In spring, partially submerged boulders bordering the koi pond disappear beneath a dense planting of irises (yellow flag and various Japanese irises, below), sweetflags (Acorus calamus variegatus), and variegated cattails. A wide spreading Oakleaf hydrangea (Hydrangea quercifolia, below) sprawls at the water’s edge with exceptional vigor and unusually large leaves, with roots cooled by the…