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You've probably heard that monarch butterflies are in trouble, and that a good way to support them is to plant swaths of milkweeds (Asclepias), because these plants are the only ones the monarch larvae will eat. Perhaps you've planted some of the three species that are commonly available: orange milkweed (A. tuberosa), common milkweed (A. syriaca) and swamp milkweed (A. incarnata).
Everybody loves these classics, but perhaps you feel it's time to broaden your monarch-garden repertoire. Worldwide, there are more than 200 milkweed species currently identified. The Biota of North America Program (BONAP) lists 79 species native to North America. For starters, you might try two compact, garden-worthy species: antelope-horn milkweed (A. asperula; USDA Zones 5–9) and green milkweed (A. viridis; Zones 5–9).
Antelope-horn milkweed has exotic-looking flowers—like a cross between an orchid and sweet William—but this plant is native to the Central Plains, Interior West and Southwest regions of the United States. Each stem is topped with three or four heads of waxy flowers with green star-like petals surrounding a purple-and-white array. The flowers last for several weeks, to the delight of native bees, honey bees and bumblebees. The flowerhead transform into bulbous seedpods that are said to resemble the horns of an antelope. Plant this species and you can be the judge.
Green milkweed is a delight for lovers of green flowers. Its flowerheads are smaller and not as dramatically domed as those of other species. Their outer green petals are relatively large, giving these flowers a softer appearance. This species hails from the US Southeast.
Both of these species grow as multistemmed, rounded plants 18 to 24 inches tall and wide. They are clumping plants not apt to spread by their roots. They are easy to grow in full sun and well-drained soil.
Recommended related reading:
Gardening for Butterflies by The Xerces Society covers monarch butterflies as well as other species in need of help. It includes many facts about butterflies and explains how to create a garden that supports them as well as moths and hummingbirds, from choosing effective plants to laying out the design and maintaining the garden.
The Monarch: Saving Our Most-Loved Butterfly by Kylee Baumle covers the threats that this butterfly faces and provides nine hands-on projects that gardeners can undertake to help it and other butterflies. The book also includes thorough information about milkweeds as well as lists of nectar plants.
Asclepias viridis By peganum from Henfield, England - Uploaded by uleli, CC BY-SA 2.0
Asclepias asperula By Stan Shebs, CC BY-SA 3.0