When does Pacific trillium bloom in Washington?

Most often in April. Across 3,262 dated, research-grade observations of Trillium ovatum in Washington, the flowering season runs roughly March to May.

Peak April In flower 3,262 Examined 3,930 State Washington

Flowering 3,262 in flower of 3,930 examined

84%

97%

90%

67%

29%

Proportion of examined Trillium ovatum in Washington in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	1	3	too few examined
Mar	345	411	84%	80% to 87%
Apr	1697	1757	97%	96% to 97%
May	855	948	90%	88% to 92%
Jun	299	448	67%	62% to 71%
Jul	64	217	29%	24% to 36%
Aug	1	91	1%	0% to 6%
Sep	0	27	0%	0% to 12%
Oct	0	25	0%	0% to 13%
Nov	0	3	too few examined
Dec	0	0	too few examined

Peak flowering in Apr. Each bar is the share of Trillium ovatum in Washington 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. 3,262 of 3,930 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 4 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 Washington found Trillium ovatum in flower, not a forecast. It is computed only from observations made in Washington, 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 ovatum

Pacific trillium

Sourcesevery claim on this page

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