Chipotle peppers have their dark color and smoky flavor for a very good reason. They’re actually smoked and overripe jalapeño peppers!
While the plump and green peppers are picked early, the “runts” of the jalapeño litter are left behind, unpicked, to dry and shrivel. When the peppers have attained a dark red color and are mostly dried, they’re picked, and moved to a special chamber. There, they are smoked for days, until they attain a prune-like appearance. According to Wikipedia, it takes about 10 pounds of jalapeños to make one pound of chipotle. They’re typically sold dried, or canned in (delicious!) adobo sauce.
If you were ever hoping to plant chipotles in your garden, you’ve got your work cut out for you!