In the heat, flowers need more care than usual. Firstly trim 4-5cm off each stem (slightly more than we normally advise) at a 45° angle. Pop them into a clean vase filled with tepid water and flower food as soon as possible. Watch them perk up within 24 hours after they've had a good drink.
Be sure to keep vase water topped and change the water every couple of days as your blooms will be more thirsty than normal, and trim the stems every couple of days to enjoy your blooms longer.