When does fireweed bloom in Colorado?

Most often in August. Across 782 dated, research-grade observations of Epilobium angustifolium in Colorado, the flowering season runs roughly July to August.

Peak August In flower 782 Examined 908 State Colorado

Flowering 782 in flower of 908 examined

63%

89%

95%

65%

33%

Proportion of examined Epilobium angustifolium in Colorado in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	0	0	too few examined
Mar	0	0	too few examined
Apr	0	0	too few examined
May	0	1	too few examined
Jun	40	63	63%	51% to 74%
Jul	332	375	89%	85% to 91%
Aug	346	366	95%	92% to 96%
Sep	61	94	65%	55% to 74%
Oct	3	9	33%	12% to 65%
Nov	0	0	too few examined
Dec	0	0	too few examined

Peak flowering in Aug. Each bar is the share of Epilobium angustifolium in Colorado observations in which someone actually recorded the reproductive state and found the plant in flower, not the raw number of flowering records. That distinction matters: people observe plants far more in spring than in winter, so a bare count of flowering records partly measures when people go outside. Dividing by the number examined removes that. 782 of 908 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 7 months have fewer than 5 examined observations, so no proportion is drawn for them. This is still a global aggregate and not a forecast for your garden: the same species flowers on different dates in different hemispheres. Where a species has fewer than 30 flowering records we do not draw this chart at all. Computed from 10.15468/dl.wt38fd.

What this is, and what it is not

This is a record of when people in Colorado found Epilobium angustifolium in flower, not a forecast. It is computed only from observations made in Colorado, so it is not the species’ global average dragged onto a map: the same plant flowers on different dates in different places, and that is the entire point of the page.

It will not tell you what your particular plant will do this year. Bloom time moves with the season, with altitude, and with the weather, and a warm February pulls everything forward. We publish the distribution and the sample size, and we refuse to draw a month that too few people examined.

The plant

Epilobium angustifolium

fireweed

Sourcesevery claim on this page

GBIF (iNaturalist Research-grade Observations). Dated flowering annotations in Colorado. Every record achieved iNaturalist quality grade Research, which is applied upstream at export. 10.15468/dl.wt38fd. Retrieved 2026-07-14.
World Flora Online Plant List. The accepted name. CC0. Retrieved 2026-07-12.