When does fringed willowherb bloom in Washington?

Most often in August. Across 171 dated, research-grade observations of Epilobium ciliatum in Washington, the flowering season runs roughly July to August.

Peak August In flower 171 Examined 389 State Washington

Flowering 171 in flower of 389 examined

12%

56%

70%

81%

65%

14%

Proportion of examined Epilobium ciliatum in Washington in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	6	0%	0% to 39%
Feb	0	4	too few examined
Mar	0	11	0%	0% to 26%
Apr	0	49	0%	0% to 7%
May	6	52	12%	5% to 23%
Jun	40	72	56%	44% to 66%
Jul	57	82	70%	59% to 78%
Aug	44	54	81%	69% to 90%
Sep	22	34	65%	48% to 79%
Oct	2	14	14%	4% to 40%
Nov	0	2	too few examined
Dec	0	9	0%	0% to 30%

Peak flowering in Aug. Each bar is the share of Epilobium ciliatum in Washington 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. 171 of 389 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 2 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 Washington found Epilobium ciliatum in flower, not a forecast. It is computed only from observations made in Washington, 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 ciliatum

fringed willowherb

Sourcesevery claim on this page

GBIF (iNaturalist Research-grade Observations). Dated flowering annotations in Washington. 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.