The Art Of The Conservation Bonfire

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Part 1: When To Have A Bonfire, Knowing Your Fuel, And Choosing Your Bonfire Site

Choosing your bonfire site

Open fires are, of course, always great fun. At least this is true for anyone born and brought up with gas central heating and limited opportunities to play with fire. If since the age of 7 you've had to scrape the ashes out, fetch coal from outside or sat all winter with your front half warm and your back half freezing cold then you might have slightly less enthusiasm for the traditional way of heating your home. But a bonfire outdoors is very different from the fire you might use to heat your front room and on dull and damp winter days almost everyone appreciates it. This article is about the conservation bonfire; it's not really the same as the camp fire or the fire you might have on Bonfire Night, because usually you want a conservation bonfire to burn as quickly as possible rather than as long as possible, you have less-than-ideal fuel and it's likely to get larger throughout the day and need active management. For many conservation volunteering projects in winter, a blazing bonfire is the focal point of the day's project. It can be the place where volunteers working in different areas meet, where some of them work all day, and often where they gather for tea or lunch breaks.

When To Have A Bonfire

Coppicing products and brash bonfire

Bonfires serve several purposes in conservation work. When habitat is being managed through scrub clearance, coppicing or pollarding, bonfires can be used to get rid of the bulky brash, the leaves, and small branches. If left, this would eventually break down and enrich the soil. For chalk grassland the low nutrient levels are important to prevent any one species becoming dominant [1]; this would reduce the opportunities for rare wild flowers to grow and instead increase the rate of scrub re-growth.

In coppice woodland, piles of dead scrub can block out the light, preventing re-growth, and obstructing paths. Some small sites being managed both for wildlife and public access simply have no room for big stacks of material and it would be a lot of work to move it off the site. Furthermore, on some sites large piles of scrub left behind can dry out over summer and catch fire in an uncontrolled way, perhaps assisted by vandals. Large piles of scrub are often of low habitat value in any case, particularly when they are not native species - precisely the sorts of things conservation volunteers are often asked to help remove.

When working on a site with ancient landscape features, it might be desirable to leave the view unimpeded by dead scrub. If there is potential archaeology, then that must be protected; it is best to remove cut material rather than allow it to rot and enrich the soil so that the re-growth sends down big roots.

Sometimes a bonfire is necessary to completely destroy a particular type of plant to prevent it seeding or because it is poisonous. Ragwort will be familiar to many people as a hazard to livestock, particularly when dried out. If willows are being removed to prevent their encroachment onto wetland, then it could be very counter-productive to leave the cut branches on the ground where they fall. They will simply start to grow and perhaps cause more encroachment in the longer term.

Some would argue that bonfires are over-used by conservation volunteers, beyond what makes sense for nature conservation. In some cases perhaps this is true, but providing it is not damaging, a bonfire can still be justified to keep volunteers warm and happy and provided with tea and baked potatoes.

Knowing The Fuel

No two bonfires are ever quite the same. Weather conditions vary, such as the amount of rain and wind. The site is very important, particularly whether it is sheltered or exposed and whether it is even a safe area to have a bonfire in the first place. Above all though, different types of material burn in very different ways and it's important to take this into consideration. There's a well known old rhyme about the best types of logs to buy:

But although people often quote this, it doesn't really help conservation volunteers with their bonfires. Firstly, you don't usually get much of a choice of wood; you have to deal with whatever is there. Secondly, it's usually just been cut so it's almost always green, and green wood burns very differently to seasoned wood. Thirdly, it's usually the side branches and brash you're asked to burn, rather than the logs. Finally, it often isn't even trees at all that you want to burn but brambles or invasive non-native plants.

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Here are some of the things you are likely to encounter doing conservation projects in the south of England and their characteristics for bonfires. The star rating is an estimation of overall ease of disposal when burning green in bonfires, not the value as fuel for the home.

