Flowering plants need nutrients to grow and survive. These nutrients are provided from the soil through a plant's root system. There are several basic nutrients that plants need and they are nitrogen, phosphorus, and potassium. Other nutrients are also required but in much smaller amounts.
When a blossom is cut from a flowering plant its nutrient supply is severed. This means the flower no longer receives the nutrients it requires to survive. Without these nutrients the flower will die within a short time after being cut from the plant.
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Plants need energy from the sun, carbon dioxide from the air, and water to live. They use all three of these elements in a process called photosynthesis. Photosynthesis is how a plant produces/creates starches and sugar or food for their survival.
When flowers are cut from a plant they no longer benefit from the photosynthesis process and it is another reason why flowers die. They are no longer a part of the complex system that keeps them alive.
Ways to Prolong the Life of Flowers
There are ways to prolong the life of cut flowers after they have had their nutrient supply severed. It is important to purchase fresh flowers (visually they should appear to be just breaking into bloom).
If you are gathering flowers from your own garden be sure to place them in water immediately after cutting them. Also try to cut them early in the morning. The early morning is a good time because they aren't stressed from the heat of the day and they are filled with moisture from the dew.
Flowers should be cut on an angle to provide as large an opening as possible for water intake. If you cut them straight across instead of on a slant the stems will sit squarely on the bottom of the vase and they will not be able to take in water. Make sure that your flowers have adequate water at all times.
Temperature also plays a role in the longevity of your flower arrangement. Be sure to keep your cut flowers away from heat such as direct sunlight, radiators, or registers. Humidity also plays a role–if your home is dry, mist your flowers in the morning and in the evening.