When does hairy bittercress bloom in Washington?

Most often in March. Across 115 dated, research-grade observations of Cardamine hirsuta in Washington, the flowering season runs roughly February to April.

Peak March In flower 115 Examined 193 State Washington

Flowering 115 in flower of 193 examined

22%

69%

77%

67%

50%

23%

Proportion of examined Cardamine hirsuta in Washington in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	6	27	22%	11% to 41%
Feb	24	35	69%	52% to 81%
Mar	50	65	77%	65% to 85%
Apr	28	42	67%	52% to 79%
May	4	8	50%	22% to 78%
Jun	0	1	too few examined
Jul	0	1	too few examined
Aug	0	0	too few examined
Sep	0	0	too few examined
Oct	0	0	too few examined
Nov	0	1	too few examined
Dec	3	13	23%	8% to 50%

Peak flowering in Mar. Each bar is the share of Cardamine hirsuta 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. 115 of 193 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 6 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 Cardamine hirsuta 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

Cardamine hirsuta

hairy bittercress

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.