When does beach knotweed bloom in California?

Most often in January. Across 256 dated, research-grade observations of Polygonum paronychia in California, the flowering season runs roughly January to December.

Peak January In flower 256 Examined 271 State California

Flowering 256 in flower of 271 examined

100%

83%

100%

98%

97%

98%

88%

96%

75%

94%

93%

90%

Proportion of examined Polygonum paronychia in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	10	10	100%	72% to 100%
Feb	5	6	83%	44% to 97%
Mar	21	21	100%	85% to 100%
Apr	45	46	98%	89% to 100%
May	37	38	97%	87% to 100%
Jun	42	43	98%	88% to 100%
Jul	22	25	88%	70% to 96%
Aug	23	24	96%	80% to 99%
Sep	12	16	75%	51% to 90%
Oct	16	17	94%	73% to 99%
Nov	14	15	93%	70% to 99%
Dec	9	10	90%	60% to 98%

Peak flowering in Jan. Each bar is the share of Polygonum paronychia in California 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. 256 of 271 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 California found Polygonum paronychia in flower, not a forecast. It is computed only from observations made in California, 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

Polygonum paronychia

beach knotweed

Sourcesevery claim on this page

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