When does garlic mustard bloom in Illinois?
Most often in May. Across 595 dated, research-grade observations of Alliaria petiolata in Illinois, the flowering season runs roughly May.
Flowering 595 in flower of 1,173 examined
Peak flowering in May. Each bar is the share of Alliaria petiolata 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. 595 of 1,173 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 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 Alliaria petiolata 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
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.