Fireweed derives from its abundance as a coloniser on burnt sites after forest fires. It has a tendency to quickly colonize open areas with little competition, such as sites of forest fires and forest clearings.

The capacity to colonize makes it a clear example of a pioneer species. Plants grow and flower as long as there is open space and plenty of light. As trees and brush grow larger the plants die out.

However, the seeds remain viable in the soil for many years; when a new fire or other disturbance occurs that opens up the ground to light, the seeds germinate. Some areas with heavy seed counts in the soil can, after burning, be covered with pure dense stands of this species and when in flower the landscape is turned into fields of color. (from Wikipedia)