When does Bull Thistle bloom in Pennsylvania?

Most often in August. Across 96 dated, research-grade observations of Cirsium vulgare in Pennsylvania, the flowering season runs roughly July to August.

Peak August In flower 96 Examined 172 State Pennsylvania

Flowering 96 in flower of 172 examined

22%

10%

87%

97%

59%

36%

Proportion of examined Cirsium vulgare in Pennsylvania in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	0	0	too few examined
Mar	0	7	0%	0% to 35%
Apr	0	11	0%	0% to 26%
May	2	9	22%	6% to 55%
Jun	2	21	10%	3% to 29%
Jul	39	45	87%	74% to 94%
Aug	36	37	97%	86% to 100%
Sep	13	22	59%	39% to 77%
Oct	4	11	36%	15% to 65%
Nov	0	7	0%	0% to 35%
Dec	0	2	too few examined

Peak flowering in Aug. Each bar is the share of Cirsium vulgare in Pennsylvania 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. 96 of 172 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 3 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 Pennsylvania found Cirsium vulgare in flower, not a forecast. It is computed only from observations made in Pennsylvania, 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 vulgare

Bull Thistle

Sourcesevery claim on this page

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