When does Antelopehorn Milkweed bloom in Arizona?

Most often in May. Across 122 dated, research-grade observations of Asclepias asperula in Arizona, the flowering season runs roughly April to May.

Peak May In flower 122 Examined 233 State Arizona

Flowering 122 in flower of 233 examined

57%

68%

51%

35%

50%

Proportion of examined Asclepias asperula in Arizona in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	0	0	too few examined
Mar	1	1	too few examined
Apr	16	28	57%	39% to 73%
May	47	69	68%	56% to 78%
Jun	34	67	51%	39% to 62%
Jul	18	51	35%	24% to 49%
Aug	6	12	50%	25% to 75%
Sep	0	5	0%	0% to 43%
Oct	0	0	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 asperula in Arizona 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. 122 of 233 examined observations were in flower, every one of them research grade. The whisker on each bar is a 95% Wilson interval. 6 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 Arizona found Asclepias asperula in flower, not a forecast. It is computed only from observations made in Arizona, 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 asperula

Antelopehorn Milkweed

Sourcesevery claim on this page

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