Freezing a flower can either preserve or destroy it, depending on how the freezing occurs and how cold the frost is. Fall frosts end the gardening season by wiping out any remaining blooms, and unexpected spring frosts often destroy early-blooming flowers such as daffodils. A bit of cold weather can actually improve the flavor of flower buds like Brussels sprouts because the frost brings out the natural sugars in the plant.
Frost tends to occur on clear nights when there is no cloud cover to trap heat on the ground. Soil is warmed throughout the day by the sun, and much of this warmth is retained at the end of the day when the air cools, especially if the soil is moist. When the air around a plant freezes, the moisture in the air turns into tiny ice crystals and settles on the plant's surface. Freezing nighttime temperatures sometimes spare flowers because the petals can retain some heat and the inside of the flower doesn't freeze.
Flowers have tiny, intricate veins that carry moisture to the petals through the sap. When a flower freezes, the water contained in the veins expands and breaks up the veins. Since the flower can no longer get moisture, it turns brown or black and dies. Flowers that have been frozen often look like they have melted because they die so quickly. Some flowers, such as crocus, snowdrops or primroses, can withstand a light frost on their surfaces and are only affected by getting frozen through in a hard, killing frost.
Freeze Drying Flowers
Florists can preserve flowers by freeze drying them. This process can save the shape and color of a flower because the inside of a freeze-drying machine is a low-pressure atmosphere created by a vacuum. When a flower is frozen at low pressure, the ice crystals are pulled out of the petals as a vapor, leaving the petals intact. The vacuum also removes the oxygen that causes cells to break down. The process of freeze drying takes several weeks because the flowers are frozen quickly, then the temperature in the machine is gradually increased. This slowly removes the moisture without damaging the flowers' structural integrity.
Another way to freeze flowers and preserve them for a short time can be done with a home freezer, and the frozen flowers can be used to decorate summer drinks. Choose sweet, edible flowers such as violets, rose petals, borage or peonies. Fill an ice tray with water and drop the flowers in. When the water freezes, the flowers' color and shape will remain the same. After the ice melts, the flowers can be eaten, although they will begin to soften and wilt right away.