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If you are cutting calla lilies yourself, give the plant a good watering the night before. This will ensure that they will be hydrated when you cut them.  Hydrated flowers will last much longer than ones that are cut when they are dehydrated.  Cut the flowers early in the morning before the day heats up to prevent them from drying early. Use a clean sharp knife to remove flowers at the base of the stem. If you have purchased or been gifted callas, unwrap them as soon as possible. Immediately after unwrapping, get them into water quickly.  If you are unable to do this, put them in a cool, dimly-lit place such as a basement until you can attend to them. Make sure that they remain cool so they do not wilt. Calla lilies are very fragile flowers. The less you can handle them, the better.  Especially avoid handling the flower heads. If these have been supplied with a protective wrapping, carefully remove it. You’ll need a deep vase for tall callas. Use a very clean vase; some florists clean vases using a combination of dish soap and a little bleach.  Rinse very carefully afterwards to remove cleaning product residue. Any remaining product residue can kill your flowers quickly because it is toxic. Measure your calla lilies against your selected vase to determine where best to cut them. Cut them to make sure that they can protrude from the top of the vase while at the same time making sure their stems will remain in the water.  Use a sharp clean knife to cut the stem at a 45 degree angle. Cutting the stems at an angle maximizes the amount of cut cell in contact with the vase water, which will help the plant take up more water. Cut them underwater so the cut surface isn’t exposed to the air. Avoid using a dull knife, or a pair of scissors, as these tend to crush as they cut. This cell damage stops the plant from being able to absorb water. Lilies won’t appreciate being crowded in a vase so avoid putting too many into one container. Add flower feed to the vase water or, if you have none, a teaspoon of sugar and a drop or two of bleach for every two liters of water.  Some people advise using a dash of Sprite or 7-up instead. Regular calla lilies will need a vase filled about 2/3 full of water. Mini callas will want much less than this, as only the lower part of the stem should stand in the water. This prevents the stem from going soggy on a mini calla.
Give the plant a good watering to ensure they stay hydrated. Unwrap purchased calla lilies as soon as possible to prevent drying. Be aware that callas bruise easily to avoid damaging them. Get a deep vase for tall callas to avoid constricting them. Compare your stem against the vase to see how long to trim it. Give your flowers something to eat to ensure their health.