Linn County Master Gardener
How did farmers and gardeners know when to plant, harvest and expect insects before calendars? They looked to the weather and temperatures, or signs from nature. Observing nature’s calendar events is the science of phenology. Agricultural scientists are recognizing the relationship between temperature and plant development. There is current research on development rates of plants, insects and other organisms so growth, blooming and harvest can be predicted during a growing season. The accumulation of “heat units” (sun energy) is measured in degree-days.
Degree days measure the amount of heat accumulated over time. The simplest way to calculate is minimum daily temperature plus maximum daily temperature, divided by 2, minus the minimum temperature. It’s easier for most of us to watch nature like our predecessors have done for centuries. We can watch bud burst, bird migration, insect hatches and animal activity. The rhythm of life around us signals when temperatures and precipitation are optimal for planting or preparing for insect invasions. Natural cues are more reliable than predicted frost dates, but frost dates are helpful too.
Watch for “firsts” in the yard and garden. First bud, first bloom, first animal migration, first appearance of different insects, first emergence of hibernating animals, and first amphibians (croaking frogs and such). These events all signal the coming of spring. Those of us who put out native bee houses and cocoons are waiting for the fruit trees start to flower so the bees will be able to find pollen for food. When the apple trees bloom it will be time to plant bush beans. When apple blossoms fall, pole beans, corn and cucumbers can go in the ground. When crabapple trees are in bud tent caterpillars will hatch. Apple maggot flies will start to attack the fruit when Canada thistles bloom.
Many flowers and vegetables have similar degree-day requirements for sprouting and growth. Some examples we can relate to are helpful when planting the spring garden. The crocus are blooming this week, so it’s time to plant radishes, parsnips and spinach. The forsythia is breaking bud so it is safe to plant peas, onion sets and lettuce. The daffodils are opening up so beets, carrots and chard can be planted. Dandelions show their color just before time to plant potatoes. Wait for quince to bloom or lilacs to leaf to plant cabbage and broccoli. When lilacs are in full bloom it will be safe to plant tender annual flowers and squashes. However, when purple lilacs bloom grasshopper eggs hatch.
Plant perennial flowers when maple trees leaf out. When morning glory vines begin to climb Japanese beetles appear and start to hunt for slug dinners. When foxglove flowers expect Mexican beetle larvae. Plant pansies, snapdragons, and hardy annuals when aspens and chokecherries have leafed out. Wait until lily-of-the valley and bearded irises bloom to transplant tomatoes, peppers and eggplant (heat lovers).
If signal plants are not growing in your area, notice other coincident events, record them and watch for them next year. To learn more about vegetable degree-day research with models to schedule planting or manage weeds and harvest, visit smallfarms.oregonstate.edu/croptime or find the free publication “EM9305 Vegetable Degree-day
Models” at catalog.extension.
oregonstate.edu.
