Chris, leeks can actually be harvested at any stage of growth. Indeed, I much prefer the "baby" leeks, myself, when they're only about index-finger thick. More of the green remains tender at that point.

Scapes usually are associated with garlic. Hardnosed varieties (and, sometimes softnecks if they've been stressed) send up a central stalk that would be a flower stalk. Except they don't flower or set seeds. Instead, a bulbous mass forms on the end, called a spathe. If left alone, the spathe will fill with bulbils---tiny, round, "cloves" that are clones of the parent).

As they grow, scapes curl. Depending on variety, they curl anything from 270 degrees to two complete turns. Then they straighten out again.

If you harvest the scapes, just before they curl, you accomplish two things: 1. You get marginally larger garlic heads; and 2. You get the great culinary treat of the scapes, themselves.

Thanks so much for your help. I may now pick my leeks before I go on vacation and use them in whatever we choose to cook while we're away :-)