Steve has given you good advice. I grow many hydrangeas, so I'll offer you my experience. If you don't mind, I'll offer this suggestion - your nursery owner will be upset. Your plant is beautiful at the moment, but when you put it in your garden, I think it will be very stressed. First, it has been forced bloomed - it is blooming out of its normal cycle. Secondly, I suspect it has been over fertilized so that the foliage is deep green. By the way, there are very good reasons for buying plants like this, especially for an event . When you put it in your garden, put it under dappled shade. In our area, hydrangeas do not do well with afternoon sun. Don't put it too close to your oak - first, because the hole you dig may damage surface roots of the oak, - secondly, the oak will suck up all of the moisture that your new, stressed hydrangea will need to survive. After the blooms turn brown, I suggest you deadhead and cut the plant back to the point where you have the greatest amount of green foliage. Don't do any cutting back of the plant after the first week in August. Next year the plant should revert to its normal growth pattern. In late winter, give it 4 ounces of 10-10-10 around the base of the plant, but not touching the plant. Do not fertilize it this year, and focus on consistent watering. Well, that's my story, and I'm stickin' to it! Hope this helps. Good Gardening.
