Big turnips flowers can be left in water with food color to create the orange, red, blue and yellow flowers seen in the photos on this blog. Before use they should be drained then lightly rinsed and drained well again to remove excess color. You will probably want to handle these with plastic gloves on to avoid dying your fingers all colors.
Leaves made of leaks and scallions are also best cut the day before as they will develop a more natural curl overnight as well.
Bowls and vases made from hard pumpkins or calabash can be carved the day before and wrapped in plastic wrap until needed. Small vases made with acorn squash will keep particularly well overnight. All hard vegetables used in this way can be rubbed with a light coat of cooking oil before use to give a nice shine and a fresh look.
Softer items like watermelon should be carved the day needed as they don't hold up as well and give up a lot of juice.
The flowers can be arranged an hour or two before use and should if possible be kept in a refrigerator until use as well. Once removed from the fridge it helps to occasionally spray them lightly with water to keep them fresh. Not that they look really bad if allowed to dry, sometimes they look quite interesting even the next day after use!
