The simple answer to the best water temperature for cut flowers is: It depends. Under some circumstances, the refreshing shock of ice water is a magic elixir. And at other times, and for other flowers, nothing but a sun-warmed room-temperature bath will do.
How Does Your Garden Grow?
Cut flowers from your own garden are the ultimate luxury. You get custom blooms and bragging rights all in one. If you act like a professional florist, those homegrown hothouse beauties will outlast the dinner party. Just-cut flowers have probably been in a temperature-controlled greenhouse or a sunny spot in the garden. They are warm, and traumatized, not the best combo for longevity. Flowers last longer in cold temperatures, so give those delicate souls an Arctic blast. Plunge them into a sink or bucket of cold water, up to but not including the blooms. Once they've cooled down, store your nice, crisp flowers in the refrigerator for about six hours at near-freezing. Flowers love 34 to 36 degrees Fahrenheit and will thrive much longer if you subject them to the Big Chill when they are freshly cut.
Grown, Flown and Overblown?
After the initial cooling-off period, treat your flowers the way florists do when they receive cut blooms and buds that have been cooled before shipping. Floral preservative dissolves better and flowers hydrate more readily in warm water. Cut flowers will absorb water between 100 degrees F and 110 degrees F. Warm molecules slip through the xylem, or channels, more easily and faster than cool water. But extreme cases may call for extreme measures. Badly wilted flowers and wilted blooms with tough, woody stems may revive in a hot tub. A bath in 180- to 200-degree water, followed by a session in the cooler could give them a second wind, according to The New York Times.
Aside from a rescue mission, don't subject most flowers to extremely hot water. Anything above 110 degrees Fahrenheit kills stem tissue and invites killer bacteria to colonize the damage. Heat, in general, speeds blooming and petal shedding. A quick, short-lived bloom is fine when you're salvaging a flower or bud already wilted, not so great for a healthy flower.
Cutting and Cooling
Cut flower stems under water so no air is absorbed to block the xylem from uptake, eventually choking the flower with "inhaled" air bubbles. Keep containers, cutting tools and water meticulously clean to inhibit bacteria. Change the water -- recutting the stems -- daily, sticking to room temperature water for almost all flowers. The exception to a daily dose of warm water for cut flowers is any flower that grows from a bulb. Those blooms, like tulips, daffodils and hyacinths, prefer their water to be consistently cold. Try the trick of popping your well-watered cut flowers in the fridge overnight, to make them last longer.