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Weeds in New Zealand

Here in New Zealand, we have some amazing plants and animals. Our unique flora and fauna have been evolving isolated from other populations for 80 million years. Over 80% of our native species are endemic (found nowhere else on Earth).

Unfortunately, that rich natural heritage is threatened by some of the plants and animals we have introduced to New Zealand.

Of the estimated 40,000 plant species in New Zealand, a whopping 37,600 come from somewhere else. Only 2,400 species are native. Many introduced plants are important for agriculture, horticulture, or forestry. Others are ornamentals we enjoy in our parks and gardens. Still others arrived accidentally in cargo or other imported items.

About 2,100 introduced plants have naturalised, or gone wild, and about 12 new species naturalise every year. Of those “wild” plants, 500 are bad enough weeds that they must be managed by law (e.g. it is forbidden to sell or spread them). Not all naturalised plants are a problem, but some can smother native vegetation, outcompete crops, poison livestock, clog waterways, or cause other damage.

Getting rid of weeds is not cheap. We spend about $350 million every year in New Zealand controlling weeds. We spend another $40 million keeping new plants out of the country. Even so, we lose about $60 million in agricultural production to weeds every year! The cost to our natural environment is harder to estimate but believed to be significant.

Fighting weeds

How do we control weeds?

Physical control

It may seem old-fashioned, but pulling weeds can be a useful way of controlling them. Other physical control methods include mowing, ploughing, or any other method that removes part or all of the plant. Physical control can be very effective, but often involves a lot of hard work. Physical control can also disturb the soil, creating the perfect conditions for new weeds to sprout.

Chemical control

Herbicides (chemicals that kill plants), are the most common form of weed control. They are quick and easy to apply, even over large areas. Unfortunately, herbicides may kill more than just the target weed. Herbicides may harm native plants or pollute waterways if they are not applied carefully, and can pose risks to human health. They are also expensive and need to be applied regularly to keep weeds under control. Over time weeds can evolve resistance to herbicides, meaning the herbicides don’t work as well.

Biological control

Biological control (or biocontrol) uses a weed’s natural enemies to keep it from causing harm. Researchers find natural enemies (usually insects or fungi) in a weed’s country of origin and, after a lot of study to make sure they’re safe, release them in New Zealand. After release, the natural enemies breed and spread naturally. Biological control takes a lot of research and time to set up, but once natural enemies are at work, they continue to work without any help from people, year after year.

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