Willow control programme underway

A large expanse of willow invading raupo bedsTe Waihora is a wetland ecosystem with significant natural and cultural values, yet invasive willows are now present within nearly one third of the lakeshore freshwater wetlands. Grey willow (Salix cinerea) in particular, is a challenge on the west side of the lake, where the area of exotic crack- and/or grey willow-dominant forest and scrub vegetation was found to have doubled, from 67 ha to 140 ha, over a 25 year period between the early 1980s and 2007 within the areas surveyed twice. Altogether, willow-infested vegetation areas (sparse willows through to close-canopy forest) were found to have invaded around 170 hectares as of a 2007 vegetation survey.

 

willow control 11Grey willow is considered one of the top ten most invasive weed species in seven of the thirteen Department of Conservation conservancies around New Zealand and willow spread has been one of the main causes of reduced extent of native freshwater wetland vegetation around the lake shore over the last 25 years. The 2007 survey also identified sites where willows are now spreading into native freshwater wetland vegetation but are not yet dominant. Vegetation at risk from further spread includes any of the remaining flax and raupo stands.
For Frances Schmechel, senior biodiversity advisor for Environment Canterbury and manager of the Immediate Steps Flagship Programme, Te Waihora (initiated by Canterbury Water Management Strategy), the ongoing challenge is to better manage the spread of willow around the lakeshore wetlands. Now in its third year, the programme is targeting grey willow and, to a lesser extent, crack willow (Salix fragilis), and Frances reports that around 200 hectares of vegetation with willows in or adjacent will have been controlled or sprayed.

“We have a budget that we work to but we try and achieve as much as we can within that. Most of the willow we are targeting is located around the west side of the lake and we have a strategy for how we attack it based on eliminating scattered pockets of willow and willow stands that are invading native vegetation.

There are many species of introduced willow trees in New Zealand but these two – crack willow and grey willow (also known as pussy willow), can be a serious environmental problem in wetland habitats, on lake edges and in riparian planting zones, especially where they threaten existing native vegetation. Crack willow favours riparian zones, lake edges and wetlands associated with rivers and lake margins. Grey willow also grows in these locations but also invades a greater range of wetland habitats.

Frances says the control measures taken depends on the location and size of the willow stand.

Staff monitoring willow at Wards“For broad spectrum cover of large dense stands we spray the common and safest herbicide, glyphosate from helicopters. We can also spot spray from helicopters using a wand or nozzle; and DOC ground crews are also used every year, to work across a range of methods from stem drilling and poison and cutting and pasting (poison on the stumps) to knapsack spraying.”

willow control 1

Typically, it takes two to four years to achieve total control in an area but reseeding is often a remaining problem if adjacent landowners are not controlling willow on their land. To gauge progress, monitoring is undertaken to assess how well willows have responded to spraying and if there have been any changes in native vegetation.

“The control problem has been a challenge given the size of the problem but it’s great to be doing something. It’s a huge step forward in terms of addressing the problem and it’s great that we have a strategy in place now to focus on the most important areas. It is an ongoing issue but we are slowing the worst of the spread.”

The Environment Canterbury team contacts landowners and works with them to develop a strategy for willow control and Frances says the response is generally favourable. It’s important she says, that site conditions such as human and livestock access and the possible requirement of resource consents are considered before any work is undertaken.
“We also need to ascertain what functions the willows have been performing on the site, such as bank stabilisation, weed suppression and shelter; what species they are and what problems they may be creating,” says Frances.

“At some sites, rather than the wholesale removal of willows, it may be better to allow a willow canopy to remain to facilitate natural regeneration of native plants, or survival of plantings. Selective removal of female grey willow or staged removal of canopy willows can take place as the native understory grows.”

Anyone interested in willow control round the Te Waihora/ Lake Ellesmere lakeshore is welcome to contact the Biodiversity Team, as they may be able to provide assistance.