When does water smartweed bloom in Minnesota?

Most often in September. Across 170 dated, research-grade observations of Persicaria amphibia in Minnesota, the flowering season runs roughly July to September.

Peak September In flower 170 Examined 210 State Minnesota

Flowering 170 in flower of 210 examined

26%

90%

94%

97%

71%

Proportion of examined Persicaria amphibia in Minnesota 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	0	0	too few examined
May	0	2	too few examined
Jun	9	34	26%	15% to 43%
Jul	43	48	90%	78% to 95%
Aug	79	84	94%	87% to 97%
Sep	34	35	97%	85% to 99%
Oct	5	7	71%	36% to 92%
Nov	0	0	too few examined
Dec	0	0	too few examined

Peak flowering in Sep. Each bar is the share of Persicaria amphibia in Minnesota 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. 170 of 210 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 Minnesota found Persicaria amphibia in flower, not a forecast. It is computed only from observations made in Minnesota, 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

Persicaria amphibia

water smartweed

Sourcesevery claim on this page

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