Poisonous plants in New Zealand

There are over 100 poisonous plants in New Zealand. As children grow up, they often come into contact with plants that have poisonous properties. However, children usually don’t eat enough of a poisonous plant to cause serious illness or death.

Animals and toxic plants

Many animals die from eating toxic plants, for example, ragwort can cause death in cattle and horses and more recently (with free-range farming) pigs and chickens. These animals would normally avoid ragwort, but if grass is sparse, they eat plants they wouldn’t normally eat.

Early British settlers suffered major losses of stock when their cattle, sheep and horses ate tree tutu and ngaio. Strathmore weed, an open-country shrub, is toxic to horses and cattle but seems to have little effect on sheep. Cattle and horses can experience nerve disorders after feeding on bracken. Acorns are toxic to cattle and sheep. Pregnant cows are likely to lose their calves if they feed on macrocarpa.

Some microscopic fungi living at the base of pasture grass produce toxins that cause liver damage in grazing animals, and facial eczema develops as a result. Grass staggers (stumbling and muscle spasms) in stock are caused by a toxin produced by an endophyte growing within the grass.

People and poisonous plants

About 10% of the poison calls to the National Poisons Centre are about exposure to plants. Very few people in New Zealand have died from plant toxins, but about 75 people need hospital treatment each year.

The plants (and the poisonous parts) that seem to cause the most problems include:

• the berries of black nightshade
• tutu
• karaka
• the leaves of oleander, hemlock and foxglove
• the beans of laburnum and castor oil plant
• the stinging hairs of the tree nettle (ongaonga).

These plants are all reasonably common and relatively toxic. People can be affected by eating some or all of the plant or by skin contact with the plant. Some people may be relatively unaffected by particular plants, but others may become seriously ill. Symptoms vary from toxin to toxin and from person to person. These may include burning in the mouth and throat, skin rashes, vomiting and diarrhoea.

Plants can become poisonous at different stages of their life cycles. The season and stage of growth influence the amount of toxin present in the plants. Other plants, such as kōwhai and laburnum, are only toxic in certain circumstances, for example, when the seeds are ground up first and then swallowed.

Fungi

There are many fungi in New Zealand. Some of them are poisonous. It is difficult to identify the toxic fungi from the non-toxic ones because they can look very similar. We often refer to the edible species Agaricus as mushrooms and all other fungi with caps and stems as toadstools. Scientifically, though, there is no real difference. Mushrooms and toadstools are both names that refer to the fleshy, spore-bearing fruiting body of a fungus.

Native fungi have not been investigated enough yet to identify poisonous varieties, but these are some poisonous introduced fungi:

Death cap

• One of the most poisonous fungi known.
• Has a yellowish or greenish-white cap.
• Usually grows beneath oak trees.

Fly agaric

• Very poisonous if eaten.
• Red or orange-red toadstool with a cap that is speckled with white warty lumps.
• Found where birches, oaks, pines and native beech grow.

Roll rim

• Has a brown cap with the margin rolled under.
• Often grows with fly agaric in colder parts of the country, especially under birch trees.

Some poisonous fungi cause vomiting and diarrhoea, others cause slowing of the heart rate and lowering of the blood pressure, some cause hallucinations and seizures, and others can cause life-threatening liver and kidney toxicity.

Why some plants and fungi are poisonous

Plants and fungi make toxins to ward off bacteria, insects and animals that may eat them and to protect their area from competitors.