She would drink until the trembling stopped. Then she would wilt over the piano like one of Celia's spinaches when Tam Lin forgot to water the garden.
In this passage, the character describes her method of coping with her emotions by drinking heavily until her trembling ceases. This act of self-medication highlights her struggle with inner turmoil and her attempt to find relief through alcohol.
After drinking, she slumps over the piano, comparing her state to a wilted spinach plant belonging to Celia, which has withered due to neglect. This comparison emphasizes her exhaustion and the sense of being drained, both physically and emotionally.