Pest plants

What is a pest plant?


A weed is simply a plant that is growing in the wrong place. There are literally hundreds of plants like this that thrive in Northland's warm climate.

However, to become a pest plant, a weed must also represent a threat to the region because of its ability to invade or take over land that is productive or has important ecological or cultural values. These serious pest plants are weeds that the Northland Regional Council (NRC) has officially declared as pest plants in the Northland Regional Pest and Marine Pathway Management Plan. The Council has the legal responsibility for managing these pest plants in Northland.

Photo of lantana with its distinctive pink and yellow flower.Lantana, a pretty but smelly and toxic pest plant, is not allowed to be sold, propagated or distributed in Northland.

Many pest plants have been in Northland since the region was first settled. Gorse, for example, was brought to New Zealand to use for hedges on farms. The English settlers did not realise that the prickly hedges that gorse formed in the cold climate of England would run rampant in Northland's subtropical climate. Gorse has rapidly spread throughout the region.

Other plants, like alligator weed and Manchurian ricegrass, were introduced accidentally in ballast rocks and soil from ships. Nassella tussock and Australian sedge arrived as impurities in grass seed mixes.

Pest plants have also thrived in Northland because their natural predators, diseases and control agents - usually tiny insects - which kept them in check in their native country were not present here as well. In some cases, control agents have been imported to help with control. The decision on whether to import new biological control agents has to be made carefully as they might cause more problems in a new country. Government experts have to decide if a biological control agent will eat only what it is supposed to and not affect other plants, animals or insects.

Many plants have been brought to New Zealand by gardeners because they are pretty. Some plants need special help to survive in New Zealand's climate - it might be too hot, cold or wet to do well and require a lot of help from the gardener. However, some plants find the warm, wet climate absolutely perfect and they grow even better than in their home countries. When these spread they can become pest plants.

Northland Regional Council's role and policies

The Northland Regional Council (NRC) has adopted the Northland Regional Pest and Marine Pathway Management Plan, which declares certain weeds as pest plants.

We have the responsibility under the Biosecurity Act for managing the pest plants in our region, apart from five plants which are considered nationally important. The responsibility for the control of these five Class A plants rests with the Ministry of Agriculture. The five Class A plants are cape tulip, Johnson grass, salvinia, water hyacinth and water lettuce. Cape tulip, salvinia and water hyacinth are present in Northland.

Under the Biosecurity Act, we develop our own strategies for other pest plants that are a problem in Northland. We have asked the public to nominate plants that they want controlled. The NRC has put all 73 pest plants nominated for inclusion by the public through a screening process based on the requirements of the Biosecurity Act. The Act requires weeds to be assessed for the impact they have economically, environmentally and culturally (Māori).

The Council classifies weeds on their current and potential impact to Northland on the basis of the way in which they affect cultural values, ecological values, human health, soil and water, production, public infrastructure, public safety, recreation and international trade. The most serious weeds usually feature in many categories, but not necessarily all of them.

The plants considered serious enough for inclusion as Northland pest plants are:

  • African feather grass;
  • aquatic weeds (five species);
  • Bathurst bur;
  • broom; 
  • forest invader plants (32 species);
  • gorse;
  • mistflower;
  • lantana;
  • Manchurian ricegrass (also known as Manchurian wild grass);
  • Nassella tussock;
  • nodding thistle;
  • pampas grass;
  • privet;
  • ragwort;
  • spartina; and
  • wild ginger.

Unwanted plants have been listed as: eelgrass, hydrilla, nardoo, old man's beard, rhamnus, Senegal tea, skeleton weed, water poppy, fringed water lily, entire marshwort, houttuynia and needlegrass.

The plants listed as unwanted are either not present in the region, or are extremely rare. Every effort is to be made to keep them out. The Northland Regional Council wants to hear from anyone who thinks they have seen any of these plants in the region. In addition, a list of 94 plant species has been compiled and classified as "nationally banned from sale''. These lists will change, over time, as new threats are recognised.

