Old fashioned hydrangea, Hydrangea macrophylla, were somewhat predictable. Those that developed color bloomed pink in alkaline soil or blue in acidic soil. White hydrangea only bloomed white. Nowadays, some cultivars bloom better in pink while others bloom better in blue. Pink cultivars in acidic soil or blue cultivars in alkaline soil may be purple.

Floral form has also evolved significantly. Old fashioned hydrangeas bloom with big and rounded trusses of many small flowers. Such trusses are about four to eight inches wide. Some modern cultivars bloom with flatter or conical trusses. Some produce tiny florets in the center with bigger florets around them. All hydrangeas bloom for summer or autumn.

Big hydrangeas can grow about six feet high and wide. Most modern cultivars are more compact. The deciduous foliage is lush. Individual leaves are about six inches long, with serrate edges. Hydrangea perform better after dormant pruning during winter, to remove older canes. Younger canes that grew during the previous season will be more vigorous. Although bulky, hydrangea blooms are delightful as cut flowers. All parts of hydrangea, though, are toxic.