Phenology continues by noting the duration of these events and trying to connect seemingly disparate events in logical ways. Migrating birds, like redwing blackbirds , fly north following available food sources. Forsythia blooms are dependant on ground temperature-the same temperature that crabgrass finds most comfortable for germination. Pre-emergent herbicides, applied when the forsythia blooms, will stop crabgrass seeds in their tracks. If those same herbicides were simply applied sometime during early spring, they might lose potency before they had a chance to stop sprouting crabgrass seeds.
Some crops depend on a certain range of soil temperatures for consistently good germination.
Those temperature ranges are determined by phenological observations, observations that you can take advantage of when you start seeds. Many gardeners watch the emerging oak leaves and refuse to plant their gardens before the leaves reach a certain size. They've learned, through experience and trusting the experience of others, to trust the fact that summer gardening weather has finally arrived when oak tree leaves are the size of dimes-or whatever objective standard they use to chart the tree's progress. The Japanese beetles should emerge soon. I don't like them, but their arrival seems to coincide with the time that morning glories stop sulking and start energetically climbing the walls.