The NRC varies its approach to weed control depending on the species of weed, where it is growing and how it is spreading. If it is one of the most serious weeds, the NRC biosecurity officers will be called in to remove the problem plants at no cost to the owner. For other serious weeds, land owners may be required to keep them under control and away from boundaries to reduce the chance of spread. For some plants that are difficult to identify, or are at very low levels of infestation, the Council carries out control work. Garden centres are also checked to make sure they are not selling any banned plants.

Find out more: Weed and pest control

The Council's work

The Council has three staff working on weed control in the region. They are each allocated an area within the region for which they are responsible. They are kept busy tracking down pest plants and implementing the region's pest plant management strategies for each type of plant.

The known sites of pest plants are being mapped using a global positioning system so that staff now and in the future can keep track of where they are, and can work towards eradicating them.

Our biosecurity officers work with landowners and the public to get rid of our region's worst weeds, and help make our environment a better one. Check out the pests and weeds in our Pest Control Hub

Why should you care?

Weeds matter because they are changing the natural balance of plants, animals and insects in Northland. Many of Northland's native plants and animals cannot be found anywhere else in the world. With the changes in habitat over the hundreds of years since humans settled New Zealand, many native plants and animals are now endangered. There is a chance that they might become extinct.

Possums are helping weeds take over native bush. When possums attack a tree for food and the tree dies and falls over, a gap in the canopy is created. Weeds can become established in this opening and begin to choke out native plants. Every weed that replaces a native plant means less food for a wood pigeon, tui or fantail.

Bush that is choked with weeds is difficult to walk through, and the natural beauty of the bush is affected.

Some weeds have pollen or strong scents that trigger asthma attacks in some people, or have poisonous sap that causes skin rashes. This could ruin a trip in the bush if you are susceptible to asthma or other allergies.

  
A school student planting native seedlings.Help the native bush by planting native seedlings as Cherie Smith of Parua Bay School, above, is doing.

Northland - a weedy wilderness

The Northland Regional Council has some great pamphlets available on many weeds in Northland. These range from aquatic weeds that can choke waterways to plants that will invade native bush or affect pasture and poison animals.

Find out more: Pest Control Hub

You won't have to look far to find some weeds to study. Check the roadside outside your school or home, around the school property or in a patch of bush nearby. Study weeds to see why they are so successful, but take care when touching them, as some weeds have poisonous sap or berries.

 
How do weeds spread?

Weeds have many ways of spreading, the same as any plant in the garden or in the bush. Some weeds are spread by seed. Birds eat the fleshy berries with the seed inside and carry them in their stomachs when they fly away - which is how weeds have been deposited on many of Northland's coastal islands. Seeds can also be carried in the wind in floating pods, or by being ejected by such things as gorse pods popping open. Other seeds have hooks or burs that are caught in the coats of animals.

Some weeds send up new plants from rhizomes that spread underground. These form dense clumps that tend to choke out other more desirable plants. Some are creepers, which grow over the top of other plants and trees and choke them in the process.

Some weeds are difficult to eradicate because new plants can grow from any piece of the plant that is broken off. The most successful weeds use a combination of different methods to spread into new areas.

People can accidentally help some weeds to spread by carrying seeds or pieces of plant on dirty farm machinery or boats and boat trailers. The next farm or the next lake can soon become infested if care is not taken to clean machinery or boats properly.

Weeds on land

There are hundreds of weeds growing on land in Northland and the most serious kinds threaten to invade or take over productive land, or land which has important ecological or cultural values. Weeds thrive in Northland's warm climate and soils. They are more likely to become established on land that is disturbed often, such as quarry sites, roadsides and erosion spots. One example of a pest plant of the land is ragwort.

Ragwort:

This pest plant has bright yellow flowers that look like daisies - it does belong to the daisy family. It was introduced to New Zealand from England in about 1874, and has quickly spread throughout the country. Ragwort is very difficult to control, because it produces many thousands of seeds and can grow from root fragments if any are left in the ground.

Bright yellow flowers of ragwortThe bright yellow flowers of ragwort.

Scientists estimate that one large ragwort plant can produce more than 50,000 seeds, and these can remain viable for eight to 10 years. Wind or water can spread the seeds. The plant can quickly invade pasture and is toxic to livestock such as cattle and horses. However, sheep can tolerate it in a mixed diet, although some deaths can occur.

