I’m curious to know when’s the best time of year to plant hydrangeas?
I’m not sure what climate you live in but, the goal is to avoid extreme heat or frost. Your best bet is Fall or early Spring (when you sure the risk of frost has passed). Once the Hydrangea’s root system is more established, it will be much more prepared to deal with weather-related stress.
It’s important to remember that hydrangeas are sensitive to pH- for example, hydrangeas have an amazing deep blue color on Cape Cod where I spend the summer- this is because the soil tends to run at a higher acidity.
Fun fact about hydrangeas- in some Asian cultures, they use hydrangeas for herbal teas.