Blooming lilac bushes reminded both Sue and JB of their Catholic school days. The nuns would not allow them to bring lilacs into the classsroom because of their alleged soporific powers. I'd never heard that. In The Wizard of Oz it was the poppies that put you to sleep.
The Ad de Vries Dictionary of Symbols and Imagery says lilacs can symbolize youth, first love, spring (duh!), and mourning. It's unlucky to bring them into the house, but there's nothing about them putting you to sleep. Perhaps the nuns were using "put you to sleep" as a euphemism for "kill you," since they are bad luck, and one is particularly warned not to bring them into a hospital.
Please enjoy these safe flowers that will enhance your alertness!