When does large white trillium bloom in Wisconsin?

Most often in May. Across 1,199 dated, research-grade observations of Trillium grandiflorum in Wisconsin, the flowering season runs roughly May to June.

Peak May In flower 1,199 Examined 1,389 State Wisconsin

Flowering 1,199 in flower of 1,389 examined

50%

94%

76%

15%

45%

Proportion of examined Trillium grandiflorum in Wisconsin 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	0	too few examined
Apr	58	115	50%	41% to 59%
May	1039	1111	94%	92% to 95%
Jun	93	123	76%	67% to 82%
Jul	4	27	15%	6% to 32%
Aug	5	11	45%	21% to 72%
Sep	0	1	too few examined
Oct	0	1	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 grandiflorum in Wisconsin 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. 1,199 of 1,389 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 Wisconsin found Trillium grandiflorum in flower, not a forecast. It is computed only from observations made in Wisconsin, 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 grandiflorum

large white trillium

Sourcesevery claim on this page

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