in Thoreau’s Journal:  

I am surprised to find Walden completely open. When did it open? According to all accounts, it must have been between the 6th and 9th. Fair Haven must have opened entirely the 5th or 6th, and Walden very nearly at the same time. This proves how steadily it has been melting, notwithstanding the severe cold of the last half of March; i. e., it is less affected by transient heat or cold than most ponds.

The flowers have blossomed very suddenly this year as soon as the long cold spell was over, and almost all together.