What a degree day is
A degree day is one day spent one degree above a base temperature. Plants and insects develop in step with heat rather than with the calendar, so a cool spring holds everything back and a warm one pulls it forward. Counting degree days from a start date tracks that development, which is why growers use them to predict harvest and bloom, and extension services time crabgrass pre-emergent, pest scouting and sprays by them.
How each day is counted
Each day adds its average temperature, (high + low) / 2, minus the base, and never less than zero. Models with an upper cutoff (corn at 86 °F, codling moth at 88 °F, seedcorn maggot at 84 °F) cap the day's high at the cutoff. Corn and sweet corn also raise the day's low to the base first, the method Purdue uses for corn; insect models don't, since a cold night slows them down. Some pest models use a sine curve through the day instead, which can differ by a few percent.
Chill hours
Fruit trees, many berries and nut trees need a winter's worth of cold before they bloom well, and nurseries rate each variety in chill hours: hours spent between 32 and 45 °F. Too few and buds open late and unevenly; that's why growers in mild winters pick low-chill varieties. Switch Count to Chill to tally this winter's hours from October 1 against the average winter. Each day's hours are estimated from its high and low, with the temperature taken as a smooth curve through the day, so a day from 30 to 50 °F counts about 11 hours. Chill works for US locations for now, using weather records and the average winter.
Too much chill doesn't hurt; a rating is a minimum. What stops a fruit crop in a cold place is the cold itself, so chill mode also shows the coldest nights: this winter's so far, and the typical and coldest of the past ten winters, against how cold the crop takes. A variety rated for your zone, snow cover and a sheltered spot all help, and plants that meet their chill early can leaf out in a warm spell and lose the year's flowers to a late frost.
Where the numbers come from
- This season, in the US: daily highs and lows from the PRISM climate grid (about 4 km), served by the NOAA Regional Climate Centers' ACIS. Yesterday is the latest day.
- A Bizon sensor: the air temperature logged where the sensor sits, so a greenhouse or a cold valley counts its own heat. Days without readings are filled from the average year.
- The average year, in the US: the last ten full years of daily highs and lows from the same PRISM grid, counted exactly the way this season is, so the two compare like for like.
- The average year, elsewhere: ten years of hourly ERA5 temperatures for your location, adjusted for elevation (contains modified Copernicus Climate Change Service information). Available anywhere in the world.
Sources for the models
- Vegetables, herbs and flowers. 68 garden plants use a Bizon estimate, the same one the garden planner uses: the seed packet's days to maturity read as heat, about 20 °F·days a day above 50 °F for warm-season plants and 18 above 39 °F for cool-season ones. Put in your own packet's days, since varieties differ. Sweet corn uses the extension model below.
- Berries and grapes. Grapes: UC Agriculture and Natural Resources, Grapes for Home Gardeners; Strawberry: University of Delaware Extension
- Sweet corn and field crops. Corn: Purdue heat units and hybrid maturity; Sweet corn: UConn Extension, Scheduling Sweet Corn Plantings
- Lawn & turf. Crabgrass pre-emergent: Purdue Turfgrass Science; Turf growth regulator: Kreuser & Soldat, University of Wisconsin-Madison (Crop Science)
- Pests. Alfalfa weevil: Nebraska Extension CropWatch; Apple maggot: University of Wisconsin-Madison Extension; Bagworm: Rutgers Plant & Pest Advisory; Black cutworm: Iowa State University Extension; Bluegrass billbug: Penn State Extension; Cabbage maggot: University of Wisconsin-Madison Extension; Codling moth: Utah State University Extension; Colorado potato beetle: University of Wisconsin-Madison Extension; Eastern tent caterpillar: University of Wisconsin-Madison Extension; Emerald ash borer: NDSU Extension; Euonymus scale: University of Wisconsin-Madison Extension; European corn borer: University of Wisconsin-Madison Extension; Japanese beetle adults: Iowa State University Extension; Obliquebanded leafroller: University of Wisconsin-Madison Extension; Onion maggot: University of Wisconsin-Madison Extension; Pine needle scale: University of Wisconsin-Madison Extension; Plum curculio: Cornell IPM; Seedcorn maggot: Iowa State University Extension; Spongy moth: University of Wisconsin-Madison Extension; Squash vine borer: University of Wisconsin-Madison Extension
- Chill ranges. UF/IFAS Extension (apple, fig, peach and nectarine, persimmon, pomegranate, blackberry, southern highbush and rabbiteye blueberry, strawberry, pecan); Midwestern Regional Climate Center (apricot, cherry, mulberry, European and Asian pear, European and Japanese plum, quince, northern highbush blueberry, currant, gooseberry, grape, kiwi, raspberry, almond, chestnut, hazelnut, walnut).
- Winter hardiness. NC State Extension Plant Toolbox: the coldest USDA zone listed for each species, read as that zone's low (zone 4a: −30 °F). Kiwi: Ohio State University, Kiwifruit and Hardy Kiwi. Cherry and Concord grape flower buds: Michigan State University Extension, in Fruit Growers News. Quince, blackberry, southern blueberry, currant, gooseberry, strawberry and almond have no figure yet.
Models are regional guides. Confirm timing with scouting and your local Extension office before spraying.