Some people swear by aspirin, sugar, or even dettol. It's a good idea when you first prepare them to crush the ends of the stems so they get a really good drink to start with
Change the water the day you leave and again when you return. Be sure to remove all leaves that would be under water. Crush stems of woody plants and give a fresh cut on an angle of about 1" to the bottom of each stem each time you replace the water.
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Newt
Newt