Here are a few tips to help your cut flower arrangement have the longest life possible.
Keep out of heat and direct sunlight
Light and heat will speed the biologic activity of the blooms, causing them to age quickly and die faster. Choose a cool site, out of direct sunlight.
Give them a fresh cut
If you’ve purchased a bouquet, vs. an arrangement in a vase, give the stems a fresh cut and immediately plunge them into tepid/lukewarm water. If you can cut the stems in/under water, that is better. This prevents air bubbles from getting trapped in the tissues of the stem that take up water (those bubbles prevent water uptake). Be sure to use a sharp knife, and cut on a 45 degree angle.
Change water daily
The leading cause of cut flower death is bacteria that grow in the water and on plant tissues. These little nasties exude a slime that clogs the drinking vessels of the plant tissue, so the flower can no longer get the water it needs. Change the water daily. Use tepid, not cold, water (unless you are working with bulbs!).
Use floral preservative
If your blooms came with a packet of floral preservative, mix it according to packet directions and use it! This contains carbohydrates the flowers need, as well as antibacterial compounds to suppress the growth of stem-clogging bad guys. If you don’t have preservative, then you can use one drop of bleach (literally, one drop – more is not better) in gallon of water. This kills the bad guys and that is the most important part.
Keep it clean
Don’t allow leaves, petals, bark bits, etc. to hang around in the water. This is bacterial breeding ground. Cut away any leaves that fall below the water line and keep the water clean.
Compost it when you’re done!
Remember to toss your bouquet in the compost, not the trash, when it is done. And just in case you’ve gotten your flowers from another florist who uses that nasty green foam, remember it is not biodegradable. Put it in the trash, and wash your hands after handling it!