Plant advice from The Morton Arboretum: Big-leaved hydrangeas are beloved for their showy flowers. However, these small shrubs from Japan are generally not hardy as far north as Chicago. Cold winters in Zone 5 and colder usually kill their flower buds, which are set on the previous year's growth in most selections. In recent years, some cultivated varieties have been developed that are somewhat hardier, although they will do better in areas where they have the insulating protecting of reliable snow cover. Blue flowers are one of the plant's appeals, but big-leaved hydrangeas only have blue flowers when planted in acid soils. In alkaline soils, such as those in most of the Chicago region and the Great Plains, the flowers will be pink.