In Thoreau’s Journal:

May 23, 1852:  The buttercup season has arrived here.

May 23, 1853:  At first we had the lighter, paler spring yellows of willows (cowslips even, for do they not grow a little darker afterward?), dandelion, cinquefoil, then the darker (methinks it is a little darker than the cowslip) and deeper yellow of the buttercup; and then this broad distinction between the buttercup and the krigia and senecio, as the seasons revolve toward July.  Every new flower that opens, no doubt, expresses a new mood of the human mind.