Leaves are full grown approximately 1 month after emergence. Flowers bloom June-September. Fruits mature approximately 1 month later. Seeds are released beginning in August and continue to be shed after shoots have died from frost injury. Foliage will turn color with limited water availability in the late summer and fall.
Seeds germinate late summer or fall, and seedlings overwinter as a rosette.
The primary and secondary roots of seedlings may develop buds which overwinter. Shoot buds form in the fall on lateral roots and overwinter just below the soil surface Fireweed production may vary with severity of fire. Severe fires remove organic soil layers, exposing mineral soil which is an excellent seedbed for fireweed. Therefore, cover and density are greatest on severely burned areas because of good seedling establishment. By seed, Planting rhizomes may speed colonization of a disturbed area.