Once the forsythia begins to bloom, for instance, it's time to renew your war against crabgrass.
When to fertilize the lawn? Think apple blossoms falling. Time to set out tomatoes? Yes, if dogwood trees are in flower.
"Phenology makes us more aware of our environment," said Robert Polomski, a horticulturist and arborist at Clemson University in Clemson, S.C. "Associating gardening tasks with flowering times is a neat way to look at how nature really functions."
Forsythia grows most everywhere in Zones 5-8. Its yellow blossoms are among the most recognizable signs of early spring, making this member of the olive family one of the best seasonal indicators for gardeners. Turf grass specialists often use the bloom time of forsythia as a bellwether for scattering pre-emergent herbicides on crabgrass-prone lawns.
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"A garden weed preventer or pre-emergent kills the seeds before they can grow into seedlings," Polomski said.
Phenology blends science with legend. It charts plant and animal development, and how those are influenced by climate change over long periods of time. It also includes the observations of people who have worked the ground for generations.
Scientists know, for instance, that soil temperatures must reach at least 35 degrees before onion and lettuce seeds will germinate. But Felder Rushing, a former extension horticulturist, 10th generation American gardener and folklorist from Jackson, Miss., puts it in a more homespun and equally correct way: "When fishermen are sitting on the riverbank instead of on their bait buckets, the soil is warm enough to plant."
Some other reliable natural markers compiled by University of Wisconsin-Extension:
— Plant potatoes as the first dandelions bloom, and peas when the daffodils flower.
— Transplant eggplant, melons and peppers when the irises bloom.
— Start looking for trouble from squash vine borers when chicory flowers open.
— Put seed corn in the ground when oak leaves are about the size of a squirrel's ear.
— The time is right for planting tomatoes when lily-of-the-valley is in full bloom.
— Seed morning glories as soon as the maple trees leaf out.
— Grasshopper eggs hatch roughly at the same time that lilacs bloom.
— Prune roses when crocuses begin to flower.
Gardeners aren't the only ones who read signs of the seasons for practical reasons. Bird watchers use them for timing migrations, fly fishermen for signalling the insect hatch and farmers as clues in weather forecasting.
Phenologists monitor one species as a reliable way to track changes in another. Birds head north, for instance, just as the insects begin to appear in their summer breeding grounds. Insect populations build when their host plants produce leaves.
Native tribes in British Columbia used the arrival of buds and blooms from certain berry-producing shrubs to signal when it was time to fish for halibut or spawning salmon. That gave them a competitive leg up over other animals consuming the same, often limited resource.
"People good at observing things can often predict when the purple martins start arriving," Rushing said. "It becomes part of the local lore."