Know thy weeds

There’s weeds in them there paddocks – and it’s a good idea to know which weed is what.

This is even more important given the fact that earlier this year, what is possibly the world’s worst weed was found growing in the Waikato.

Velvetleaf, otherwise known as butter-print, was found growing out of a maize silage stack in February. Photographs were shown to AgResearch weed scientist Trevor James who says he had no idea, at first, what he was looking at.

“Then, when I realised, panic buttons went off. Because I knew from the literature that this is a seriously bad weed.”

A member of the mallow family, the plant has small, yellow flowers and large round leaves. It grows upright in a manner similar to hollyhock and can get up to three metres tall when growing in a maize crop.

Velvetleaf is said to be the worst weed in America. Uncontrolled, it can reduce crop yields by up to 34 per cent, costing hundreds of millions of dollars per year.

“If it’s the worst weed in America, it’s going to be the worst weed nearly anywhere.”

While the plant is probably laying low over the winter, Dr James says it’s important for people to be aware of what is growing on their properties.

Based at the Ruakura campus, he has been studying weeds for more than 30 years, and with Ian Popay and Paul Champion last year published the third edition of a guide to New Zealand’s weeds.

The book, with about 1800 photographs, covers about 600 species. It also includes some native plants which can be confused with weeds. There are, he says, around 2300 plants foreign to New Zealand that have escaped from gardens. “And there’s about 10 times that many plants, most safely in gardens. It’s estimated that 10 new ones escape every year.”

Weeds can be thought of in two ways – those which affect agriculture, such as ragwort and thistles, and those that threaten the natural environment.

“Many have been here for 100 years, and thought to be safe. Then something changes and they are suddenly found outside of where they were planted.”

What is considered a weed can be tricky ground, sometimes. “Some people love agapanthus, and some varieties are sterile. There are real concerns with one in particular however, you only need to go to Whakatane and look at how it’s colonising the sand dunes and the cliff behind the town. Sometimes we get quite a hammering for killing wild, pretty flowers.”

The National Plant Pest Accord lists about 113 plants that are not allowed to be sold, propagated, moved or given away.

“If they’re already existing in the garden, that’s okay. But they can’t be given to a neighbour or sold, that’s against the law.”

Included on the list are such familiar plants as Mexican daisy, aluminium plant, blue morning glory, blue iris, ice plant, pampas, pussy willow, Italian jasmine…

“We still have problems with road side stalls and market days. And the most common way for these plants to become environmental weeds is when people dump trailer loads in parks and reserves. It’s quite surprising just how far they will drive to do it in fact.”

• An Illustrated Guide to Common Weeds of New Zealand, third edition, is available online from Manaaki Whenua Press, $55.