Degree Days
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.
Daily calculation
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, alfalfa weevil at 90 °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. UConn's sweet corn example uses the plain average, so totals can differ a little from it. Some pest models use a sine curve through the day instead, which can differ by a few percent.
Data sources
- 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.
Model sources
- 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, with the region cutoffs from Winkler et al., General Viticulture (1974); Strawberry: University of Delaware Extension
- Sweet corn and field crops. Corn: Purdue heat units, emergence 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
Models are regional guides (sources). Confirm timing with scouting and your local Extension office before spraying.