Conservation Comment: When a plant becomes a pest

Margi Keys

PHOENIX palm, sometimes called Canary Island palm (Phoenix canariensis), is among the largest palm species in the world. It has been formally recognised as a pest plant in Auckland, which means it can no longer be legally propagated, sold, moved, planted, distributed or exhibited. This species of palm, like some other exotic palms, spreads freely into gardens, roadsides, dunes and bush, threatening native ecosystems.

When an exotic plant species spreads, botanists say the species has naturalised. Garden escapes that naturalise can become problematic. Montbretia, buddleja and old man's beard (Clematis vitalba) are examples of plants that have naturalised in our district.

Sometimes we don't mind weeds and sometimes we do. It all depends on how invasive they are. Palms produce large quantities of fruiting bodies which rats and kereru love to eat, thus helping to distribute the seeds. Water is another vector.

The big beef I have about phoenix is their toxic spines. An accidental jab by a spine is a nasty experience that is extremely painful.

It will lead you to the emergency department where a doctor will give you massive doses of intravenous antibiotics. The treatment is expensive. And when low-hanging fronds are not trimmed back, they are hazardous to the eyes of passers-by.

Then there is the cost of controlling mature phoenix palms; the work is dangerous and removing a sizeable tree can cost thousands of dollars. Arborists don't like this work.

The phoenix palm is potentially one of Whanganui's environmental threats. In the suburb of Springvale, there are more than a dozen mature specimens. To keep them looking good, they require regular maintenance.

Toxic pest: A local phoenix palm tree. Photo/supplied

On the other hand, phoenix palms are hardy, stable and can resist high winds. They look spectacular along the Whitianga waterfront and in Eketahuna's main street.

Two other invasive palm species are still widely available. The Chinese windmill or fan palm (Trachycarpus fortuneii) is not yet on the pest list and is still readily available through garden centres and nurseries, even though it is rapidly spreading into bush reserves. It is very common on Waiheke Island. The bangalow palm (Archontophoenix cunninghamiana) is also still for sale, despite its spread. This species appeals to those wanting an instant garden; it grows much faster than our nikau.

Introduced exotic palms have long been popular with gardeners and property developers. They are trendy, grow quickly and sell well, which is why growers and garden centres have, up till recently, strongly opposed some exotic palms being on the pest plant list.

But the community has to fund the work needed to remedy the environmental harm caused by these plants. The cost of maintaining the bush in a pest-free state is already high because of other invaders.

It would be great if gardeners and landscapers stopped buying and planting known invasive plants. I would like to see the troublesome palm species formally recognised as pest plants and their sale banned. However, a ban will be ineffective unless it is reinforced by a proactive community awareness programme on how to deal with such pests; a systematic removal programme must follow, to stop further spread.

At this stage, not all palms are invasive but if you don't want to be responsible for adding to the problem, plant a nikau (Rhopalostylis sapida), the world's southernmost palm. It is the only palm species endemic to mainland New Zealand. Nikau's pink inflorescences are stunning.

�Margi Keys, the co-ordinator of Conservation Comment, has lived in Whanganui since June 2015. She is a member of Sustainable Whanganui, Bushy Park Trust, Tongariro Natural History Society, Forest & Bird and the Green Party.