When does butterfly milkweed bloom in Pennsylvania?

Most often in July. Across 258 dated, research-grade observations of Asclepias tuberosa in Pennsylvania, the flowering season runs roughly June to August.

Peak July In flower 258 Examined 320 State Pennsylvania

Flowering 258 in flower of 320 examined

20%

81%

97%

79%

54%

Proportion of examined Asclepias tuberosa in Pennsylvania in flower, by month
Month	In flower	Examined	Share	95% interval
Jan	0	0	too few examined
Feb	0	0	too few examined
Mar	0	0	too few examined
Apr	0	0	too few examined
May	1	5	20%	4% to 62%
Jun	110	136	81%	73% to 87%
Jul	99	102	97%	92% to 99%
Aug	34	43	79%	65% to 89%
Sep	14	26	54%	35% to 71%
Oct	0	8	0%	0% to 32%
Nov	0	0	too few examined
Dec	0	0	too few examined

Peak flowering in Jul. Each bar is the share of Asclepias tuberosa in Pennsylvania 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. 258 of 320 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 Pennsylvania found Asclepias tuberosa in flower, not a forecast. It is computed only from observations made in Pennsylvania, 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 tuberosa

butterfly milkweed

Sourcesevery claim on this page

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