When does western ragweed bloom in California?

Most often in October. Across 113 dated, research-grade observations of Ambrosia psilostachya in California, the flowering season runs roughly August to October.

Peak October In flower 113 Examined 251 State California

Flowering 113 in flower of 251 examined

37%

46%

84%

78%

86%

68%

21%

Proportion of examined Ambrosia psilostachya in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	7	19	37%	19% to 59%
Feb	1	17	6%	1% to 27%
Mar	0	7	0%	0% to 35%
Apr	0	19	0%	0% to 17%
May	2	22	9%	3% to 28%
Jun	1	19	5%	1% to 25%
Jul	11	24	46%	28% to 65%
Aug	26	31	84%	67% to 93%
Sep	25	32	78%	61% to 89%
Oct	24	28	86%	69% to 94%
Nov	13	19	68%	46% to 85%
Dec	3	14	21%	8% to 48%

Peak flowering in Oct. Each bar is the share of Ambrosia psilostachya 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. 113 of 251 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 Ambrosia psilostachya 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

Ambrosia psilostachya

western ragweed

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.