Melt butter in a medium saucepan over low heat. stir in the flour to make a roux. Stir in the milk a little at a time until smooth. Mix in the dandelions, celery seed, thyme, and bay leaf. Simmer until the flowers are tender, 15-20 minutes.
(The book notes that Oliver Wendall Homes likens the color of dandelion to "sparks that have leapt from kindling sun's fire.")