Poplars and willows for erosion control
Sleeved kawa poplars helping stabilise up a pastoral hillside.
Funding support for soil conservation initiatives – 2016
Northland Regional Council, as part of its soil conservation initiatives and Waiora Northland Water Programme, offers a limited number of poplar poles and willow material each winter to help prevent and control erosion in Northland.
These are partly funded through the council's Environment Fund with the balance being picked up by the landowner who carries out the planting and establishment.
Poplars and willows are exotic fast-growing species ideally suited to both prevent and control erosion, due to their extensive and deep root network. Current varieties distributed via regional councils and private nurseries have been selected and developed in New Zealand over decades to promote desirable characteristics of erosion control, fodder, decreased palatability to possums, drought tolerance and even timber. Problematic traits such as vigorous suckering, brittleness of timber, and poor growth form have largely been eliminated.
The plant material is provided to the recipients if they agree to have a planting plan produced for their erosion issue. They also must agree to maintain the planted trees, including protecting them from damage by stock, ensuring they remain firm in the ground and pruning and thinning them in future years so that they continue to perform their intended soil conservation function.
Numbers are limited and in order to qualify for the subsidy a planting plan must be in place before planting season (June – early August).
Interested individuals, groups and organisations from the Northland region are encouraged to contact the council's land management team to indicate their requirements.
When it comes to planting your poplar and willows poles, getting things right at the beginning will give you the best results in the long term.
In Northland, the best time of year to plant poplar and willow poles is during the winter months of June, July, and early August. Remember to plan ahead and allow a few months for ordering, delivery and creating a site plan. To order poles through Northland Regional Council, get your orders in before the end of May of your planned planting year.
It's best to plant before any erosion occurs – prevention is better than cure. After an erosion event it can take up to 20 years for an area to recover, but never to the level of pre-erosion. A little foresight can save a lot of time and effort in the long run, and prevent loss of production.
Willow plantings can be used to help stabilise stream banks, as their fibrous root systems help knit and bind the banks. Poplars should be planted a little further back from the edge of the bank.
Planting densities range from one to 20 metres apart with consideration given to the severity of the erosion and long-term maintenance.
It's important to think before you plant – your poplar and willow poles will develop into large trees so placement is key. If planted too close to the stream (with the exception of shrubby willow) the trees can, over time, become too heavy and collapse, causing further erosion.
A good rule of thumb is to set willow planting back at least three metres from the edge of your bank, with poplars further back.
Poplars are perhaps the best species for stabilising unstable hillsides and slips, and it's best to tackle erosion early.
When planting on hill slopes:
- For gentle slopes, space plants at 10-12 metre spacings
- On steeper slopes, use 5-6 metre spacings
- For earthflows, slumps and slips, plant poles at 5-10 metre spacings, using closer spacings at the toe of the slope and wider spacings towards the head
- Avoid planting on ridges or high spots as the trees can be damaged by high winds
- Choose the best site for each pole – look for depressions and low spots, small channels where water flows or pools as these are spots where erosion is likely to occur and where poles will thrive
- Avoid exposed windy sites – instead plant part way down the slope at 8-10 metre spacings.
Willows are perhaps best for controlling gully erosion, but poplars also have their place. It really depends on the type and size of gully erosion you are trying to control.
It's best to plant poplar or willows poles in pairs. Pair-planting causes the trees' root systems to overlap across the gully, preventing further down-cutting of the gully bottom and slumping of the sides.
Before you start planting, put protective sleeves on your poplar or willow poles – this increases the survival rate of the poles and makes it difficult for possums to climb them. Made from recycled plastics, the sleeves also protect the poles from stock damage and reduce moisture loss.
There are a couple of options available for use. Sleeves are available from the regional council – contact a land management adviser on 0800 002 004.
On a three metre pole, slide the sleeve on at the butt end and make sure the bottom of the sleeve is 60-70cm from the butt end. Some sleeves are designed to split and fall away as the tree grows – if this doesn't happen, remove the sleeves carefully when they become too tight.
Cattle should be excluded from planted areas for at least 12-18 months to allow time for root development. Cattle rubbing or scratching up against the poles will cause root damage and the likely death of your pole.
Land management advisors are available to offer advice on:
- Siting and planting plans
- Existing tree identification
- Crack willow eradication removal advice
- Replacement advice for existing trees in sensitive areas
- Mixed poplar/willow/native plantings.
Dynex© Tree protector sleeves designed to protect against stock and possum damage are available for purchase and are not subsidised. These are available in 1.7m lengths and are made of recycled plastic with easy-tear perforations that split for removal as the trees grows. Sales are available via the council's Water Street office. Check current pricing for these when ordering your poplar and willow poles online.
A frequently asked question is, do all pole plantings require sleeves? The simple answer is no. If for example you plant poles behind a fence and stock are excluded and provided the poles are planted firmly in the ground, your chances of pole survival will be high. Other considerations for sleeves are whether possum numbers are high in your area? Could sheep or goats have access? A few of the benefits of sleeves are listed below.
Benefits of Dynex Sleeves
- Poles are less susceptible from rubbing by light stock, however its best to exclude any stock except sheep for the first 2-3 years of establishment.
- Smooth enclosed sleeve deters possums climbing up.
- Deflects light debris during floods.
- Bark cannot be chewed by stock, sheep or goats.
- Conserves moisture in pole during dry months
Harvested poplar poles ready for planting.
Interested individuals, groups and organisations are encouraged to contact:
Land Management Advisor
Poplar and willows coordinator
Northland Regional Council
Phone: 0800 002 004