When does Italian thistle bloom in California?

Most often in May. Across 455 dated, research-grade observations of Carduus pycnocephalus in California, the flowering season runs roughly April to June.

Peak May In flower 455 Examined 705 State California

Flowering 455 in flower of 705 examined

14%

19%

83%

95%

81%

47%

17%

29%

13%

18%

Proportion of examined Carduus pycnocephalus in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	2	29	7%	2% to 22%
Feb	8	58	14%	7% to 25%
Mar	13	70	19%	11% to 29%
Apr	213	256	83%	78% to 87%
May	144	151	95%	91% to 98%
Jun	52	64	81%	70% to 89%
Jul	14	30	47%	30% to 64%
Aug	2	12	17%	5% to 45%
Sep	2	7	29%	8% to 64%
Oct	1	3	too few examined
Nov	1	8	13%	2% to 47%
Dec	3	17	18%	6% to 41%

Peak flowering in May. Each bar is the share of Carduus pycnocephalus 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. 455 of 705 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. One month has fewer than 5 examined observations, so no proportion is drawn for it. 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 Carduus pycnocephalus 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

Carduus pycnocephalus

Italian 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.