And, while cut flowers don't last forever, you can extend the length of time you'll enjoy them by employing a few simple tricks, including adding lemon juice and sugar to the water, or by lining a vase with lemon slices for visual interest (or, OK, to sometimes hide the stems). Here's really why you might want to put lemons in with your flowers.
Reason #1: To make your own flower food.
Often when you purchase a bouquet from the store, it comes with a little flower feeder packet. This is a combination of three substances: citric acid, sugar, and an antimicrobial agent. They work together to keep the cut blossoms going strong. (The citric acid helps open up those stems to allow water in, the sugar provides carbs, and the antimicrobial prevents fungus growth inside the vase.)
However, if you are using flowers from your own garden (or don't get the little packet of feeder), you can make your own mixture to keep those blossoms happy and looking good. Here's where you have to be careful about what you read on the internet: neither aspirin nor vinegar will help your plants.
Instead, make a mixture that duplicates the ingredients in the little packet. Mix 2 tablespoons of lemon juice (for the citric acid), 1 tablespoon sugar, and 1/4 teaspoon bleach (for the antimicrobial agent) into a quart of distilled water. The mixture both offers fertilizer for your cut flowers and opens their stems to water intake. Finally, it helps to reduce the microbes in your flower water, which slows down the decomposition of the stems.
Reason #2: For visual interest.
Lemon slices are pretty, in and of themselves, yellow and shiny with juice. Creative folk make use of their decorative qualities by placing them carefully in a large vase or pitcher. They cut the lemons into slim slices about 1/3 of an inch thick, then line the inside of the vase with them.
Do all those lemons make the water too acidic? They might if they were actually in the flower water. But here's the trick. In order to accomplish this cool look you use a slender vase inside the larger vase or pitcher.
Put cut flowers and their water (as well as cut-flower feeding mixture) into the slender vase. Then place this vase inside the larger vase or pitcher. The lemon slices get positioned between the two vases. You can see them from the outside but they are totally independent of the flowers and water.