Flower pots, they are not. But Stephen Procter’s large stoneware garden vessels, some as tall as 5 feet and incorporating 250 pounds of clay, are nevertheless functional pottery — even without the soil and the plants. In the clay world, he said, there’s always talk about functional versus nonfunctional pottery, attempting to draw a line between the two. Procter, a Vermont-based ceramist, has seen his and other such outsize garden sculptures in action, though. “An object that invites contemplation, and inspires, and offers this kind of mysterious sustenance is functional in a deep and important way,” he said. “Not functional that you’re going to drink your coffee out of it, but the work has high purpose in the landscape and in the world.” A substantial sculptural element can perform a variety of garden-design jobs, he added, strengthening the structure of the landscape by “calling attention to its junctures — the entry, or the transition point, or the destination.” Procter has watched the space-transforming powers of ceramics at work for roughly 20 years, since his first …