The NRC strategy for ragwort includes requiring property owners to keep boundaries and roadsides clear of the plant. Landowners can be prosecuted if they don't follow these rules.

Other pest plants of the land include African feathergrass, pampas, gorse, privet and lantana. 

 

Weeds in water

Aquatic weeds are like stealthy invaders because they quickly spread to occupy the available space in water bodies in a very short time. Aquatic weeds include alligator weed, oxygen weeds and parrot's feather. They all reproduce rapidly from pieces that break off and form floating mats. Some, like Manchurian ricegrass, live partly in the water and partly on the land. 

Manchurian ricegrass  (Zizania latifolia):

One of the worst aquatic weeds in Northland is Manchurian ricegrass. This pest plant arrived accidentally near Dargaville more than 100 years ago in ship's ballast.

The invasive Manchurian ricegrass has now spread far and wide in the Northern Wairoa River and its tributaries. The tall grass, which originally comes from China, Japan and Korea, forms dense mats and has deep roots and thick rhizomes (underground stems) which spread widely. Ricegrass usually grows about 2-3 metres high, but can grow to five metres. The leaves are a dull grey-green, and make a loud rustling noise in the wind.

Ricegrass is considered a major plant pest because it completely takes over all other vegetation, blocking drainage in waterways and destroying stopbanks with its deep roots. It also wrecks native plant and fish habitats, and is extremely difficult to eradicate because any root or rhizome fragments will regrow. It invades waterways and farmland. Herbicides are the most effective control measure, but use of these is restricted because many chemicals can affect waterways. Special Council Resource Consents are required before use. 

Manchurian ricegrass.Manchurian ricegrass.

The Northland Regional Pest Management Strategy for Manchurian ricegrass involves prohibiting any distribution or sale of the plant and working with landowners and contractors to prevent the spread of the grass through dirty machinery. NRC biosecurity officers eradicate any ricegrass plants found outside the main Kaipara infestation area in a bid to stop the pest plant spreading further into other areas.

 

 

 

 

 

 

Weeds in bush

Northland's forests are special because of their unique native species. Bush invaders are a serious threat because they can take over the habitat of rare and endangered species, prevent native seedlings growing and reduce the diversity of plants. One example is wild ginger.

Wild ginger:

There are two species of wild ginger that cause problems in Northland. The most common is kahili ginger (Hedychium gardnerianum). Yellow ginger (Hedychium flavescens), is the other.

The strikingly beautiful yellow flowers make kahili ginger easy to spot on roadsides and in bush areas, the large yellow flowers with red stamens appearing between January and March. Yellow ginger flowers later, between April and July, and its flowers are creamy yellow.

Kahili ginger's yellow flower.Kahili ginger's yellow flower.

There are major infestations of kahili ginger in Whangaroa, Herekino, Kohukohu, Panguru, Rawene, Waimamaku-Waipoua, Kerikeri, Opua-Paihia, Helena Bay, Whangarei and McLeods Bay. Yellow ginger is found at Kaeo and Whangarei.

Both types of wild ginger are capable of forming thick infestations, taking over every other type of vegetation. The rhizomes form dense beds up to one metre deep. All underground parts of the wild ginger plants can regrow, making it difficult to eradicate completely. Herbicides are used to kill plants completely. Kahili ginger seeds are carried long distances by birds, creating new infestations. Yellow ginger does not produce fruit or seed in New Zealand, making it less of a danger. However, it can still take over areas in which it is growing.

The Regional Pest Management Strategy for wild ginger includes banning these plants from sale, propagation or distribution in any way. The Council may also require control measures to be taken, where a local community is actually controlling wild ginger.

Other forest invaders include climbing asparagus, Himalayan honeysuckle, jasmine, wandering jew, velvet groundsel; climbing vines such as moth plant, banana passionfruit and blue morning glory; ground covers such as African club moss; and shrubs and small trees such as white monkey apple. 

The dirty dozen

These plants are considered so serious to our environmental safety that if they are found in Northland the NRC will remove them at no cost to the landowner.

Any sightings should be reported to NRC biosecurity officers.

1 Fringed water lily, (alias water snowflake)

Photo of fringed water lily.Fringed water lily. 
 
