When does milk thistle bloom in Oregon?

Most often in June. Across 68 dated, research-grade observations of Silybum marianum in Oregon, the flowering season runs roughly June.

Peak June In flower 68 Examined 225 State Oregon

Flowering 68 in flower of 225 examined

39%

80%

57%

Proportion of examined Silybum marianum in Oregon in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	14	0%	0% to 22%
Feb	0	17	0%	0% to 18%
Mar	0	15	0%	0% to 20%
Apr	2	32	6%	2% to 20%
May	16	41	39%	26% to 54%
Jun	45	56	80%	68% to 89%
Jul	4	7	57%	25% to 84%
Aug	0	0	too few examined
Sep	0	4	too few examined
Oct	0	8	0%	0% to 32%
Nov	0	13	0%	0% to 23%
Dec	1	18	6%	1% to 26%

Peak flowering in Jun. Each bar is the share of Silybum marianum 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. 68 of 225 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 Silybum marianum 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

Silybum marianum

milk 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.