When does California milkweed bloom in California?

Most often in May. Across 396 dated, research-grade observations of Asclepias californica in California, the flowering season runs roughly April to June.

Peak May In flower 396 Examined 570 State California

Flowering 396 in flower of 570 examined

45%

55%

73%

85%

77%

39%

19%

Proportion of examined Asclepias californica in California in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	5	11	45%	21% to 72%
Mar	23	42	55%	40% to 69%
Apr	108	147	73%	66% to 80%
May	158	186	85%	79% to 89%
Jun	86	112	77%	68% to 84%
Jul	13	33	39%	25% to 56%
Aug	3	16	19%	7% to 43%
Sep	0	20	0%	0% to 16%
Oct	0	3	too few examined
Nov	0	0	too few examined
Dec	0	0	too few examined

Peak flowering in May. Each bar is the share of Asclepias californica 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. 396 of 570 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 4 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 California found Asclepias californica 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 californica

California 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.