When does marsh marigold bloom in Illinois?

Most often in September. Across 225 dated, research-grade observations of Caltha palustris in Illinois, the flowering season runs roughly April to September.

Peak September In flower 225 Examined 295 State Illinois

Flowering 225 in flower of 295 examined

20%

50%

90%

88%

100%

Proportion of examined Caltha palustris in Illinois in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	1	5	20%	4% to 62%
Mar	22	44	50%	36% to 64%
Apr	132	147	90%	84% to 94%
May	58	66	88%	78% to 94%
Jun	0	18	0%	0% to 18%
Jul	0	1	too few examined
Aug	2	2	too few examined
Sep	6	6	100%	61% to 100%
Oct	3	4	too few examined
Nov	1	1	too few examined
Dec	0	1	too few examined

Peak flowering in Sep. Each bar is the share of Caltha palustris 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. 225 of 295 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 6 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 Caltha palustris 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

Caltha palustris

marsh marigold

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.