But they don't have to. Just ask artist Shirley Tsoi Pui-yu. She uses a special preservation technique called "L'ecrin flower" - from the French for a "treasure trove of flowers". With its help, she keeps beautiful blooms intact, complete with their natural colours and shapes, inside airtight containers.
Tsoi learned the technique from its pioneer, Japanese pressed-flower expert Nobuo Sugino.
"Bouquets preserved with the L'ecrin flower technique are impeccable and full of life," says Tsoi, who first started using the craft in 2008. "They can easily be mistaken for [fresh] ones."
The key to the technique is "dehydration sand", she explains. Its tiny particles resemble the moisture-absorbing beads used to prevent fogging and corrosion of valuable electronics and cameras.
She gently and skilfully covers flowers in the sand and leaves them to dry out for at least three days.
Another preserving technique, popular in Europe, is freeze-drying. It works best with deep-coloured blooms that have moisture-rich petals like purple orchids. It is not so suitable for roses, though.
The L'ecrin flower technique can preserve all types of flowers, vines and leaves across the colour spectrum - from dark green to teal. It also stops blooms from shrinking too much or shrivelling up. Best of all, the flowers' fragrance survives the preservation process.
"When I take out boxes of L'ecrin flower materials for workshops, students say my studio smells like a garden in spring," Tsoi says.
Her favourite flower to preserve is the rose. "There is some sort of magic about the intricate layered arrangement of its petals that mesmerises the beholder," she says.
But a hybrid called Rieger begonia has put her skills, and those of fellow flower artists, to the test. Tsoi spent two weeks trying to dry the begonia using the L'ecrin flower technique - all in vain.
"No matter how long they sit in the dehydration sand, their petals still feel soft," Tsoi says. "And their peach-pink colour fades to grey."
Tsoi says pressed-flower artists have greater room for creativity - they can use dried petals for elements in paintings such as a blue river or a yellow vase. But she insists the three-dimensional results of L'ecrin are more pleasing to the eye.
Her brightly coloured works are a fusion of East and West. Japanese artists like Sugino tend to favour simpler designs, while their Taiwanese counterparts often use bright reds, pinks and purples.
Tsoi is planning to put her skills to practical use. She will launch a new service for brides who want to preserve their wedding bouquets. She will photograph, disassemble, dry and reassemble each bouquet. The process may take up to a month.
Tsoi's works will be on show at the Landmark North shopping mall in Sheung Shui until May 14