When does white snakeroot bloom in New York?

Most often in September. Across 1,174 dated, research-grade observations of Ageratina altissima in New York, the flowering season runs roughly August to December.

Peak September In flower 1,174 Examined 1,628 State New York

Flowering 1,174 in flower of 1,628 examined

12%

81%

93%

87%

70%

81%

Proportion of examined Ageratina altissima in New York in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	3	too few examined
Feb	0	0	too few examined
Mar	0	0	too few examined
Apr	0	58	0%	0% to 6%
May	1	69	1%	0% to 8%
Jun	0	72	0%	0% to 5%
Jul	11	89	12%	7% to 21%
Aug	166	204	81%	75% to 86%
Sep	419	452	93%	90% to 95%
Oct	493	565	87%	84% to 90%
Nov	63	90	70%	60% to 78%
Dec	21	26	81%	62% to 91%

Peak flowering in Sep. Each bar is the share of Ageratina altissima in New York 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. 1,174 of 1,628 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 3 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 New York found Ageratina altissima in flower, not a forecast. It is computed only from observations made in New York, 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

Ageratina altissima

white snakeroot

Sourcesevery claim on this page

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