Practicing phenology: The art of gardening in rhythm with nature


MADISON, Wis. – “When the daffodils begin to bloom it’s time to plant peas.” A gardener who follows this kind of natural cuing for planning a garden schedule is practicing phenology.

Karen Delahaut, an extension integrated pest management outreach specialist works at the University of Wisconsin, works with phenological observations in monitoring the emergence of insect pests that commercial vegetable growers worry about.

Phenology is a field of science concerned with the timing of natural living processes with weather events over a period of years. Studies in phenology focus on when plants bud, flower and fruit; when insects emerge; when frogs get active in spring; when lakes freeze and thaw; and when birds migrate.

“You’ve probably heard the adage “Don’t plant corn until oak leaves are the size of squirrel’s ears.” Delahaut said. “Native Americans made the correlation that when oak leaves were the size of squirrel’s ears, the soil temperature had warmed enough to promote rapid seed germination and consequently a higher corn yield, so it was the ideal time to plant corn.”

Looking for frost.

Phenology is useful for gardeners who want to know when the last spring frost and the first frost in the fall will occur in their area.

Spring events are good indicators for gardening phenologists. When the crocus and forsythia are blooming, for example, it’s time for rose pruning and crabgrass prevention. Phenology can also be used to determine when to plant herbaceous and woody plants and when to hunt for morel mushrooms.

Phenology can also play an important part in insect control.

Delahaut, a member of the extension urban agriculture/horticulture team, is currently working with vegetable growers to help control the squash vine borer. The adult borer moth is active for about two weeks, during which it lays its eggs on susceptible plants.

Chicory flowers.

Using natural signals like the flowering of chicory, Delahaut can alert growers when to go on the defensive against the moth.

Phenology can provide other beneficial information, such as helping allergy sufferers determine when their particular allergens are likely to be a problem. Because allergies to plant pollen are often seasonal, reactions can vary depending on the weather and the rate of plant development.

“Trees typically pollinate in May, grasses in June, and ragweed in August,” Delahaut said.

Delahaut has posted an article on phenology on the Wisconsin urban agriculture/horticulture team web site at

To learn more about phenology, you may also want to visit the Wisconsin Phenological Society web site at


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