When does Herb Robert bloom in California?

Most often in August. Across 824 dated, research-grade observations of Geranium robertianum in California, the flowering season runs roughly January to December.

Peak August In flower 824 Examined 860 State California

Flowering 824 in flower of 860 examined

97%

98%

96%

93%

94%

98%

100%

98%

79%

100%

95%

Proportion of examined Geranium robertianum in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	33	34	97%	85% to 99%
Feb	57	58	98%	91% to 100%
Mar	82	84	98%	92% to 99%
Apr	144	150	96%	92% to 98%
May	120	129	93%	87% to 96%
Jun	101	107	94%	88% to 97%
Jul	95	97	98%	93% to 99%
Aug	56	56	100%	94% to 100%
Sep	50	51	98%	90% to 100%
Oct	23	29	79%	62% to 90%
Nov	28	28	100%	88% to 100%
Dec	35	37	95%	82% to 99%

Peak flowering in Aug. Each bar is the share of Geranium robertianum in California 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. 824 of 860 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 California found Geranium robertianum in flower, not a forecast. It is computed only from observations made in California, 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 California. 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.