Ash ***** Hazel Beech Oak Sycamore Birch ****	These all burn very well indeed, even when green, particularly Ash. Ash and Hazel are often coppiced for nature conservation and the materials have a range of uses, so as a conservation volunteer you just have to be careful not to get carried away and burn material that you should be keeping for stakes or cord wood. Oak is very slow growing and valuable, so it's rare to cut much of it. If you have forgotten the newspapers and fire-lighters then Birch bark is very useful and can be easily lit with a match. People like to cut down Sycamore because it may not be a native species [2] but the logs are good quality and shouldn't be wasted.
Yew *****	The branches of Yew trees make a very satisfying noise as they burn and give off a thick black smoke. You need to be very careful of the sparks and avoid choking in the smoke. The logs are highly prized for burning or wood turning. Traditionally it was mainly planted in churchyards rather than on farmland, because it is poisonous to cattle (Holywell Cemetery in Oxford is a good example of a churchyard containing a lot of Yew). That's not the only place that conservation volunteers find it, since sometimes there is woodland made of little else and it's deliberately removed to increase diversity. This has been done by the National Trust at Watlington Hill in South Oxfordshire.
Willow ****	This is so fast growing that it is often grown for firewood, but it isn't useful for much else because it's a very soft wood. It's much better when it's been left to season, but we don't usually have that luxury. It's regularly burnt by conservation volunteers as a result of pollarding, and presents few difficulties.
Pine *****	Sometimes conservation volunteers get involved in clearing areas of Scots Pine plantations in order to allow native species to return. Supposedly, Pine branches can explode when the resin in them becomes very hot. Pine makes a satisfying noise when it goes on the fire and burns quickly, but it can spark a lot. Pine cones are excellent for starting fires; if they're dry you can light them with a match. Deep beds of pine needles can create the risk of fires spreading, particularly after a spell of dry weather. In fact, it's very possible to accidentally set fire to a bed of pine needles without realising it, which is unlikely to get your group of volunteers invited back to a site.
Hawthorn Blackthorn ***	In Oxfordshire these are seen a lot on chalk grassland and they are used very widely in hedging. Although the wood burns very well indeed, the structure of these trees mean that the fuel often can't be got near enough to the fire. If you pile a lot of them on, the middle will burn out and the tangle of thorns will prevent you even noticing that the fire is starved of fuel until too late. You'll have to pull off the branches and start again. But when the fire gets really hot, Blackthorn branches will disappear as soon as you put them on. So the best policy is to hold them back until the fire is hottest, or cut them up into very small pieces.
Bramble **	The main problem with bramble is that it burns too quickly and can often be tangled and difficult to get to the fire. Dead brambles can be very handy for starting fires, but you need a lot of them because they go up so quickly. A fire made with just green bramble may have difficulty getting hot enough, and the core might burn out, starving the fire of fuel. In this situation you really need some logs to burn first to provide a bed of hot ashes.
Ivy Bind Weed Woolly Thistle Himalayan Balsam Ragwort **	Volunteers are often asked to clear Ivy from gravestones. There's also the view that Ivy is parasitic on trees and kills them, but this is largely a misconception [3] so it's unlikely to be burnt on its own in woodland. If plants such as these are all you have, it will be hard to get the fire hot enough and keep it going. You'll really need some decent logs to provide a good bed of embers.
Elder Elm *	Elder can be an absolute nightmare, particularly for starting a fire. Although the twigs snap easily and it seems dry, it refuses to burn with a decent flame even when dry and just slowly smoulders. Larger branches are similarly troublesome. The best thing is to leave it until a fire is going well and mix it in gradually. There is less Elm around, following Dutch Elm disease, but it is said to present much the same problem.
Leaves *	Piles of leaves, especially when green or wet, have a tendency to smoulder, creating huge amounts of smoke but few flames. Even if they look dry, they are generally almost useless as tinder. Fortunately there are few good conservation reasons for burning dead leaves; it's more likely to be something done in gardens or parks for aesthetic reasons.
Gorse Heather ****	These are rare things that burn not only when green but in their normal growing state. Needless to say the main risk of such a fire is it getting out of control. But if you had a small isolated clump to dispose of, you could probably just drop a match on it. There's unlikely to be very much of either in Oxfordshire because they favour acidic soil. Acidic soil is rare in Oxfordshire: Tackley Heath is an example, but here the dominant species are mainly bracken rather than heather [4]. True heath is much more common in other parts of the UK.

