I’m a Phenologist—Are You?
Gardeners become incredible observers of nature and love to share this knowledge with each other. Some observations may be anecdotal, however, and are not based on repeated observations. An example of this is the belief that the width of the black band on, the banded woolly bear caterpillar (Pyrrharctia Isabella) predicts the severity of the upcoming winter. Differences in the caterpillars are actually related to their age! Wooly bear caterpillars molt several times and the older caterpillars have more black than young ones. Wooly bears that feed where the fall weather is wetter have more black hair than caterpillars from dry areas.
However, we all recognize that plants (and animals) have life cycles. Perennials usually regrow. Annual seeds sprout. Plants flower. The leaves of deciduous trees form an abscission layer in the autumn and fall off. Evergreen trees also lose needles (leaves really), but not all of them at once. Tulips bloom in spring. How do plants know when to do these things?
The study of how these events occur is called “phenology.” The word was coined by the Belgian botanist Charles Morren in 1849. It comes from the Greek language and means “to show” and “study.” Phenology is really the study of when biological happenings reoccur from year to year. These happenings (the earliest opening of buds, growth, behaviors, the flight of different butterflies and bumblebees, the first re-appearance of migratory birds in spring and the movement south of migratory birds, and the beginning of leaf color change in the autumn) depend on rainfall, available sunlight, and air and soil temperatures. Phenological events vary in their timing from one year to the next. You may have heard people say to prune your roses when forsythia bloom; the buds of roses swell around the same time as the flower buds of forsythia open. They don't say to prune on April 15.
Many phenomena are very sensitive to small variations in climate especially temperature. Thus, phenological records are useful in the study of climate change.
Paying attention helps us decide when to plant seeds, when to plant seed potatoes, when Japanese beetles hatch. Japanese beetles don't look at their watches and say, “Well it is July 5. Time to emerge and eat Carol Ann's roses.” What does trigger them? One answer is found in Growing Degree Days.
Growing Degree Days (GDD) are one way of determining behaviors of plants, insects, and disease. This is important because it means that it is not the date that is important but a number of factors including temperature. Beginning on March first and each day thereafter, the average of the highest and lowest air temperature for each day is calculated and fifty is subtracted from that result. If the answer is less than zero (a negative number) the number of growing degree days is zero. Got a number greater than zero? Write it down. Each day's growing degrees are added to those that came before. Each species has a certain number that needs to be reached (heat accumulation) in order for certain behaviors or growth patterns to occur.
Forsythia begin to bloom when they have accumulated between 1 and 27 GDD. Crab apples bloom when they have accumulated between 50 and 80 GDD. Elderberries need to accumulate between 300 to 400 GDD to bloom. That's one reason these three plants don't all bloom at the same time. Farmers can also make use of GDD to time the use of pest controls.
You can be a phenologist too! Here are some timing tips.
When the buds break on your crab apple, tent caterpillars are hatching, as their accumulated GDD overlap.
Plant your potatoes when your shadbush flowers. This also means that the soil will be warm enough.
When your lily-of-the valley bloom, plant your tomatoes.
When the roadside weed chicory blooms, watch out for squash vine borers.
(By the way, the number of GDD for Japanese beetles is between 1029 and 2154.)
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