The burning, not only of heather and grass, but also gorse, bracken and bilberry is controlled by the Heather and Grass, etc. (burning) Regulations 1986.
Burning is allowed only between 10th October and April 15th in upland areas, DNP recommends no burning after March 31st to prevent harm to nesting birds.
Quite often there seems to be a prolonged dry spell in March and so it proved this year. The tell tale columns of smoke could be seen rising from different sections of the moor over the last few days. Swaling is not an exact science, but the skill and experience of commoners and farmers makes the process as safe and controlled as possible. Before burning can commence, Devon Fire and Rescue, DNP, Natural England and the Dartmoor Commoners Council must be informed. Occasionally, well intentioned visitors see the fires and call the Fire Service, who in turn check where burning is to occur and act accordingly.
Burning after dark is not common but when it does occur it makes for dramatic viewing.