Boutonnieres last for a surprisingly long time, but they don't last forever. By carefully selecting flowers, planning ahead and storing the finished boutonnieres properly, you can ensure that you have lovely living flowers for your special occasion.
The flowers in a boutonniere are out of water, and different types of flowers will stay alive and looking good for different lengths of time. A florist can give you options for longer-lasting flowers, but even standard boutonniere flowers like roses and carnations will last for several days if kept in the right conditions. Some flowers will be just fine for up to a week or more. Boutonnieres are generally a single flower with a bed of greenery behind. They should be tied together tightly with wire or florist tape (even if the stems are left loose at the bottom) to ensure that the arrangement stays together and can be easily fastened to a lapel.
Finished boutonnieres should be kept moist and cool until the time they are needed. You can heavily mist the flowers and seal them up in a plastic bag or box, which is then stored somewhere cool, like a refrigerator (but not a freezer). Check to make sure they are still moist occasionally. Storing them in a cool place and misting them every few hours can work, as well.
While boutonnieres can survive for up to a week, it really is best to make or pick them up close to an event. The day before or early on the day of the event is usually ideal. The fresher the flower is, the better it generally looks. If you're getting boutonnieres for several people, it is also a good idea to get or make a spare just in case. When you are making the boutonnieres yourself and want to make them some time in advance, it might be worth it to make a test boutonniere a few weeks ahead of time and see how long it lasts. That will give you a good idea of how long the flowers you are working with will live and look attractive. Make sure to use the same kind of flowers and greenery you intend to use for the final boutonniere.