Widespread fires can be generated after large impact events by atmospheric heating caused by the reaccretion of high-energy, vapor-rich plume material. We examine the threshold irradiance levels necessary for spontaneous and pilot ignition of various types of vegetation and define three specific cases for investigation: (1) 51 kW/m2 for a period of at least 2 min to spontaneously ignite wood; (2) 20 kW/m2 for a period of at least 20 min to ignite wood in the presence of an ignition source; and (3) 28 kW/m2 for a period of at least 1 min to ignite foliage, rotten wood, and forest litter. The threshold ejected plume mass for continent-wide spontaneous ignition of wood is ~2 to 6 ¿ 1015 kg, independent of impact location but dependent on the details of the ejecta speed distribution. The threshold ejected plume mass for global spontaneous ignition of wood is in the range ~1 to 2 ¿ 1016 kg. The threshold plume masses for continent-wide and global fires are very nearly the same for piloted ignition of wood, while the threshold plume masses for continent-wide and global ignition of leaves and forest litter are significantly lower, by a factor of ~2 to 3. Impact craters of at least 85 km diameter are needed to produce continental-scale fires, and craters of ~135 km diameter are needed for global-scale fires.