How to read growing degree days

Pest forecasts are full of numbers like this one: bronze birch borer adults begin to emerge at about 450 growing degree days.¹ It reads like an instruction. It isn’t one yet.

In the Twin Cities this spring, 450 growing degree days arrived on May 24. Or May 27, or May 30, or June 1, 2, 3 or 4. Every one of those dates is correct. Which one applies depends on how the heat was counted and where it was measured — and a horticulturist who knows which one the forecast meant protects the specimen birches in the right week instead of the wrong one.

In the last Field Note we argued that plants keep time in heat, not dates. This one is the manual: what a degree day measures, the recipe behind every number, how fast heat arrives, and the places where degree days quietly mislead.

What a degree day measures

Plants, and the insects that feed on them, develop faster as it warms, above a floor below which development all but stops. That floor is the base temperature. A degree day is one degree above the base, held for one day.

The common way to count it is the simple average: average the day’s high and low, subtract the base, and count zero if the result is negative. A day that runs from 48°F to 72°F averages 60°F; at a base of 50°F, that’s 10 degree days. Add each day’s count from a fixed start date and you have the running total that thresholds are written against.

So 450 growing degree days isn’t a number of days. It’s an amount of warmth: forty-five days that average 60°F, or fifteen that average 80°F.

Every number is a recipe

A degree-day threshold only means something alongside the recipe it was counted with. There are five ingredients, and each one moves the date.

The base temperature. Fifty degrees Fahrenheit is the usual base for landscape insects and woody plants, a reasonable approximation for many species rather than a law.² Wisconsin’s oak wilt pruning tool, built on a study of the sap beetles that carry the disease from our last Field Note, counts from 41°F.³ The national emerald ash borer forecast counts from 54°F.⁴ A thousand degree days above one base is a different amount of heat than a thousand above another, and it arrives on a different day.

The method. Averaging the high and the low is the simplest way to count a day’s heat, not the only one. The sine method fits a smooth curve through the day’s low and high and counts the part of it above the base.⁵ The 86/50 method, standard for field crops, raises any low below 50°F to 50 and caps any high above 86°F before averaging.⁶ On a warm day they agree. On a day that straddles the base — a cold morning and a mild afternoon — they don’t, and in the Twin Cities that describes most days in April.

One April day, two answers

Minneapolis–St. Paul, April 20, 2026: a low of 31°F and a high of 64°F

High 64°FLow 31°FBase 50°F

12 AM6 AMNoon6 PM12 AM

• Temperature through the day
• Daily average, 47.5°F
• Heat above the base

Simple average

0 degree days

(64 + 31) ÷ 2 = 47.5°F, below the base, so the day counts nothing.

Sine method

4.1 degree days

The shaded afternoon above 50°F counts.

One day’s disagreement is small. A season’s isn’t: by June 1, 2026, Minneapolis–St. Paul had banked 512 degree days by the simple average, 582 by the sine method and 651 by the 86/50 method.⁷

The start date. Most models count from January 1 or March 1. In Minnesota the choice rarely matters — by the simple average, the airport banked nothing at all before March in 2026 — but farther south it can. Some insect models start instead at a biofix, a biological event such as the first moths caught in a trap, and count forward from there.⁸

The place. A count is only as local as its thermometer. By June 1, 2026, Minneapolis–St. Paul International Airport had banked 512 degree days. Crystal Airport, about 14 miles northwest, had banked 410. The University of Minnesota’s St. Paul campus, about 8 miles north — where much of the DNR’s Twin Cities phenology record is observed — had banked 392: nearly a quarter less heat, in the same metro, in the same spring.⁷ Some of that gap is the city’s own heat, which University of Minnesota researchers have measured at several degrees over downtown on the strongest nights;⁹ some is where and how each station takes its readings.

The units. Degree days can be counted in Fahrenheit or Celsius, and the two aren’t interchangeable: it takes nine Fahrenheit degree days to make five Celsius ones.¹⁰ With an equivalent base — 10°C is 50°F — a threshold of 250 in a Canadian or European table means about 450 here.

Put the ingredients together and one threshold becomes a spread of dates:

One threshold, 11 days of answers

The day 450 growing degree days (base 50°F) arrived in the Twin Cities in 2026

One station (MSP airport), four methods

• 86/50 methodlow raised to 50°F, high capped at 86°FMay 24
• Double sinethe bronze birch borer forecast’s methodMay 27
• Single sineMay 27
• Simple averageMay 30

One method (simple average), six stations

• MSP airportMay 30
• Flying Cloud airportJun 1
• New HopeJun 2
• Crystal airportJun 3
• Chanhassen (NWS)Jun 4
• U of M, St. Paul campusJun 4

May 24May 28Jun 1Jun 5

The answer isn’t to hunt for the one right recipe. It’s to count each threshold the way it was published — and published forecasts don’t agree among themselves. The USA National Phenology Network counts bronze birch borer with a double-sine method from January 1, and pine needle scale with the simple average from March 1.¹¹ Counted the bronze birch borer forecast’s own way, the airport reached 450 on May 27. Read the method off the source, count the same way, and use the station most like the property.

