Pest Plant Control

Willow control in the Te Henga (Waitākere) Wetland

Over the past few years, it has become evident that one particular pest plant, the European or Crack willow, is threatening our homes, built infrastructure, and the health of Te Henga wetland because their roots trap soil and create blockages in the riverbed. Read more about the why, how and what here:

Impact of Willows on our Homes, Roads and Services

Willows trap silt and debris around their roots and trunks. This blocks river flow and raises the water level, making flooding and damage to infrastructure worse. During significant heavy rainfall, the Waitākere River rises and bursts through the willow blockages with a major increase in force and speed. Removing the willows will significantly reduce obstructions in the river and slow the flow of the water, resulting in reduced damage to homes, infrastructure, and habitats.

We in the Waitākere Valley have experienced serious damage from floods in the past few years. Severe flooding has caused destruction and loss to people’s property, we’ve lost the Surf Club, the road and other services, and has had serious impacts on people’s lives as well as causing millions of dollars’ worth of damage.

We can’t stop the rain, and the flooding will happen again, but removing the willows will help lessen the flood damage.

Wetlands are now rare and those left are in poor condition

Of the original wetland area that existed in the North Island of pre-settlement Aotearoa New Zealand, only 4.9% remains, one of the highest extent of wetland loss in the world. It is imperative to restore what remains of our wetlands to good health. Te Henga Wetland, the largest mainland wetland in the Auckland region, is a very valuable taonga.

Wetlands provide essential ‘ecosystem services’. These include sequestering carbon and returning nitrogen to the atmosphere, protecting and improving water quality by filtering water, storing and slowing floodwaters, replenishing groundwater aquifers, and supporting an immense variety of native microbes, plants, insects, amphibians, birds and fish, many of which are endangered.

Native birds: Wetlands are home to many native species

Willows spread aggressively and outcompete native wetland plants, as well as degrading the habitat for wetland species such as pāteke (brown teal), mātātā (fern bird) and pekapeka touroa (long-tailed bats).

The matuku-hūrepo (Australasian bittern) relies on the Te Henga Wetland for its survival, and with fewer than 700 individuals remaining in New Zealand, it is even rarer than the brown kiwi. Matuku, as visual hunters, need clear water to spot their prey. Raupō (bullrush) and other native wetland reeds filter out silt and keep the water clean. Areas of willow do not maintain the same optimal habitat for matuku, and these birds struggle to find sufficient feeding areas and prey in murky water. This poses a serious threat to their survival.

Frequently Asked Questions

There are several FAQs below in which we consider different aspects of willow control. We hope this information will allay any concerns.

Q1: Why are willows in Te Henga a pest or a problem?

The pest willow species in Te Henga wetland is the crack willow (Salix fragilis). It traps silt in its roots, creating islands in the stream bed. In flood, this is a hazard as the water will try to find a new path. This new path will be onto roads where it floods in several places, or onto people’s properties. The willows can act as a ”plug”, with the force and volume of water building up behind it until it crashes through with much more force than if the water could have flowed naturally, thus causing more damage.

Q2: How can we remove or get rid of willows in the Te Henga wetland?

A: There are four methods used in NZ, each with their own benefits and issues of concern.

Method	Benefits	Concerns
Cut & Burn or Chainsaw	Precise, individual targeting of willows	Very labour intensive Difficult to access willows in deeper water Hazardous for the operators All cut stems have to be removed or they will regrow Leads to other invasive weed growth following control Expensive
Drill & Inject	Precise, individual targeting of willows	Slow, very labour intensive Higher concentration of herbicide and more herbicide required Hazardous for the operators Expensive
Helicopter Spraying	Precise application Requires 10 to 20 times smaller amount of herbicide than Drill & Inject No negative effects to water quality or macroinvertebrates Significant increase in native plant growth following death of willows as shown by 3 years of monitoring in Te Henga wetland by Boffa Miskell	Noisy at the time of operation Risk of helicopter accidents Minimal spray drift is expected if operation is undertaken in ideal weather conditions – needs low to no wind, ideally early morning Te Henga community doesn’t like the sight and sound of helicopters
Drone Spraying	Precise application Requires 10 to 20 times smaller amount of herbicide than drill & inject Uses exact amounts of measured herbicide, minimising environmental contamination Minimal spray drift Can fly very low and target each tree individually Quiet Efficient A drone is much less intrusive and can do more precise spraying than a helicopter	More costly than helicopter spraying Operation needs to be done in ideal weather conditions – needs low to no wind, ideally early morning

Q3: How often do we need to undertake willow control?

A: We’re aiming to control all willows in the Te Henga wetland in one operation timed for spring/summer. After the first year of control we expect there to be a small “mop up” at the end of the summer to treat any willows that were not controlled in the main operation. Monitoring post-operation will continue for two years to detect any reinvasion of willows and to measure the impact on water quality and biota.

Q4: What about the spray drift from the herbicide?

A: The herbicide to be used is Polaris 450, a glyphosate formulation approved for use over water. Because the drone will be flying very low over the foliage and in low wind conditions, very limited spray drift is expected. New Zealand Food Safety’s view is that there isn’t a risk to humans where this herbicide is applied at concentrations complying with the New Zealand maximum residue levels. The method used here complies with this requirement. For more information on glyphosate, head to the bottom of this page.

The herbicide is a formulation registered for use over waterways.

Q5: What has been done about willows in the Te Henga Wetland in the past?

A: In the late 1980s local contractors eradicated about 20 grey willow (Salix cinerea), weeping willow, and controlled crack willow alongside the river in the upper part of the wetland by drilling and injecting trunks, and painting stumps with glyphosate.

