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Fires caused by 'spontaneous combustion of compost'


Case 1

How this fire happened


A fire station was contacted by a householder informing Kent Fire and Rescue Service (KFRS) that she had a small solar light in a plant pot which had apparently ‘spontaneously’ caught light.

She had extinguished the fire herself using water from her garden hose.

A member of the KFRS Fire Investigation and Research Team (FIRT) visited the occupier and examined the plant pot; the tree that was planted in the plant pot, the remaining compost in the plant pot and the remains of the solar garden light that the occupier stated was located in the plant pot before the fire.

The pot was of ceramic construction and showed no signs of fire heat or smoke damage. There was however evidence of the original level of the compost and the burnt remains of compost that had partially smouldered to ash.

The bottom of the apple tree had smouldered to charcoal.

There was an ants’ nest in the compost with live ants below the pot when it was lifted.

The solar light was the self contained type. It was designed to use a single solar cell to recharge a battery during daylight and then automatically turn on the light after dark.


The light was designed to be inserted into soil using a plastic spike under the light housing.


The solar light showed signs it had burnt from outside to in, from below, with no apparent damage to the circuitry.

It is believed this fire had not been caused by the solar light at all, but had in fact been produced by spontaneous combustion of the compost within the ceramic pot, made worse by the presence of an ants’ nest within the compost.

Ants in soil or compost aerate the soil, thereby increasing ventilation. They also change the PH (acidity) of the soil and introduce microorganisms (bacteria). 

These factors can lead to an increase in temperature of the soil, or in this case compost. It is believed this allowed the compost to reach smoulder temperature and thereby cause this fire.

The effect it had


The occupier felt that it had been useful to share this information with KFRS as; had she not been in, or the pot closer to the wooden cladding on her house and patio, the consequences could have been worse.



This case study was contributed by Kent Fire & rescue Service.

Safety message​

  • Smokers should discard butts and  matches in appropriate disposal containers, rather than planters or garden beds. The danger can be reduced by providing receptacles for smokers.     

  • Planters should keep well watered to reduce flammability and remove dead plants to lessen available potential fuel for a fire.

  • Planters should not rest on or against flammable surfaces such as wooden decks or siding.

  • Remove and dispose of dead plants promptly.

  • Where possible, use non-combustible planters and flowerpots.

  • Should a fire start call the Fire and Rescue Service.

Compost can spontaneously ignite given the correct circumstances.

If smoke is seen rising from planters, ensure copious water is applied immediately.

Should this not deal with the problem call out the fire service.

Case 2

How this fire happened


A small fire occurred on roof terrace which was confined to two planters.

It was discovered by security staff, who raised the alarm.

None of the planters had been watered for some considerable time and the foliage was dead and very dry.

According to the site staff, there was no event running on the terrace during the preceding evening or in fact any of the days before.

There being no other ignition source, the cause of fire, is thought to be spontaneous combustion.

East Sussex Fire & Rescue Service attended the incident and dealt with the fire.

A number of paving slabs were removed to check the spread and there was evidence of damage to the expanded polystyrene insulation board and the roof liner membrane below.

Peat moss, a common organic ingredient in potting soil and soil conditioning products, presents a fire hazard under certain conditions.

The presence of peat moss in planters and flower beds provides a flammable medium when conditions are particularly dry.

Some fires of this type have been attributed to spontaneous combustion.


Although peat moss can spontaneously ignite, to create the conditions under which this is possible would usually require a much larger quantity of material than would normally be found in even a large planter or flower pot.

A depth of several feet is usually necessary to allow sufficient heat build up to result in ignition.

Fire most often occurs when an external heat source makes contact with the peat. For example, a discarded cigarette butt generates temperatures in excess of 350 degrees C.

Smokers should avoid discarding butts in planters.

Fires which begin in planters can burn unnoticed for several hours.

Peat smoulders rather than bursting into flame and can burn down to the base of the container with no more evidence than a thin smoke plume and pungent odour.

Any flammable materials near the container can help spread the fire.

Examination of the CCTV footage from the previous 12 hours indicted that no one had accessed the area, making application of an external heat source i.e. cigarette butt, unlikely.

The effect it had


This small fire caused damage not only to the planters on the roof terrace but also the paving and insulation.

In all, approximately £7,000 will be spent on remedial work.

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