Master Gardener committed to growing native milkweeds for our plant sales; also teaching others how to grow from seed. Trying to become expert at differentiating Asclepias linearis versus A. verticillata. People share seeds labelled both but not sure of provenance and accuracy. These two still stump me. What morphological traits do you look for when making a positive ID?
it looks like you’re in Central Texas. the Botanical Research Institute of Texas (BRIT) offers an electronic (or print, if you prefer) version of the Illustrated Flora of North Central Texas here: https://fwbg.org/science-conservation/brit-press/illustrated-flora-of-north-central-texas-online/. if you look in the Dicots A-E volume, you can find a key for Asclepias, complete with illustrations and short descriptions. the key seems to indicate that Asclepias verticillata is unique among North/Central Texas species in having leaves mostly in whorls of 3-4. (leaves for A. linearis are opposite.)
if you need more detailed descriptions without illustrations, i often turn to Flora of North America. their key might be harder to use because it covers a broader geographic area, but it can provide more completed, standardized descriptions for A. linearis and A. verticillata.
not everyone agrees with every key. so you can also look for other keys to see what they say. you can choose which one you think works best for you, or you can try to look for overlap in distinguishing characteristics noted between different keys.
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If more of us internalized the principle this implies, maybe we wouldn’t have as much collective angst about identifications.