Lemon Balm, like so many other famous healers, comes from the mint family. It has a citrus-y scent and flavor that lends it just as easily to culinary applications as it does to medicinal and magickal ones.
The spell is a simple tea, mostly chamomile, inspired by the book to help her sleep.
I don't know about your household, but mine can use all the calm it can get right now. Thankfully, for a kitchen witch, calm and healing are only a bubbling cauldron away.