Heat arrives at different speeds

Degree days tell you how far away an event is. On their own, they don’t tell you how soon.

How fast heat arrives

Average growing degree days banked per day, by week, at Minneapolis–St. Paul, 1999–2025

0102030

AB

MarAprMayJunJulAugSepOct

• AMid-April: 4.2 GDD a day that week; 50 more typically took 15 days.
• BMid-July: 25.5 GDD a day that week; 50 more typically took 2 days.

View the data

Week of	GDD a day	Days to bank 50
Mar 1	0.2	47
Mar 8	0.4	40
Mar 15	0.6	34
Mar 22	0.6	28
Mar 29	0.9	23
Apr 5	1.6	18
Apr 12	4.2	15
Apr 19	2.8	13
Apr 26	4.5	9
May 3	8.3	7
May 10	8.7	6
May 17	10.8	5
May 24	14.5	4
May 31	17.2	3
Jun 7	19.3	3
Jun 14	21	3
Jun 21	22.6	3
Jun 28	24.1	3
Jul 5	25.1	2
Jul 12	25.5	2
Jul 19	25.5	2
Jul 26	25.4	2
Aug 2	24	3
Aug 9	22.4	3
Aug 16	21.4	3
Aug 23	21.8	3
Aug 30	19.9	3
Sep 6	16.8	4
Sep 13	15.3	4
Sep 20	12.1	5
Sep 27	9.7	7
Oct 4	6.7	12
Oct 11	3.2	Not by Dec 31
Oct 18	3	Not by Dec 31
Oct 25	1.2	Not by Dec 31

In mid-April the Twin Cities bank three or four degree days a day; in mid-July, about twenty-five. Over the twenty-seven springs from 1999 to 2025, a window 50 degree days away was typically about two weeks off in mid-April and two days off in mid-July. “We’re close” means something different in each.

The other half of reading a threshold is remembering that most are ranges. Pine needle scale crawlers hatch across roughly 300 to 450 degree days,¹¹ which at the airport has typically spanned about a week and a half, from late May into early June — as little as a week, as much as three.⁷ The start of a window, its peak and its end are different numbers, and a treatment timed to one isn’t timed to the others.

Where degree days mislead

Degree days are the best clock we have for spring. They’re still a model, and a model has edges.

The station isn’t the property. A south-facing brick wall, a frost pocket at the foot of a slope and a lakeshore can each run days ahead of or behind the nearest weather station. No regional count sees that. The plants on the property do.

Winter heat doesn’t count the same. Many woody plants need a winter’s worth of chilling before warmth can push them toward budbreak, and they respond more to spring warming than to winter warming — so a February thaw doesn’t advance spring the way the same heat in April would.¹²

Fall runs on day length. As the days shorten, most temperate trees and shrubs stop growing, set buds and begin to harden for winter in response to day length more than to heat; apples, crabapples and some of their relatives in the rose family are notable exceptions that take their cue from cold instead.¹³ Degree days will tell you when a spring pest emerges. They won’t tell you when a shrub is ready for winter.

Soil isn’t air. Crabgrass germinates when the top inch or two of soil holds around 50 to 55°F for a few days running.¹⁴ Air degree days, and the bloom of forsythia, are stand-ins for that.

Hot days overcount. Above an upper threshold, which varies by species from the mid-80s into the 90s, development stops speeding up, but a simple average keeps adding heat.¹⁵ In the Upper Midwest the threshold is rarely exceeded for long enough to matter; in a July heat wave, it can.

Precision isn’t accuracy. A degree-day forecast can name a Tuesday. Biology runs on a range. In the Twin Cities lilac record, predicting first bloom from accumulated heat missed by about three days on average — far better than the calendar’s seven, and still not zero.¹⁶ Plan in windows, then watch.

Let the plants have the last word

Long before anyone tracked degree days, gardeners timed work by what else was in bloom. It still works, for the reason this whole post points to: a plant growing on the property has already added up that property’s heat — the wall, the slope, the lake — in a way no weather station can.¹⁷

Daniel Herms’ years of records at Ohio State put the bloom of common landscape plants and the activity of landscape pests on the same degree-day scale, and some pairings held every year. Pine needle scale eggs hatched just as common lilac reached full bloom. Bronze birch borer adults began to emerge just as black locust bloomed — in seasons when the degree-day count at emergence, counted his way, ranged from 475 to 654.² The count wandered; the pairing didn’t. The USA National Phenology Network built its national Spring Index on lilac and honeysuckle for the same reason.¹⁸

Choose indicators that are reliable where you work. In Minnesota, the flower buds of non-hardy forsythia are often killed over winter above the snow line, so a hardy cultivar makes a far better clock.¹⁹

The working pattern uses both: degree days to see an event coming and plan the crews, and the plants on site to confirm it has arrived.

