Best Time To Plant Hydrangeas. Hydrangeas grown in containers outside in nurseries or garden centres can be planted during any season, but midsummer plantings should be avoided, if possible. Mopheads are the hydrangeas to grow if you want bold colour.
Ideally spring after frost is a good time. Hydrangeas need plenty of time to establish a healthy root system before blooming without freezing or drying up. “hydro,” meaning water, and “angeion,” which means vessel.
The best time to plant hydrangeas like most shrubs is in spring or autumn, planting at this time will give them an easier time to establish themselves.
I do not recommend planting earlier than this time, as august may be too hot. To avoid frost damage, leave the faded flowerheads on the plant and prune at the correct time. Fall/autumn is the best season to plant hydrangeas, followed by early spring. Hydrangeas are extremely popular plants to have in your flower garden, and for good reason.