Blooming in spring and summer, the hydrangea is considered a shrub. Can be large showstoppers in your garden and low maintenance as tend to grow all by themselves. Can grow up to 15 feet in height, they grow quickly and often fills in a space in just one summer. You’ll find hydrangeas growing in hardiness Zones 3 to 7 as perennials. Starts to flower in spring and last throughout summer, hydrangea flowers can be the foundation plant of your landscape.