This is probably the most commonly grown of all milkweeds, and is a great nectar source for all sorts of butterflies and other insects, but monarchs seem to prefer to lay their eggs on just about any OTHER species of milkweed. Native across nearly the entire country, it can grow in any kind of well-drained soil, wet or dry, and prefers full sun. Probably the best species to grow for people who like their flower beds neat and tidy. Produces orange flowers up to 3 feet tall. It does not spread by rhizomes but develops thick and strong taproots. As an aside, people who need neat and tidy gardens likely aren't doing nearly as much to benefit wildlife as they like to think they are. Manicured lawns and weedless fields are essentially biological deserts.
Swamp Milkweed (Asclepias incarnata)
As its name implies, this one is meant for areas that stay wet most of the time. Lovely deep pink clusters of blooms. Tolerates shade. Native to most of the country. If you have a swampy bottomland or other area that stays wet, this is the one for you. For the other swamp species, see A. lanceolata below.
Common Milkweed (Asclepias syriaca)
Extremely common and easy to grow milkweed grown across the country. Pink blooms on fairly tall plants, wants full sun on average mesic to dry soil. Not recommended for gardeners who are new to milkweeds, though, as it can be extremely aggressive and just doesn't work well in small or manicured gardens. Recommended for larger, more naturalistic areas.
Whorled Milkweed (Asclepias verticillata)
This milkweed has a single stem up to 2 feet tall with narrow and almost pine-like leaves. White blooms. It likes dry soil and is shade tolerant. Native to the Midwest, East, and South. You wouldn't think it to look at it, but this species is highly favored by monarchs. Garden.org administrator Kent Pfeiffer has seen the caterpillars mow down entire patches of it.
Showy Milkweed (Asclepias speciosa)
An attractive and easy to grow milkweed for the garden, with generously thick bluish leaves and light pink fragrant flowers. The thick leaves provide plenty of food for any monarchs lucky enough to gain access to it. Can get tall and requires full sun and moist soil. Native to the Western US. Not recommended for small, neat or tidy gardens.
Spider Milkweed (Asclepias viridis)
Another one with nice fleshy leaves. This one has unique green flowers and wants full sun. Native to the southern, mid-south and great plains regions where it mostly grows in upland prairies, especially prairies with a lot of rocks at the surface.
Purple Milkweed (Asclepias purpurascens)
This is your purple blooming milkweed. Rare and hard to find, it's worth growing. Full sun and dry soil. Its reluctance to set seed pods contributes to its rarity. Native to the eastern half of the country.
Generally considered to be an indicator of oak savanna in the Midwest. It's rare because oak savannas, which used to be the most extensive Midwestern habitat type, are virtually gone now. Kent Pfeiffer reports that they started burning a remnant oak savanna about a decade ago and it was amazing just how much purple milkweed reappeared, how well the plants flowered, and how many pods they set. When they started the restoration project, a botanist friend told him he'd never seen it in bloom in his whole life. Now, blooming plants are as common as dirt in those woods.
Redring Milkweed (Asclepias variegata)
Very white tight clusters of striking blooms. Wants dry soils and is one of the more shade tolerant species of milkweed. Native to the Eastern and Southern states.
Antelope horns (Asclepias asperula)
Produces unusual pale green flowers with seed pods look like antelope horns. Give it full sun with any moisture conditions. Native from the Midwest over to the Southwest.
Narrow-Leaf Milkweed (Asclepias fascicularis)
Long thin wispy leaves with white blooms. Native to the Western states and Pacific Northwest. Give it full sun with typical soil conditions.
Tropical Milkweed (Asclepias curassavica)
The tropical milkweed is controversial. As with all milkweeds, the monarchs make full use of this plant, but if it is allowed to grow into the winter, it can discourage monarchs from their normal migration further south. If you do grow this one, make sure you remove it from your garden by Thanksgiving. It is a lovely and easy to grow milkweed with brilliant red/yellow blooms.
Poke Milkweed (Asclepias exaltata)
Native to the East and South, this one looks just like pokeweed. It has nice big leaves and likes partial or dappled shade. Perfect for growing in your wooded or otherwise shady environments. Can grow over 5 feet tall.
Prairie Milkweed (Asclepias sullivantii)
A quintessential tallgrass prairie species. Only seen in high quality prairie remnants and in road distches adjacent to current or former prairies. Attractive bluish and rounded leaves, more suitable for gardens than A. syriaca. It moves around, but not nearly as quickly as A. syriaca. Native to the Midwest.
Fewflower Milkweed (Asclepias lanceolata)
This is another one of the swampland species, with very thin leaves. Native across the South, it can grow in either dry or wet conditions, liking full sun but tolerating partial shade just fine. In its native habitat it is usually found growing in wet forested bottomlands.
White Milkweed (Asclepias perennis)
Very easy to grow and quick to bloom species, this is small in size with cute little white clusters of flowers. Native to the South and Mid-South. Likes plenty of moisture and can tolerate shade.
Bluntleaf Milkweed (Asclepias amplexicaulis)
An interesting species with thick leaves that have deep waves all along the edges, with a pink midrib and stem. Native all over the Eastern half of the US. Likes dry sandy soil in full sun.
Green milkweed (Asclepias viridiflora)
This is another green-blooming milkweed, native everywhere in the US except the far west and pacific northwest. Nice vertical erect stems, can be grown in either dry or wet conditions.
Pinewoods milkweed (Asclepias humistrata)
Native to the Deep South, this is a really neat one to grow. The leaves are very thick with distictive purple veining. The name humistrata means that it is low growing. Stems will lay almost flat on the ground. Tolerates very sandy and dry soil. Can be grown in high forests.
Fourleaf milkweed (Asclepias quadrifolia)
As its name implies, this milkweed has a central stem which produces four leaves all oppositely arranged. Loose clusters of pink blooms. It looks like a forest plant, and that is the environment it favors. Native through most of the East and South.
Texas milkweed (Asclepias texana)
This is the species native to Texas, but keep in mind that Texas is a big country with a huge variety of ecoregions. The region where this species is found is the Hill Country, northwest of San Antonio, a semi-arid region. Texas Milkweed often found in dry, rocky soil under the shade of juniper trees. Produces strikingly lovely white clusters of flowers.
Longleaf milkweed (Asclepias longifolia)
Native to most of the East, Midwest, and South. Has long thin leaves and wants full sun with usually wet conditions.
Indian Milkweed (Asclepias eriocarpa)
The first one to mention of the desert milkweeds, this one is really fuzzy. The hairs give the plant a grey appearance; it looks a lot like Lamb's Ear. Native to California, tolerates poor soil and is very drought tolerant. An excellent choice for the Four Corners and all other dry regions.
Rush Milkweed (Asclepias subulata)
For those gardeners in the dry western states, this is your milkweed! Extremely tolerant of drought and poor soils. Native to Arizona, Nevada and California.
White-stemmed Milkweed (Asclepias albicans)
This is the other desert milkweed. It is native to the Mojave and Sonoran Deserts. Can reach over 12 feet tall. As with many desert plants, the stems are coated in a thin waxy and slightly pubescent (hairy) layer.
California Milkweed (Asclepias californica)
Fuzzy leaves, dark purple blooms, very drought tolerant) Central and Southern California
Zizotes milkweed (Asclepias oenotheroides)
This one is quite interesting. Native across the southwest, you can see in the photo that the blooms have tall hoods. More than just being a conversation piece in your garden, this one can tolerate drought conditions and poor soil. It is a short plant, under 2 feet tall, that absolutely requires full sun. Has slight pubescence (hairs) on its stems and leaves.