This section is from the book "Commercial Gardening Vol4", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is the best known. It grows 20-30 ft. high, and has peculiar bluntly heart-shaped notched leaves and clusters of bright - purple, pale - rose, or whitish flowers in May. It requires protection in cold places. 0. canadensis has bright-rose flowers. Increased by seeds and layers.