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.