Since early 2000’s people have continued to “drill and inject” willow trunks in an attempt to control large areas of crack willows. However, the willows were found to grow faster than they could be dealt with.

This led to the then Waitakere City Council and Rodney District Council helicopter spraying glyphosate in a part of the Mokoroa Arm in 2009. This successfully removed willows from the target area but opposition from a few local residents led to this programme stopping.

Boffa Miskell was contracted to monitor the potential impacts on water quality, fish life and macroinvertebrates from the spraying, and reports in 2011, 2012 and 2013 showed no harm to water quality or biota.

Download report 2011 part 1 and 2011 part 2

Download report 2012

Download report 2013

Q6: How will we protect desirable or non-target plants (native trees, private owner’s plants)?

A: Willows next to desirable plants will be treated with the ‘drill and inject’ method to avoid any possibility of spray drift onto desirable or non-target plants.

Q7: Are council, iwi and landowners supporting willow control?

A: Yes, all landowners with willows in the Te Henga wetland on the attached map (the light yellow areas are willows), support willow control to protect their properties from future floods and help establish a flourishing wetland. Te Kawerau ā Maki, mana whenua for the valley also support the programme. Landowners can voluntarily remove willows (and other weeds) from their own property. Auckland Council has granted a Permitted Activity Notice to control all willows in and around the wetland.

Q8: Who pays for this?

The Department of Conservation has funded the Matuku Reserve Trust to remove willows to restore the wetland as habitat for endangered and threatened species including matuku-hūrepo (bittern), pāteke (brown teal), mātātā (fernbird), and pekapeka tou roa (long tailed bats).

Q9: What about effects on honey bees?

A: Honey bees collect both nectar and pollen from willow flowers. Crack willows flower during September and October, and spraying will not happen over this time. The surfactant will be Aquakynde which makes the herbicide stick to the willow leaves – there is a risk that bees can get stuck as well. Spraying of herbicide will only take place on dry days with medium to low humidity when it ceases to be sticky after one hour to a maximum four hours. Spraying in early morning still air, when bees are less active also minimises the risk to bees.

If you keep bees in the valley, please notify the Matuku Reserve Trust at hello@matukulink.org.nz and we will keep you informed of the control operation. You can take appropriate measures such as closing the hive or if possible move it further away from the wetland.

Q10: How will spray drift be minimised?

A: Spraying will only occur in the right weather conditions to avoid spray drift (low to no wind). This will also avoid wasting herbicide which is expensive.

Low flight elevation of the drones (one meter above the canopy) maintains a limited spread of the area sprayed. The drone operator will always have visual control over the drone. The drone has specific low drift air induction nozzles to create big droplets, not a mist. The droplets will be medium 32 drops per cm to coarse 35 drops per cm or 200 – 300Mu. with maximum drift of 1 m with air velocity of 5kph and height at application of between 1m to 3m.

Q11: Who will do the work?

A: Phylogeny Ltd has been contracted to do the work. Manager David Hall lives locally and has significant experience in wetland drone spraying. The drone will need 4 – 6 hours of fine weather (no rain, low wind speeds and low to medium humidity) over approximately 20 flying days. A temporary pontoon will be used in the wide parts of the wetland as an emergency landing platform and refilling site, with one or two “spotters” who will keep visual contact with the drone to check its path and keep an eye out for other potential risks. Phylogeny’s Chief Pilot is Part 102 Certified. Phylogeny’s Chief Pilot Has current Pilots Chemical rating, and is a qualified person to apply herbicides over or near waterways. More information about Phylogeny here.

Q12: Are all willows bad or a problem?

A: No. All willow species have been introduced to New Zealand since the 1930’s. The Latin name for willows is “Salix”. The invasive unwanted species are Crack willow (Salix fragilis) and Grey Willow (which has been eradicated at Te Henga in the eighties). These are invasive and are on the Auckland Pest Plant register.

The other willow cultivars are not a problem.

Q13: Where are the targeted willows in the wetland?

A: The yellow areas are willows – all downstream from 127 Bethells Road. Most willows are in the north-west corner in the Mokoroa stream, which is where most of the work for 2025 will be done.

Q14: What will happen to the trees?

A: The willow trees will die while upright and slowly rot away – they will not end up as slash on the beach like pine trees. This approach has two key benefits: it minimises the chance of broken willow branches taking root in the wetland (a significant risk with manual or helicopter removal) and provides time for any species living in the willows to relocate to other trees.

Q15: What products will be used?

A: The herbicide used is Polaris 450, a glyphosate formulation approved for use over water. For drill-and-inject Glyphosate 510 and Glyphosate Dry 800 WSG will be used. The surfactant is Aquakynde or an organosilicon. A blue marker dye is also used to clearly show the drone operator and spotters where the herbicide has been applied. These products have been proven to be very effective for willow control in wetlands with no detrimental effects on water quality or native biota. See Boffa Miskell monitoring reports in Q5.

The products will be used in diluted form, in accordance with guidelines for use over wetlands. For examples of successful willow eradication in Aotearoa, see the Further research section at the bottom of this page.

Further resources:

Boffa Miskell Vegetation and Aquatic Ecology Monitoring Reports following helicopter spraying on the Mokoroa arm, Te Henga

Download report 2011 part 1 and 2011 part 2
Download report 2012
Download report 2013

Willow control:

Glyphosate:

Effects of willow spraying in other wetlands

Effects of willow removal in the Para wetlands
Images showing effects of willow control in the Pekapeka wetland:

Pekapeka wetland before willow control

Pekapeka wetland after willow control

Other resources:

Wetland Restoration. A handbook for New Zealand freshwater systems: Manaaki Whenua Landcare Research