Asclepias hallii aka Hall's Milkweed

Asclepias hallii is known as Hall's milkweed or purple silkweed. This perennial lives for more than one year. It belongs to the dogbane family, Apocynaceae. It is native to the west-central United States. Records come from Colorado, Utah, and southern Wyoming. Other records come from central Nevada and northern Arizona. Its range also includes southwestern New Mexico. It grows in sandy prairie soils and along roadsides. Kew places it in a region with a temperate climate.

Plants grow about 3.3 feet (1 metre) tall. They spread about 2 feet (0.6 metres) wide. This size suits a place in a garden bed. Flowers bloom from June through August. Seeds ripen in September. Each flower has both male and female organs. Bees, butterflies, moths, and other insects help pollinate the flowers. The plant can also set seed through its own pollen. The supplied evidence gives no detailed account of its leaves. It gives no detailed account of its flowers either.

Choose a sunny site where the soil drains well. Plants also grow in light shade, such as open woods. Both sand and loam suit this plant. PFAF suggests light, rich soil or soil rich in peat. The soil can be slightly acid, neutral, or slightly alkaline. The plant grows in dry or moist soil. PFAF lists USDA hardiness zones 3–7. It gives a separate UK zone number as well. That number does not establish an RHS hardiness rating.

Take care with roots; the plant dislikes having them disturbed. Put young plants in their final sites while still small. Sow fresh seed in autumn, or sow in late winter. PFAF reports that early spring sowing under glass also works. Stored seed may need two to three weeks of cold treatment. At 18°C, seeds can take one to three months to sprout. Keep seedlings under glass for their first winter. Plant them out while they grow in late spring or early summer. You can also divide plants in spring, but handle them with care.

PFAF says to guard young plants from slugs while they take root. It does not flag this species as a weed. Its accounts of food and medicinal uses concern another milkweed species. The same is true of its accounts of material uses. These do not prove such uses for Hall's milkweed. The source also warns of toxic compounds across the genus. It gives no toxicity findings for this species alone. Its safety thus remains unclear here.