Flowers drink a lot of water! It is not uncommon for a large flower arrangement to suck up all the water in a vase within the first day or two you have it at home. Keep the vase full to ensure the flowers do not dry out and wilt. Flowers are also highly susceptible to bacteria that builds up as stems sit in the water. By changing the water in the vase every few days, even if the water hasn’t been used up, will help keep your flowers fresh longer (and avoid that horrid rotten smell that develops if you let them sit a long time). For large formal arrangements, carefully tip the vase over a sink to let the water drain without disturbing the design. Then re-fill the vase by gently pouring water in at the top of the flowers.
As flowers sit out of water on your ride home, the ends of the stem dry out and the cells die, making it difficult for the flowers to absorb water. By cutting the stems just before placing them in water again, you expose fresh tissue that can suck up the water much more efficiently. When you trim stems when you change the water in the vase a few days later, you remove tissue at the tips that may be breaking down and once again expose fresh tissue that absorbs more water.
Sometimes people think they should set their vase of flowers in a sunny windowsill since that is where a plant would be happiest. However, cut flowers are actually the opposite of potted plants. They are at their peak of perfection. Sun and heat will encourage them to “mature” and thus quicken their demise. Instead, keep your cut flowers in a cool dark spot if you would like them to last as long as possible.
After you throw out your last arrangement, be sure to wash the vase/container very thoroughly in hot soapy water or, better yet, in your dishwasher. Bacteria build up in dirty vases and do not go away just because the vase dries out. As soon as you add water again, the vase will once again be full of bacteria and your new bouquet will be subjected to the same bacteria that killed the last bouquet. Give your flowers a fresh clean environment free of bacteria and they will last much longer.
If you use dull old scissors or snips to trim your flowers, you are often smashing, and thus damaging, the tissue/cells at the end of the stem. Damaged cells cannot absorb water as effectively as healthy cells. Sharp scissors ensure a clean cut that leaves cells unharmed (except the poor few that inevitably get sliced).