How can I tell if a plant is milkweed or dogbane?

Dogbane (Apocynum cannabinum) looks strikingly similar to common milkweed (Asclepias syriaca). However, monarch caterpillars cannot eat dogbane. This look-alike can leave people disappointed if they're expecting monarchs on their milkweed, but it turns out to be the wrong plant! Here are a few ways they're similar and how to tell them apart.

Similarities: Both plants have milky sap if you break off a leaf, and their growth pattern is very similar. Both have opposite and oval-shaped leaves.

Differences: Common milkweed has a hairy stem, unlike dogbane. When mature, the dogbane stem branches in the upper portion of the plant. The flowers also look quite different. Common milkweed flowers are pinkish, large and ball shaped, whereas dogbane flowers are whitish green and in small clusters.