When does Cobwebby Thistle bloom in California?

Most often in June. Across 2,054 dated, research-grade observations of Cirsium occidentale in California, the flowering season runs roughly May to August.

Peak June In flower 2,054 Examined 2,750 State California

Flowering 2,054 in flower of 2,750 examined

28%

18%

42%

62%

78%

93%

90%

74%

52%

50%

22%

27%

Proportion of examined Cirsium occidentale in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	7	25	28%	14% to 48%
Feb	10	57	18%	10% to 29%
Mar	83	198	42%	35% to 49%
Apr	296	480	62%	57% to 66%
May	661	851	78%	75% to 80%
Jun	607	653	93%	91% to 95%
Jul	287	319	90%	86% to 93%
Aug	73	98	74%	65% to 82%
Sep	17	33	52%	35% to 68%
Oct	8	16	50%	28% to 72%
Nov	2	9	22%	6% to 55%
Dec	3	11	27%	10% to 57%

Peak flowering in Jun. Each bar is the share of Cirsium occidentale 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. 2,054 of 2,750 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 Cirsium occidentale 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

Cirsium occidentale

Cobwebby Thistle

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.