There have been a number of reports lately about fires occurring in planters or flowerpots in which potting soil has been used. The fires are of a smouldering nature and, if the planter or flowerpot is plastic, can also result in melting the container. If other combustible material is in close proximity to the planter or flowerpot it may become involved resulting in a flaming fire. In some cases dead plants have contributed to the fuel load.

via London Fire Department.