A note about growing hydrangeas for you to consider before you buy them- Old fashioned hydrangeas flower on stems the fall prior. People like to cut them back because they don't like the brown stems all winter but what they do not know is that they are cutting off the flower buds for the following year, and when this happens, they have to wait 18 months for that plant to flower again. If that homeowner cuts back that hydrangea every year, that plant will never flower. Ever.
New hydrangea plants have been hybridized to flower on "new wood," so even if you mow the plant down in March, it will still flower that same summer. I tell people to NEVER prune hydrangea plants- there really is no need to. If the plant is too tall, move it. If you don't like the brown stems in the winter, choose another plant for that spot, or get used to it.
Hydrangeas push out flowers all summer if they are happy. Making them happy includes good soil, commercial grade fertilizer, adequate sunlight, and of course, water when its hot out. Flowering for a plant is like giving birth for a human- it takes a lot of energy from the parent, so plant hydrangeas in either full to part sun. I consider full sun to mean 5 hours a day of full sun, or three hours a day of full sun then filtered light all day. In full shade you might get good flowering if all other needs of the plant are met.
Hydrangeas are grown in really lightweight soil at the nursery, so you really need to pay close attention to the plants for the first year after you plant them to assure the plant grabs hold of it's new home asap.
If the plant gets dessicated, they usually rebound with closer attention with water, and an apology. When they begin to dry the first year, they "candycane," drooping their flexible stems downward, Almost as soon as you water them, they perk right back up. Spraying them with water from the hose cools them off and buys time for the roots to start to drink up the water you just offered them. Drape a light sheet over them to shade them if the wilting is severe. Best thing is not letting them dry out in the first place.
New varieties are shorter than the old ones, ranging in size from 2-3-4-5' tall which makes them great for foundation plantings, decks and patios. If you want them in an above ground container, plant it in the ground for winter because the roots will not survive the cold temps. Bringing the pot into an unheated garage solves the problem.
Some new varieties have foliar characteristics that extends their attractiveness way beyond the flowering weeks. Red foliage and stem color are two traits that are lush and unique. Some have red stems, some are black. Some of the new white ones have the very whitest flowers on the very darkest green leaves, the contrast between the two being a stunning attribute. Some of the newest cultivars have double or triple petals on each flower, or two-toned blossoms.
There's one thing that can be said about the horticultural industry- research and development is a continual focus, with new varieties coming into the marketplace yearly for discerning gardeners and people who just want something different in their yard.