When does Herb Robert bloom in Washington?

Most often in June. Across 926 dated, research-grade observations of Geranium robertianum in Washington, the flowering season runs roughly May to July.

Peak June In flower 926 Examined 1,565 State Washington

Flowering 926 in flower of 1,565 examined

30%

48%

83%

90%

81%

48%

53%

43%

44%

32%

Proportion of examined Geranium robertianum in Washington in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	19	63	30%	20% to 42%
Feb	1	45	2%	0% to 12%
Mar	8	85	9%	5% to 17%
Apr	125	261	48%	42% to 54%
May	350	423	83%	79% to 86%
Jun	145	161	90%	84% to 94%
Jul	98	121	81%	73% to 87%
Aug	41	86	48%	37% to 58%
Sep	52	99	53%	43% to 62%
Oct	34	80	43%	32% to 53%
Nov	29	66	44%	33% to 56%
Dec	24	75	32%	23% to 43%

Peak flowering in Jun. Each bar is the share of Geranium robertianum 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. 926 of 1,565 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 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 Geranium robertianum 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

Geranium robertianum

Herb Robert

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.