When Plants Go Bad

Weeds are more than simply a nuisance in your garden. They have the potential to wipe out entire communities of native plants. By Ann Graeme.

Poor, persecuted weeds. We dig and plough and poison them but they keep on growing, trying to bandage the raw earth. That is what a weed would do in its natural environment. Weed species have evolved to grow in unsettled earth and damaged landscapes, and that
is the kind of landscape people make.

It may suit the weeds but we don’t want them. We want only our gardens and crops, where the soil is constantly bared and planted with unnatural but useful plant
communities. So for thousands of years we have struggled
against the plants that strangle our crops and break our
backs, until the word “weed” has taken on a malignant meaning.

But it is not the weeds’ fault. A weed is only a plant we think is growing in the “wrong” place. But for every weed species there is a “right” place, in its own ecosystem where
it contributes to a living community, where it covers the scars of natural disasters and is overtaken and subdued by other plants in a natural succession.

Weeds are pioneers. They grow quickly, set seed in abundance and spread over the soil surface to beat the competitors. And when people shift species from country
to country, a weed has another advantage. It has come from an ecosystem where it was nibbled by insects and kept in check by diseases and competitors. Now it is in a new land,
leaving all that baggage behind and is free to become a botanical thug - aggressive, invasive and dangerous. As well as spoiling crops, introduced weeds can wreck whole
ecosystems.

Some native plant communities are particularly vulnerable because disturbance – the weeds’ friend – is a natural part of their ecosystem. Marram grass takes root on storm-battered dunes, agapanthus clings to eroding cliffs, and willow and lupins choke the ephemeral islands in braided river beds.

Wilding pines are a threat to the high country. These Northern Hemisphere conifers can grow beyond the native beech forest, high up the mountains into the native shrub
lands and herb fields where there is no natural competition for tall trespassers.
This is where people must step in. Volunteer pinepullers led by Waikato Forest & Bird weeded the slopesof Ruapehu for more than a decade.

Now, with the seed source turned into chips, the mountain is free of pines. Volunteers are tackling the wilding pines of the South Island too but it is an extensive and huge challenge.
Tall, intact plant communities are slightly less at risk from weed invasions. A native forest is normally difficult for nonnative plants to penetrate but where pests have degraded
it, weeds can sneak in.

The sunlit gap where a tree fell and deer and goats have eaten the native seedlings offers an opportunity to a wind-blown seed of old man’s beard or a honeysuckle seed in a bird’s dropping. However, even when the canopy is tight, shade-loving weeds like wandering willie and African club moss can take root and smother native ground plants. In time, the loss of
replacement native seedlings will bring a thinning of the canopy, which has the same impact as browsing mammals, letting in exotic vines, shrubs and trees.

The global advance of weed species is leading towards a more homogenised world, where specialised and local species are driven out by Jacks-of-all-places. There will continue to be plenty of life covering the globe, says Stephen Meyer in his book, The End of the Wild, but that life will be different and less diverse. “The wild will give way to the predictable, the common and the usual.”

If you think such gloom and doom is an exaggeration, consider these statistics about plants in New Zealand:

There are 2345 native plant species (that’s flowering
plants, cone-bearing plants and ferns)

There are about 35,000 exotic (non-native) species

2375 exotic species have naturalised and live in the wild – that’s more than all the native species

15-30 new and some not-so new species go wild every
year

More than 400 exotic species are serious environmental
weeds

In the early 1990s botanists, noxious plant officers and conservationists realised that some native ecosystems like dunelands, streamsides and offshore islands might be more
endangered by weeds than they were by animal pests. This was a radical idea. Many gardeners and nursery owners furiously repudiated any suggestion that they and their
precious plants were the source of most of these invasive weeds.

Forest & Bird led the way. Guided by long-time member Jack Craw, now head of biosecurity at Auckland Council, they initiated the Forest Friendly campaign, rewarding garden centres that did not stock plants known to invade native ecosystems. Despite ruffling a few nursery owners’ feathers, the campaign was very successful and marked the beginning of a new awareness of the potential dangers posed by garden plants.

The Forest Friendly Awards were also the catalyst for legislative change. Not everyone complied with the voluntary award so regional councils and MAF were forced to take official action. Today the National Pest Plant Accord regulates and maintains high standards for nurseries and retailers and the nursery trade is working towards adopting
good sustainable practices, thanks in great part to the actions of Forest & Bird.

Regional council biosecurity officers follow strategies to control listed pest plants and keep a sharp look out for new invaders. Recently Bay of Plenty pest officers found and destroyed a sprawling specimen of kudzu, a species ranked as one of the worst weeds in the world. Just imagine if that had got away.

I know a lot about weeds. I have nurtured so many in my garden. There was ladder fern (which I thought was native), the arum lilies, the toetoe that turned out to be pampas, the pretty agapanthus – the list is long.

It is people like me who are the reason that weeds are so widespread. Eighty per cent of all environmental weeds originated as garden escapes or have been dumped in parks and waterways.

We amateur gardeners like plants that are vigorous and easy to grow but these are the very characteristics a plant needs to become a future weed.

The conflict between garden favourite and invasive weed will continue, particularly as warming climate speeds some garden escapes. But if you care about protecting native New Zealand you will abandon those rampant old friends and consider the alternatives. There are other plants, both native and exotic, which can replace your treasured weeds.

And as conservationists we need to be alert to any strange plant turning up in the bush reserve or on the dunes. We can’t leave it all to the biosecurity officers. They need our eyes as well, to spot and root out or report intruders and prevent a pioneer weed becoming a plague.

Ann Graeme