Lilac

Why do the leaves of my French lilac fold up? It lives outside the garden fence, in full sun, in rocky soil which we have not improved as we did the garden soil. It has small leaves and double dark purple flowers. We do water it regularly.

When a plant folds its leaves, it is defending itself from some kind of stress. In this climate, the common stresses are lack of water and cold weather (usually, but not necessarily, in winter). Your French lilac is probably trying to hang onto as much of its limited water as possible by folding its leaves. By doing so, it exposes only half its leaf surfaces...