Lessons from a plant barbecue
Get the right plants, and you give yourself a big advantage in a forest fire.
Dr Tim Curran has a special barbecue, but he doesn’t use it for cooking his favourite sausages. In the great Kiwi tradition, it’s a 44-gallon drum cut in half lengthways with a gas grill that maintains the temperature at 150°C.
The senior lecturer in ecology and his team use the ‘plant barbecue’ to test the flammability of plants, roasting over 200 native and exotics so far.
Finding out the flammability of common plants is vital as NZ starts to experience climate change, which is bringing longer, drier summers to many regions. NZ has already had two major fires in rural areas in the last two years: Christchurch’s Port Hills in 2017, and the Tasman District in 2019.
Fire & Emergency NZ is encouraging people to take long-term steps to protect their blocks, such as protective landscaping and year-round maintenance.
One option is low-flammability plants which might slow down a fire or protect a property in some situations.
Tim Curran’s team from Lincoln and the University of Auckland is working out which plants are best. They heat 70mmthick cuttings on the plant barbecue, then ignite them with a blow torch.
“This process mimics fire approaching a plant, initially drying off moisture as it gets closer, and then igniting it,” says Tim. “During the experiment, the researchers recorded the ignition time; the maximum temperature at which a sample would burn; how much of the plant burns; and the total burn time.”
They found that applying a blow torch to pre-heated branches of manuka, kanuka or eucalyptus quickly drove out flammable oils, and the resulting fire was almost explosive.
Of the 60 species tested in the original experiment, both natives and exotics, 27 showed low flammability.
See page 28 for options
But Tim’s team found it wasn’t as simple as finding species which did or didn’t burn. Their work showed the flammability of cultivars and sub-species of common plants could be quite different. For example, flax (and mountain flax) has a low flammability level, but a variety called Dark Delight didn’t burn on the barbecue at all.
It will also depend on the condition of your plants, says Tim. Young, well-maintained flax plants are more difficult to ignite than older plants, which tend to have more straggly, dead leaves at their base.
There are many factors which lower a plant’s flammability level. These include:
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the stage the plant is in its life cycle;
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how much dead material is trapped or retained in its structure;
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the moisture content of its leaves;
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the presence of volatile oils;
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if the plant is stressed by drought.
How flammable a plant is will also depend on its condition.
Replanting Tasman’s farmland
Publicity around low flammability plants doesn’t seem to have made a significant impact on people affected by the Tasman fire.
Oliver Dean, a director of Dean’s Nursery on the outskirts of Richmond in Tasman, says his customers just wanted the landscape ‘greened up’ with trees and shrubs as quickly as possible.
“In many instances, they’re not fussed about the plants being fire-resistant,” he says. “They just want quick, tough shelter. They want it looking green again.”
Some people were more conscious about what they were planting, he says, but they still wanted manuka and kanuka. For many, the stronger imperative is resistance to drought. Landowners are taking note of the changes in climate and looking for plants which will withstand a long, dry spell like the one they experienced in summer 2019.
Tim Le Gros, from Titoki Nursery in Brightwater, is also aware of the plants which have been identified and discussed since the fires.
“We have had some inquiries from the public wanting to plant fire retardant species nearer to housing. However, on a large revegetation scale, it’s important not to skew the natural forest composition by completely leaving out (more flammable) species such as manuka and kanuka. They’re important species in the ecosystem.”
Tim Curran agrees.
“There will always be a place in restoration for flammable species like manuka and kanuka. It’s just that we should be careful about where in the landscape we plant them to minimise the fire hazard.”
Tim suggests planting low flammability plants on the outer perimeter of higher flammability plants. Another option is to intersperse both types to reduce the possibility of flames taking hold.
Fire is less likely to travel horizontally through well-maintained flax with spaces, preferably soil, between each plant.
Plants which help a fire track upwards into the canopy of trees are classed as ‘ladder fuels’ (see page 30) and can start a crown fire, which is more severe.
“There are a whole host of decisions to be made when planting,” says Tim. “We just ask that flammability is one. We aren’t telling people what to plant; we’re just providing the information, and they can make a decision based on it.”
Ian Reade is the principal rural fire officer for Fire & Emergency NZ in the Tasman-Nelson region. His team, including volunteers, are encouraging rural people to think about how they can protect their properties from a big fire.
Ian was on the frontline fighting the Redwood and Pigeon Valley fires and says they were a huge wake-up call for many people.
“We are entering a new era. Climate change is very much part of our planning and is one of the principles behind what we are doing now.”
Australian and US forests need fire to regenerate and trigger new growth. New Zealand forests and bush don’t, says Ian. Americans and Australians are also brought up to be more prepared in their fire-prone environments. That’s a rare attitude in this country, says Ian.
“People are more vulnerable because they’ve never been in this situation before.”
Ian wants people to build resilient communities. These are where landowners carry out planting and landscaping with fires in mind, and routinely practice good housekeeping to keep their properties safe (see pages 29-30).
Most people think of flames roaring up to their home... but the greatest risk is from embers, which can fly up to 2km from the main flames.