To grow milkweed, you need at least half-day sun. Also, many varieties prefer their roots a bit damp. What's in it for you? Dramatic flowers, interesting foliage, great seedpods, and nectar-seeking butterflies. If you're really lucky, you'll also be delighted by holes in the leaves and ragged edges, the sure-fire sign of baby monarchs. Look for fat caterpillars resembling tiny, horned tigers.
The curious thing about milkweed is not that monarchs are crazy about it and won't eat anything else; it's that no one else can stand it. So, if someone's munching on your milkweed, you can be certain that you've hit the monarch jackpot.
Other animals, regardless of species, think milkweed tastes awful and get sick from eating it. It is life threatening in large quantities but tastes so bad that you have to be starving to eat that much of it. Native Americans did eat the young asparagus-like shoots but had to boil them twice to get out the bad juices. Technically, the bad juices are said to be cardiac-active steroids (whatever they are). In many parts of the Americas, milkweeds are (very carefully) used for medicinal purposes.
Monarchs, as you've probably read, also taste bad. In fact, so bad that once a bird has tried one, it will shun the species for life. And as you've probably also read, Viceroy butterflies look like monarchs to fool birds into thinking that they taste bad too. Viceroys, though, get the best of both worlds: defense against birds and something tastier to eat. The monarchs, of course, get the bad taste by storing the yukky cardiac-active steroids in their bodies.
The milkweed pictured about is the North American "common" milkweed (asclepias syiaca), now in flower along the Mill River near where I live in Connecticut. There are about 200 native milkweeds, about 100 of them from North America, and countless garden hybrids. Most milkweed nectar will attract humming birds as well as butterflies.
If you want to foster baby monarchs (a very good thing, indeed), note that monarchs only like some milkweeds. The butterfly web sites recommend the common milkweed (above), and another perennial North American native called the swamp milkweed (asclepias incarnata). If you really want to roll out the red carpet, monarchs' all-time favorites are reported to be certain tropical milkweed hybrids.
The fluffy milkweed seeds are good for stuffing things like pillows and floatation devices. The seedpods, which are silky inside like seashells, look wonderful in the garden and in dried arrangements. The milkweed stems contain fiber which can be used like hemp or flax.
For baby monarch pictures, see
http://home.neb.rr.com/monarchrose/milkweed.htm. Sites for more information, pictures, and plant sources: