A mix of willows and poplars planted for erosion control east of Dannevirke. Photos courtesy Mair Owen, Horizons Regional Council
Willows are the workhorses of rural New Zealand's landscape and often an unsung hero, says Plant and Food senior scientist Ian McIvor.
McIvor is also general manager of the New Zealand Poplar and Willow Research Trust.
"Every day willows are working on our behalf around New Zealand.
They protect river banks by acting as a buffer to absorb flood water and deflect strong currents from the river edge, stabilise hill sides and gullies, provide fodder for bees and stock in times of drought, as well as shade and shelter, not to mention enhancing the landscape."
He says it's important to get the right variety of willow for the right job.
"Regional councils are in a good position to provide advice and there are fact sheets, frequently asked questions and how-to videos available on the poplarandwillow.org.nz website. The best thing to do is consult your local regional council or a commercial nursery near you for more advice on clone selection for particular sites or purposes.
The experiences of others can save you making bad choices."
Willows are versatile trees that can be established from cuttings, in species ranging from large trees to small shrubs, and grow in a wide range of habitats.
"They are tolerant of periodic flooding, are great at drying boggy areas and have an extensive root system."
Willows can help farmers meet environmental targets around reducing nutrient loss and reducing water pollution.
"All farmers can empathise with this. Meeting ever more stringent environmental standards and especially around improving water quality is a reality of doing business today and here willows can be a really powerful tool in so many ways for stabilising streams and riparian planting. They are easy to plant and establish and their root system binds exposed soil.
"At the same time, they provide a great nursery environment where other less adaptable trees and shrubs can establish, survive and grow."
Willows shade out aggressive competing species such as grasses and blackberry and give a shady, high-humidity environment for native plants.
"At the same time they help filter subsurface N and P run-off, shade and cool the water, feed stream organisms from leaves and leaf animals falling in to the stream [about 20 per cent of fish diet] and their shade improves stream habitat for fish and aquatic insects."
McIvor says both willows and poplars have been employed in field trials to better manage dairy shed effluent.
"This may well become an important low cost technology on some dairy farms where surface and subterranean run-off into water bodies is a significant risk.
Tree-pasture systems offer potential environmental benefits over straight pasture systems and can supply supplementary fodder - by browsing and/or cut and carry - on smaller land areas.
Animal welfare is another issue where willows can help farmers. Willow shelterbelts establish quickly, grow fast, are cheap to replace if deaths occur, are easily trimmed and provide ready drought forage. Willows are particularly useful as east-west shelter because their deciduous nature minimises shading through winter. The tree bark is easily protected from browsing behind an electric fence.
On dairy farms, cows in sheltered areas have up to 17 per cent estimated increase in dairy milk production (sourced from Dairy Australia).
"Cows also appreciate shade in the height of summer and utilise any available shade.
Shade provided by trees reduces animal heat stress and reduces feed requirements.
Australian research has shown that on a 27 degree day unshaded cows have 26 per cent less milk production than shaded cows. Trees can reduce summer heat load in cows by 50 per cent."
Willows provide a highly palatable fodder source for stock in times of summer and autumn feed shortage.
"Willow fodder is nutritious for all stock types. Cattle strip leaves and bark and eat stems up to 10mm diameter, while sheep can manage leaf and small stems up to 5mm diameter. Although for this reason, stock should be kept away from newly planted poles."
He says willows grown for summer fodder are best managed with regular pruning (called pollarding for isolated trees) and quickly regenerate new stems.
"Pollarded willows harvested every three to four years maintain their vigour for years. Mature pollarded willows maintain root networks sufficient for soil conservation and continue to offer shade for stock."
Both poplars and willows are popular choices for slope stabilisation on blocks with hilly terrain and eroding gullies.
"Willows are planted in gullies because they develop a fibrous root mat that separates soil from eroding water.
Increasingly, dairy conversions are taking place on hilly land while in some regions dairy cows and heifers are grazed on run-offs that are in hill country areas and many young dairy bulls are finished on hill country.
Preservation of these land areas from erosion is important both in terms of avoiding loss of valuable soil as well as in maintaining soil quality. The risk is greater when the land is intensively stocked with heavy animals."
The superior ability of poplars and willows to stabilise soils on slopes and gullies as well as around streams compared with native species is due to their much greater root length at the same age. On farms, willows can be used to dry out troublesome wet areas and improve pasture quality while being managed for tree fodder.
"Willows are efficient because they adapt to wet soil, have a high evapotranspiration rate when in leaf, their lateral root extension reduces the effects of compaction and their size is easily managed by coppicing or pollarding."
Willows provide bank protection and flood management for rivers all over the country.
"Many farms would be inundated if it wasn't for willows stabilising river banks, slowing water flow and protecting stop banks from the full brunt of the current in flood."
Willows also play a key role in reproduction and population growth of bee colonies after their winter rest. By summertime every year, a bee colony must reach the population size needed for pollination services and honey harvesting. Wherever willows are abundant, bee colonies rapidly build up to maximum strength because willow pollen is plentiful and easily accessible to bees.
McIvor says the arrival of the giant willow aphid is problematic for willows.
"The aphid is now widespread through the country and is adapting well to our climate.
Researchers are working on solutions; biological control of the aphid coupled with development of willows that are less favoured by the aphid. In the meantime, we must manage this pest as best we can using non-biological options."
A serious previous pest incursion, the willow sawfly, which repeatedly defoliated our tree willows 15 to 20 years ago seems to have reached an equilibrium.
"Its devastating effects are rarely seen across the country now. We must hope as a result of our research and other factors that the giant willow aphid will also reach such equilibrium."
The New Zealand Poplar and Willow Research Trust was formed to ensure financial funding for research in breeding and applied science for poplars and willows for public good. The trust's poplar and willow breeding programme is committed to initiatives that maintain the productive potential of hill country and the protection of flood plains and vulnerable urban areas.
Three main willow types are grown in New Zealand.
Tree willows, such as Tangoio, are grown for soil conservation, shade, shelter, fodder and stabilising river banks. They grow up to 20m tall mostly with a single trunk that can grow to 90cm in diameter.
Osier or basket willows grow as medium-sized shrubs (such as Booth, Irette, Kinuyanagi and Salix viminalis Gigantea). They have multiple stems.
Sallow or shrub willows are low shrubs or small trees with multiple stems and stout branches, such as pussy willow. Osier and Sallow willows are used as windbreaks and for slope, roadside and stream bank stabilisation and as pollen sources for bees.
Basketry is another ancient use for willows - a practice that is still widespread. Certain varieties (Salix tiandra, Salix viminalis, Salix purpurea) are coppiced in midwinter when three years old.
"Then there is willow for cricket bats," McIvor adds.
In the 19th century James Crowe took 40 years to select the one willow (Salix alba var. caerulea) to make the consummate cricket bat. It must be grown the right way in the right conditions - even today, only 5 per cent of the caerulea wood offered is accepted by leading bat makers.
There are 93 horses still facing an uncertain fate.