When does Yellow Star-Thistle bloom in California?

Most often in July. Across 850 dated, research-grade observations of Centaurea solstitialis in California, the flowering season runs roughly June to December.

Peak July In flower 850 Examined 947 State California

Flowering 850 in flower of 947 examined

74%

33%

54%

31%

68%

93%

99%

97%

92%

93%

Proportion of examined Centaurea solstitialis in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	20	27	74%	55% to 87%
Feb	4	12	33%	14% to 61%
Mar	7	13	54%	29% to 77%
Apr	12	39	31%	19% to 46%
May	43	63	68%	56% to 78%
Jun	136	146	93%	88% to 96%
Jul	221	223	99%	97% to 100%
Aug	149	154	97%	93% to 99%
Sep	122	126	97%	92% to 99%
Oct	65	67	97%	90% to 99%
Nov	45	49	92%	81% to 97%
Dec	26	28	93%	77% to 98%

Peak flowering in Jul. Each bar is the share of Centaurea solstitialis 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. 850 of 947 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 Centaurea solstitialis 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

Centaurea solstitialis

Yellow Star-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.