When does creeping thistle bloom in Oregon?

Most often in July. Across 160 dated, research-grade observations of Cirsium arvense in Oregon, the flowering season runs roughly July to August.

Peak July In flower 160 Examined 272 State Oregon

Flowering 160 in flower of 272 examined

68%

88%

73%

50%

41%

25%

Proportion of examined Cirsium arvense in Oregon in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	5	0%	0% to 43%
Feb	0	1	too few examined
Mar	0	5	0%	0% to 43%
Apr	0	12	0%	0% to 24%
May	1	16	6%	1% to 28%
Jun	26	38	68%	53% to 81%
Jul	57	65	88%	78% to 94%
Aug	51	70	73%	61% to 82%
Sep	16	32	50%	34% to 66%
Oct	7	17	41%	22% to 64%
Nov	2	8	25%	7% to 59%
Dec	0	3	too few examined

Peak flowering in Jul. Each bar is the share of Cirsium arvense in Oregon 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. 160 of 272 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 2 months have fewer than 5 examined observations, so no proportion is drawn for them. 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 Oregon found Cirsium arvense in flower, not a forecast. It is computed only from observations made in Oregon, 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 arvense

creeping thistle

Sourcesevery claim on this page

GBIF (iNaturalist Research-grade Observations). Dated flowering annotations in Oregon. 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.