 

 

 

 

 



Photo of marshwort.Marshwort. 2 Entire marshwort (alias floating heart)

Both species are bottom rooted aquatic plants, with small water lily-like leaves (less than 10 centimetres wide), and distinctive yellow flowers with five lobes.

The petals on both types of flowers have fringes on the outside margins.

These plants are rare or absent from Northland. Both species block waterways, causing flooding, and will ruin water quality.

 

 


Senegal tea.Senegal tea.3 Senegal tea (alias temple plant, costata)

This perennial herb grows to 1.5 metres high in marshland or water. It has pointed leaves with wavy margins, and distinctive, fluffy white flowers.

The plant looks like alligator weed and is extremely rare in Northland. It invades damp ground and still or slow-moving water where it forms floating mats. It is poisonous and induces allergic reactions.

 


Nardoo, four-leafed water clover.Nardoo. 4 Nardoo (alias four-leafed water clover)

This is a bottom-rooted aquatic plant that floats on the top of the water.

It has distinctive clover-like leaves with four lobes and is uncommon in Northland.

Nardoo invades still water and is poisonous.


 

 


Water poppyWater Poppy. 5 Water poppy

This bottom-rooted, top-floating aquatic weed has fleshy leaves. It also features a large poppy-like yellow flower with a purple centre.

This plant is very rare in Northland. It invades waterways and marshland, forming dense mats. It ruins water quality and causes flooding.



 



Hydrilla.Hydrilla.6 Hydrilla (alias water thyme)

Hydrilla is a submerged aquatic weed, bottom-rooted, that grows up to four metres long and has many branches. Hydrilla looks like other oxygen weeds except for toothed leaf margins and small sharp buds.

This is a very invasive but rare underwater weed. It is unlikely to be found in Northland.


 

 


Eelgrass.Eelgrass.7 Eelgrass (alias Vallisneria spiralis)

This is a submerged fresh water grass, bottom-rooted, with strap-like leaves.

It may be small or up to six metres long. Eelgrass is rare or absent from Northland.



 

 

 

 


Houttuynia.Houttuynia.8 Houttuynia (alias chameleon)

This deciduous creeping groundcover has heart-shaped leaves that feature many colours - yellow, green, pink and bronze. 

It is rare or absent in Northland. Houttuynia becomes rampant in gardens, damp areas, wetlands and bush fringes. All fragments can resprout.

 

 

 

Old man's beard.Old Man's Beard. 9 Old man's beard (alias travellers joy)

This vine is usually deciduous and can be confused with the common native clematis. Old man's beard has five leaflets per twig, ribbed stems and it flowers in summer-autumn and seeds in winter.

Clematis has three leaflets, smooth stems and flowers in spring and seeds in summer. Old man's beard will smother and kill every other plant when it invades bush. It is very rare in Northland.

 



Rhamnus.Rhamnus. has shiny red berries 10 Rhamnus (alias evergreen or Italian buckthorn)

This is a large shrub or small tree, with tough glossy leaves. It looks like native coprosma or tawapou. Rhamnus has distinctive dark, shiny red berries in spring-summer, which turn black.

It was once sold as a variegated-leaf garden shrub, but leaves revert readily to green.
Rhamnus is rare in Northland, but grows wild near Matakohe and may be present in a few gardens.

It forms dense stands, especially on the coast, and prevents regeneration of native forest. 

11 Needlegrass (alias Mexican feather grass)

A fine-leafed clumping grass, needlegrass grows to 35 centimetres high and is bright green in colour.

It looks identical to the equally invasive nassella tussock. Many needlegrass plants have been found in Northland gardens in recent years. It is a major potential weed on farmland, coastline and in gardens.
 
Skeleton weed.Skeleton weed. 12 Skeleton weed (alias naked weed)

A thin, wiry perennial weed, skeleton weed grows to 1.2 metres high from a rosette. It has erect ,interlacing stems and small, yellow, daisy-like flowers. It is unlikely to be found in Northland.

Skeleton weed is a major potential weed for crops, and chokes harvesting equipment. 

