in Thoreau’s Journal:

The early willow will bloom tomorrow. Its catkins have lost many of their scales. The crowded yellow anthers are already bursting out through the silvery down, like the sun of spring through the clouds of winter. How measuredly this plant has advanced, sensitive to the least change of temperature, its expanding not to be foretold, unless you can foretell the weather. This is the earliest willow that I know.  Yet it is on a dry upland. There is a great difference in localities in respect to warmth, and a corresponding difference in the blossoming of plants of the same species.