When does rush milkweed bloom in California?

Most often in October. Across 669 dated, research-grade observations of Asclepias subulata in California, the flowering season runs roughly May to December.

Peak October In flower 669 Examined 957 State California

Flowering 669 in flower of 957 examined

66%

49%

27%

70%

90%

75%

38%

61%

69%

90%

81%

75%

Proportion of examined Asclepias subulata in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	74	112	66%	57% to 74%
Feb	35	72	49%	37% to 60%
Mar	22	82	27%	18% to 37%
Apr	49	70	70%	58% to 79%
May	52	58	90%	79% to 95%
Jun	12	16	75%	51% to 90%
Jul	3	8	38%	14% to 69%
Aug	23	38	61%	45% to 74%
Sep	34	49	69%	55% to 80%
Oct	111	123	90%	84% to 94%
Nov	104	129	81%	73% to 87%
Dec	150	200	75%	69% to 80%

Peak flowering in Oct. Each bar is the share of Asclepias subulata 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. 669 of 957 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 Asclepias subulata 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

Asclepias subulata

rush milkweed

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.