Roses take in a lot of water after they're cut. Fill the vase 3/4 full with fresh, cool water from the tap, then arrange the flowers in the vase so that the stems are within an inch of the bottom of the vase. This way they'll be able the reach the water they need to stay fresh.1  Consider adding some flower food to the water.art 2 of 3: Preparing the Stems. 1. Trim the stems underwater. Trimming the stems underwater helps to keep them fresh by preventing air from entering the bottom of the stems. When air gets in, it immediately starts to cause flowers to deteriorate, the same way it turns apples and avocados brown and mushy.