 

 

Managing the weed invasion

We use a multi-faceted approach when working with Northlanders to combat pest plants and stop them spreading. Education and advice to the public, incentives such as supporting Landcare groups, rules for property owners and undertaking control work are all part of the NRC strategy for pest plant control.

Like any plant, weeds have differing needs for survival. They need light, air, water and a place to grow.

Our staff members can offer their knowledge and experience to help landowners decide the best methods of controlling different weeds.

 
Spray and manual control

Using machines and labourers to cut down or remove the plants can control some pest plants. However, cutting often doesn't kill plants and in some cases may make them grow faster. Sometimes spraying is needed in awkward spots or when the infestation is too large or difficult to be reached by labourers. NRC staff can help with advice on the best methods to use. Some weeds respond best to control at certain times of year. Spraying or cutting back at other times of year might not work as well and result in wasted spray, money and effort.


Biological controls

Our biosecurity officers can also offer advice on biocontrols. Biological control uses one organism or a group of organisms (usually insects or fungi) to control another.
Insects and diseases used in biological controls have been imported to New Zealand in instances in which scientists have been able to prove that the biological control agent will only affect target plants and will not affect any others. The aim is to restore the natural balance between weeds and the environment by introducing some of these natural enemies.

Many pest plants have thrived in New Zealand because of the warm climate. However, in some cases, these plants are kept under control in their own country by tiny insects and mites. Without those insects munching them back, such plants can grow much more quickly in New Zealand and become a problem plant.

Tiny gorse spider mites live in web.Tiny gorse spider mites live in webs on a gorse
bush. These are spread with the wind.

Before a biological control agent can be released, scientists conduct extensive tests. They study the weed and check if there are any native organisms already causing damage. Testing includes checking whether the control agent will affect close relatives of the weed, unrelated plants that are similar, native plants that have never been exposed to it before, and economically important plants.

If the safety tests overseas show any damage to these other plants, then the control agent is rejected. There is no guarantee that the biological control agents will be successful, but scientists breed up large numbers before they are released to help with their success rate. Biological control agents will never completely eradicate a pest plant. However, if they are successful, the weed will be kept from spreading and may reduce infestations to a manageable level.  

 

Five biological controls for one weed - gorse

Sometimes scientists introduce a range of different biological control agents to target the same pest plant so as to get a better effect. An example would be the five different biological control agents that have been imported to target one of New Zealand's worst pest plants - gorse. They are the gorse spider mite, gorse thrips, gorse seed weevil, gorse pod moth and colonial hard shoot moth.

Gorse spider mite

These tiny spider mites can cause considerable damage to gorse plants if they reach large numbers on a bush. Gorse spider mites come from Europe, and the Department of Scientific and Industrial Research first imported them in 1988. The first batch of spider mites did not do well in Northland, so scientists found other spider mites from Spain and Portugal which have similar wet, warm climates.

These mites have been more successful and they are now established throughout all regions of New Zealand.

Gorse spider mites live in communal webs on gorse bushes. The adults are brick red in colour. The females are smaller than the size of a pinhead, and the males are even tinier and more triangular in shape. The mites have sucking mouthparts that pierce the walls of gorse foliage cells and extract the cell contents. They only feed from gorse.
 
Drawing of a gorsethripEnlarged view of a gorsethrip
Gorse thrips


Gorse thrips do not have wings and spread by walking or blowing on to neighbouring bushes. The tiny insects tend to stay in one place and infestations can take years to move only a few metres. If a bush becomes overpopulated, gorse thrips will occasionally produce winged offspring that can fly away and start new colonies.

Gorse thrips tend to be successful because they keep attacking a bush for years, and the plant will suffer.

 

 

 

 

Gorse seed weevil.Gorse seed weevil. Gorse seed weevil

These insects come from Europe and they were one of the first biological control agents to be introduced to New Zealand. Gorse seed weevils were first imported between 1931 and 1946.


Gorse seed weevils are tiny - between 1.8mm and 2.5mm long - and they use their long curved snouts to chew small holes in the walls of seedpods.

Inside the pods they lay their eggs, sometimes in clusters of up to about 20 at a time. When the larvae hatch after about a month, they feed on and destroy the gorse seeds in the pods.

