. While the top part of the plant may be removed, the problem still exists if the rhizomes are left in the ground. Dense thickets of blackberry of up to 2 metres tall can prevent stock and human access as well as providing refuges 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.

About the author: Bevan Jones is biosecurity, plant control and animal control expert in Auckland, New Zealand.