When does bristle thistle bloom in Louisiana?

Most often in March. Across 130 dated, research-grade observations of Cirsium horridulum in Louisiana, the flowering season runs roughly March to April.

Peak March In flower 130 Examined 301 State Louisiana

Flowering 130 in flower of 301 examined

17%

77%

63%

35%

Proportion of examined Cirsium horridulum in Louisiana in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	21	0%	0% to 15%
Feb	4	23	17%	7% to 37%
Mar	58	75	77%	67% to 85%
Apr	55	87	63%	53% to 73%
May	11	31	35%	21% to 53%
Jun	1	3	too few examined
Jul	0	3	too few examined
Aug	0	5	0%	0% to 43%
Sep	0	2	too few examined
Oct	1	11	9%	2% to 38%
Nov	0	15	0%	0% to 20%
Dec	0	25	0%	0% to 13%

Peak flowering in Mar. Each bar is the share of Cirsium horridulum in Louisiana 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. 130 of 301 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 Louisiana found Cirsium horridulum in flower, not a forecast. It is computed only from observations made in Louisiana, 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 horridulum

bristle thistle

Sourcesevery claim on this page

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