When does prairie trillium bloom in Illinois?

Most often in May. Across 1,432 dated, research-grade observations of Trillium recurvatum in Illinois, the flowering season runs roughly May to June.

Peak May In flower 1,432 Examined 2,617 State Illinois

Flowering 1,432 in flower of 2,617 examined

14%

44%

87%

79%

Proportion of examined Trillium recurvatum in Illinois in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	0	0	too few examined
Mar	32	234	14%	10% to 19%
Apr	666	1526	44%	41% to 46%
May	704	806	87%	85% to 89%
Jun	30	38	79%	64% to 89%
Jul	0	9	0%	0% to 30%
Aug	0	4	too few examined
Sep	0	0	too few examined
Oct	0	0	too few examined
Nov	0	0	too few examined
Dec	0	0	too few examined

Peak flowering in May. Each bar is the share of Trillium recurvatum in Illinois 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. 1,432 of 2,617 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 7 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 Illinois found Trillium recurvatum in flower, not a forecast. It is computed only from observations made in Illinois, 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

Trillium recurvatum

prairie trillium

Sourcesevery claim on this page

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