1 : a branch of science dealing with the relations between climate and periodic biological phenomena (as bird migration or plant flowering)
2 : periodic biological phenomena that are correlated with climatic conditions
Here's some of the phenology advice I've come across in regards to gardening:
When red winged blackbird females return it is time to plant peas.
When the chickadees build their nests, plant peas and spinach.
When dandelions are blooming plant potatoes, beets, lettuce, spinach and carrots.
When the iris bloom, plant the peppers and eggplants outside.
When lilac is in first leaf, plant beets, lettuce, and spinach.
When lilac blooms, plant beans, cukes and squash.
Plant corn when oak leaves are the size of squirrels ears.
Plant your corn when apple blossoms start to fall.
Plant soybeans when oak leaves are the size of mouse ears.