When does asthmaweed bloom in California?

Most often in December. Across 104 dated, research-grade observations of Erigeron bonariensis in California, the flowering season runs roughly November to December.

Peak December In flower 104 Examined 176 State California

Flowering 104 in flower of 176 examined

70%

60%

29%

69%

40%

70%

57%

61%

43%

80%

88%

Proportion of examined Erigeron bonariensis in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	7	10	70%	40% to 89%
Feb	2	3	too few examined
Mar	3	5	60%	23% to 88%
Apr	2	7	29%	8% to 64%
May	11	16	69%	44% to 86%
Jun	8	20	40%	22% to 61%
Jul	16	23	70%	49% to 84%
Aug	17	30	57%	39% to 73%
Sep	14	23	61%	41% to 78%
Oct	9	21	43%	24% to 63%
Nov	8	10	80%	49% to 94%
Dec	7	8	88%	53% to 98%

Peak flowering in Dec. Each bar is the share of Erigeron bonariensis 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. 104 of 176 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. One month has fewer than 5 examined observations, so no proportion is drawn for it. 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 Erigeron bonariensis 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

Erigeron bonariensis

asthmaweed

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.