How Kloro reads them

In Kloro’s catalog, every window carries its recipe — base 50°F, the simple average, counted from January 1 — and the advisories it raises are checked against heat counted the same way. A threshold taken from a guide that counts another way is counted Kloro’s way, so its window can open a few days off the guide’s own forecast. Each property is tied to a weather station, and the dashboard shows where the season stands against the ten-year normal. When crews log the work they actually did, Kloro records the heat banked that day; when a firm’s timing consistently runs ahead of or behind a published window, it suggests a threshold of the firm’s own.

You can follow the Twin Cities season against a typical year, for free, on our Season Tracker.

Six questions to ask of any degree-day number

1. What base temperature? Fifty degrees Fahrenheit is common, not universal.
2. Which method? Simple average, sine or 86/50.
3. Counted from when? January 1, March 1 or a biofix.
4. Measured where? Which station, and how much like your property is it?
5. In which units? Fahrenheit or Celsius.
6. The start, the peak or the end? Most thresholds describe a window.

A degree-day number is a measurement, not a date. Read its recipe, translate it into days for the time of year, and let the plants on the property have the last word.

Next in Field Notes: indicator plants — the living thermometers already growing on the properties you manage.

Sources

1. USA National Phenology Network, Bronze birch borer Pheno Forecast: adult emergence at 450 growing degree days, base 50°F, counted from January 1 with the double-sine method. ↩

2. Herms, D. A. (2004), “Using degree-days and plant phenology to predict pest activity,” in IPM of Midwest Landscapes, University of Minnesota, pp. 49–59. The pairings and the 475–654 range come from five years of records at Wooster, Ohio. ↩ ↩²

3. Jagemann, S. M. et al. (2018), Environmental Entomology 47: 1152–1164, a degree-day model of the sap beetles that carry oak wilt (base 5°C, or 41°F); University of Wisconsin–Madison Extension, an online tool for timing oak pruning. ↩

4. USA National Phenology Network, Emerald ash borer Pheno Forecast: 391 growing degree days, base 54°F, with an upper threshold of 97°F. ↩

5. Baskerville, G. L. and Emin, P. (1969), “Rapid estimation of heat accumulation from maximum and minimum temperatures,” Ecology 50: 514–517; University of California IPM, degree-day concepts and methods. ↩

6. Midwestern Regional Climate Center, growing degree days. ↩

7. Kloro analysis of NOAA daily highs and lows from the Regional Climate Centers’ ACIS, base 50°F, counted from January 1 (March 1 for pine needle scale, as the forecast does). Station totals use the simple average; every station compared has a complete record for the period. Typical figures are medians over 1999–2025 at MSP. ↩ ↩² ↩³

8. University of California IPM, degree-day concepts: “The date to begin accumulating degree-days, known as the biofix date, varies with the species.” ↩

9. Smoliak, B. V. et al. (2015), Journal of Applied Meteorology and Climatology 54: 1899–1917; the team’s summary reports a warm-season heat island peaking around 1.5°C at night, and about 4.5°C over downtown Minneapolis in the strongest events. ↩

10. University of Florida IFAS Extension, AE428: “It takes nine Fahrenheit degree-days to make five Celsius degree-days.” The conversion holds only between equivalent bases and the same method. ↩

11. USA National Phenology Network, Pine needle scale Pheno Forecast: first-generation crawlers at 298–448 growing degree days, base 50°F, simple average from March 1. ↩ ↩²

12. Fu, Y. H. et al. (2012), PLoS ONE 7: e47324: the species studied “were more sensitive to spring warming than to winter warming”; Laube, J. et al. (2014), Global Change Biology 20: 170–182. ↩

13. Heide, O. M. and Prestrud, A. K. (2005), Tree Physiology 25: 109–114: “In contrast to most temperate woody species, apple and pear and some other woody species of the Rosaceae family are insensitive to photoperiod.” ↩

14. University of Minnesota Extension, lawn weeds and a guide to spring lawn tasks. ↩

15. University of California IPM, degree-day concepts; Herms (2004), above, on the Midwest. ↩

16. Kloro analysis of the Minnesota DNR State Climatology Office’s Twin Cities phenology record, 1999–2025, against NOAA daily temperatures at MSP; see Your garden doesn’t read a calendar. ↩

17. University of Illinois Extension, using phenology for ornamental pest control: indicator plants “report exposure to site-specific conditions via their phenological timeline.” ↩

18. USA National Phenology Network, Spring Indices. ↩

19. University of Minnesota Extension, forsythia. ↩