Asclepias viridiflora Raf.

I've only seen this plant in Indiana, though it also occurs in the southwest. It's uncommon in Indiana, found on limestone barrens.

An inflorescence at Baseline Barrens, Washington County, Indiana, 7 Aug 2010.

Inflorescences at Baseline Barrens, 7 Aug 2010.

Cauline leaves at Baseline Barrens, 7 Aug 2010.

Flowers at Baseline Barrens, 7 Aug 2010.

A flowering stem at Baseline Barrens, south of Salem, Washington County, Indiana, 27 Jun 2004.

Side view of flowers at Baseline Barrens, 27 Jun 2004.

Front view of flowers at Baseline Barrens, 27 Jun 2004.