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The main thing to be aware of is that if there is a lot of low-density, bulky, light, wet, or green material to burn, it might be hard to build up a big enough bed of hot embers to get the fire to be really effective. If you know you're going to be burning a lot of this type of material you will need to find a good supply of decent wood, preferably logs to go in the base of the fire to provide the embers to create the heat. Once the embers are hot enough it will be easy to burn the lighter material.

Choosing Your Fire Site

The most difficult place to start a fire seems to be at the bottom of a ditch or embankment. On a still day, and until there is a decent amount of heat, there is no air flow and things move very slowly. Nature reserves often include disused railway cuttings, quarries, steep ground unsuitable for farming, or prehistoric defensive ditches, so there is a high chance of this problem. However, if you're working on a slope it's still often best to have the fire at the bottom, so then the trees and branches can be rolled or thrown down rather than dragged or carried up. The slowness of the fire is usually of less importance than safety considerations and reducing the amount of carrying. Once the fire is going well, it creates its own air currents.

Platform protects the archaeology in the ground

Flat ground is usually best. If it's very windy, an exposed spot - such as the top of a hill - will make the fire more likely to spread, but this will only be gradual because the fire won't spread to anywhere that isn't hot enough. Hot air will still tend to rise from the bed of embers, spreading the heat built up in the embers upwards much faster than it goes sideways.

Halfway up a slope is not ideal, but if this is the only option then the best hollow in the ground should be found so there is less risk of burning material toppling over and rolling down the hill. In such circumstances it's usually safest to load the fire from alongside it, rather than above or below it.

The space around the bonfire should be cleared for safety reasons. Although people are often concerned about the risk of fire spreading, this is usually not the main concern. In England, in autumn or winter, on normal soil or stony ground without dense vegetation, the risk of fires spreading is very small indeed. But you still need to be careful when there are beds of pine needles, dead brambles or after a long, hot summer. The real reason for clearing around the fire site is to provide clear escape routes and prevent trees being damaged by scorching. Escape routes are particularly important because if the wind changes a big rush of heat and smoke can suddenly engulf someone who may be unable to see. In such circumstances you really don't want any trip hazards. It's best to clear before lighting the fire, because later on it might be unpleasant or unsafe to do so, due to the smoke.

Sometimes you see people placing rocks around an open fire. This might be to support cooking pots, or because they think it marks out the fire or prevents it spreading. In fact, this is often a very bad idea because some types of rock can shatter when they get hot. In Oxfordshire there is a lot of flint, which may be particularly dangerous, although there is some uncertainty around this issue [5]. If you see any boulders on your fire site you should certainly move them out of the way before starting the fire, if only to remove a trip hazard.

Once you've chosen the site and worked out what you're going to burn and in what order, you can get on with the business of lighting the fire. This will be covered in the next instalment of this article which will be in the summer 2010 edition of The Weasel.

Chris Skepper

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References

[1] Natural History Museum: Chalk grassland

[2] Sycamore in Britain: native or non-native? http://www.treeblog.co.uk/viewpost.php?id=216

[3] Ivy - Friend or Foe? http://www.arborecology.co.uk/article_forf.htm

[4] Oxfordshire Wildlife and Landscape Study: Tackley

[5] Bushcraft UK: Urban myth or what? http://www.bushcraftuk.com/forum/showthread.php?t=53454

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