The Collaborative International Dictionary of English v.0.48:

Fireweed \Fire"weed`\, n. (Bot.)
   (a) An American plant (Erechthites hiercifolia), very
       troublesome in spots where brushwood has been burned.
   (b) The great willow-herb (Epilobium spicatum).
       [1913 Webster]

WordNet (r) 3.0 (2006):

fireweed
    n 1: tall North American perennial with creeping rootstocks and
         narrow leaves and spikes of pinkish-purple flowers
         occurring in great abundance in burned-over areas or recent
         clearings; an important honey plant [syn: fireweed,
         giant willowherb, rosebay willowherb, wickup,
         Epilobium angustifolium]
    2: an American weedy plant with small white or greenish flowers
       [syn: fireweed, Erechtites hieracifolia]