A Primer in Monarchs and Milkweeds:
Compiled by Donna Carr Reilly from many sources.  Click on these links.
The Xerces Society
and Monarch Watch
The monarch butterfly is a wonder of nature! No butterflies migrate like the Monarchs of North America. They travel much farther than all other tropical butterflies, up to three thousand miles. They are the only butterflies to make such a long, two way migration every year. Amazingly, they fly in masses to the same winter roosts, often to the exact same trees. Their migration is more the type we expect from birds or whales. However, unlike birds and whales, individuals only make the round-trip once. It is their children's grandchildren that return south the following fall. The peak migration in our area of New York is September.

In the last 20 years, the population of monarch butterflies in the eastern US have declined 90%. A major factor for this decline is the use of herbicides, which result in the loss of milkweed across the US. Milkweeds (Asclepias species) are the only plant where the monarch butterfly lays it’s eggs and the only leaf the monarch caterpillar can eat. The use of genetically modified crops that are resistant to traditional herbicides have enabled farmers to spray entire fields, killing all the “weeds,” including Milkweeds, while sparing their crops.
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Milkweeds (Asclepias species) are the only plant where the monarch butterfly lays it’s eggs and the only leaf the monarch caterpillar can eat. 
As we lose more milkweeds, we lose more Monarch Butterflies. Conservation and restoration of milkweed populations are essential to ensuring the survival of the Monarch. There are 72 species of milkweed native to the U. S. and Canada. Milkweed is NOT a Weed. Flower color varies considerably among milkweeds and include: white, yellow, green, purple, pink, orange, and red.
There are many species of Milkweeds adapted to our area. It is suggested siting them according to their preferences. #1, Butterflyweed (Asclepias tuberosa) likes full sun in very well drained, infertile soil. #2, Swamp Milkweed (Asclepias incarnata) prefers moist, more fertile soil, in full sun. It will also do well in average garden soil. #3, Tall Milkweed (Asclepias exaltata) is more of an edge of the woods species, and likes moist soil in part sun to part shade. 
Unless you have a planting area with high invasive plant pressure, Kim Elerman from the EcoBeneficial website would avoid planting #4, Common Milkweed (Asclepias syriaca) which can be a bit of a thug. A great, more well-behaved alternative would be #5, Asclepias purpurascens (Purple Milkweed) which is similar in appearance and does quite well in full sun with average to dryish soil.
#1 Blooms: June to August         #2 Blooms: July to August              #3 Blooms: June to August       #4 Blooms: June to Aug.       #5 Blooms: May to June