While gorse seed weevils are very successful at killing gorse seeds, the problem is they can only do this in spring and summer.

In many parts of New Zealand, including Northland, gorse can flower and seed a second time when the weevil is not active.
 
Gorse pod moth.Gorse pod moth. Gorse pod moth

These little moths were brought in to attack gorse seeds when the gorse seed weevils could not.

From Europe originally, the gorse pod moth lays shiny white eggs hidden in the gorse flowers.

After about two weeks the caterpillars hatch out and chew their way into the pods. Once a pod moth caterpillar has eaten all the seeds inside a pod it will burrow back out again and find a new pod. 

 

Colonial hard shoot moth

A newcomer to the battle against gorse is the colonial hard shoot moth. This was introduced to trial sites at Ruakaka in November last year. Biosecurity officers hope the moth will become established so colonies can be introduced in other areas.

These examples show how scientists are using many different biological control agents to target one pest plant. Similar measures are being used against ragwort, which is a major problem in many parts of Northland. Biological control measures imported to New Zealand for ragwort include the ragwort flea beetle, cinnabar moth and ragwort seed fly. The latter two have not made much of an impact, but scientists hope the flea beetle will be more successful. The beetle is being distributed around the North and soon there will be sites dotted all over the region.

Biological control agents are initially released in trial sites around Northland. Once they become established, biosecurity officers distribute them freely to landowners although often the insects are successful in spreading around the region by themselves. Biosecurity officers have found that once plants have been sickened by the biocontrol agents, other insects such as lemon borer join in the attack.

More details on biocontrols are available from biosecurity officers.

Future weeds

Agapanthus.Agapanthus. There are many plants that have the potential to become pest plants in Northland - including many that are on the verge of that classification already.
These include agapanthus, which is very common throughout the district but which is causing problems in some areas where it has taken over hillsides. Gardeners are being encouraged to use varieties of agapanthus that cannot be spread by seed.

Other weeds could arrive by gardeners bringing in plant matter illegally.

Volunteers clear weeds and replant the edges of the Waiarohia Stream.Volunteers clear weeds and replant the edges of the Waiarohia Stream in Whangarei.

Hundreds of different plant varieties are imported legally every year by gardeners and garden centres. They have to go through a strict process to be allowed to bring the plants into New Zealand.

 

 

 

 

How can you help?

 

  • If any of the plants featured here are growing in your garden or your parents' garden, remove them and dispose of them carefully. Pick up a pamphlet on the weed from the Northland Regional Council, or ask for advice on how to deal with it if you are not sure. There are many other attractive plants that can be used, instead, that won't spread beyond your garden to become a problem plant.

  • Dispose of weeds carefully, as many weeds have spread when people have thrown their waste over the garden fence or on to roadsides. Problem weeds should be disposed of through composting, burying or mulching. However, be aware that some plants will regrow from fragments and these types should always be taken to a composting depot or landfill.
  • Adopt a piece of bush and monitor the kinds of weeds that can be found growing there. Remember just because a patch of bush is green does not mean that the bush is healthy. It might be that a lot of the greenery is actually weeds that are choking the native bush.
  • Get a group of friends or family together for weed-clearing working bees. Learn to identify weeds and the best ways of getting rid of them properly.
  • Pull out a few weeds each time you go for a walk through bush or on the roadside. Take care to remove them properly and dispose of any spreading kinds carefully. Carry them home in a plastic bag, if necessary, to be disposed of in an appropriate way.
  • Join a Landcare group, or form a new one if there is none in your area. Contact the Northland Regional Council or the New Zealand Landcare Trust for details on landcare groups and how they operate.
  • If you find patches of ginger, snip the flower heads off before they go to seed. Pull very small seedlings out of the ground making sure that the entire plant is removed. Flower heads that have gone to seed should be removed and put in a plastic bag. Each flower head can form about 100 seeds, each of which can become a new plant. Bigger clumps can be dug out or sprayed with Escort. 
     

Sources: NRC staff, The Biological Control of Weeds Book, Northland Regional Pest Management Strategies, NRC Unwanted Plants - the Dirty Dozen pamphlet, NRC Environmental Weeds - Delightful But Destructive pamphlet, NRC Pest facts pamphlets.