This weekend I saw that one clump of Japanese iris (Iris ensata, below) close beside the waterfall of the large koi pond was dangerously crowded by overhanging branches of an exuberant Oakleaf hydrangea (Hydrangea quercifolia). On the other side of the falls a rambunctious Winter jasmine (Jasminum nudiflorum) has invaded another clump, and both shrubs…