Place the fireweed seeds in a plastic food storage bag in late summer and seal the bag tightly. Insert the bag into a refrigerator that maintains a temperature of 40 degrees Fahrenheit and leave it there for 40 days.
Look for a planting site that has full to partial sunlight and moist to slightly soggy soil. Good locations are near bodies of water such as streams, bogs or drainage ditches or under a canopy of trees.
Pull up all weeds and other vegetation in the area. Spread an all-purpose fertilizer over the soil using the amount specified on the fertilizer label. Mix the fertilizer into the top 5 to 6 inches of the soil using a pitchfork or a shovel. Smooth the top of the soil out evenly, but do not pack it down.
Spread the fireweed seeds onto the surface of the ground, spacing them at three-foot distances. As the plants grow, they will form dense clumps and fill in the bare spaces. Cover the seeds with an additional 1/4 to 1/2 inch of soil.
Water the soil only when it feels dry or when drought occurs. If the proper planting site is chosen, fireweed rarely requires additional water. The seeds will germinate about 10 days after planting.