The science
How Kloro reads a plant’s clock
Plants develop with accumulated heat, not with the date. This is how Kloro turns a station’s daily temperatures into the care windows it advises on, where each window comes from, and where the method stops.
One day’s heat, added up
A growing degree day measures how much warmth a day offered above the temperature at which most temperate plants begin to grow, 50°F. Average the day’s high and low, subtract 50, and count nothing for a day that averages below it.
Kloro adds each day to a running total that starts on January 1. A plant reaches each stage of growth at a far steadier total than date: across 27 Twin Cities springs, predicting lilac bloom from accumulated heat missed by about three days on average, and the calendar date by about seven (the field note, with sources).
Daily highs and lows for each station’s location come from Open-Meteo’s weather archive. Minnesota properties read the Minneapolis–St Paul station today.
A June day
High 78°F, low 56°F
(78 + 56) ÷ 2 = 67
67 − 50 = 17 growing degree days
A day averaging 45°F adds nothing. Nothing is subtracted.
Where every window comes from
A window is when one kind of care suits one plant: usually a range of accumulated degree days, and a span of dates for work tied to frost or the calendar, such as cutting peonies back after a hard freeze. Kloro’s curated catalog holds 295 windows across 130 species, and each one records how it was made.
- Published threshold
- 14
- Recorded from a degree-day figure a guide publishes, sometimes counted another way. Kloro counts it its own way (base 50°F, simple average, from January 1), so its window can open a few days off the guide’s own forecast.
- 14 of 14 carry a link to their source.
- Translated from a guide
- 270
- Usually Kloro’s translation of timing a guide gives in words, such as right after bloom or before new growth, into a degree-day range at the station, or into a span of dates when the work follows frost or the calendar. A few use a guide’s degree-day figure directly.
- 266 of 270 carry a link to their source.
- Set by hand
- 11
- Recorded as set by hand from horticultural practice.
- 6 of 11 carry a link to their source.
Most cited: University of Minnesota Extension (69), Missouri Botanical Garden (55), Iowa State University Extension (37), UW–Madison Extension (31), NC State Extension (14), UMass Extension (12).
A sample of the catalog
18 plants a Minnesota estate garden is likely to hold. Each is listed with every window Kloro advises on for it and the source behind each, and a plant appears only when every one of those windows carries a working link to its source. The full catalog lives in the product.
Panicle hydrangea
Hydrangea paniculata
- Prune0–90 GDDfrom Mar 1Translated from a guide, University of Minnesota Extension
- Feed7–94 GDDTranslated from a guide, Iowa State University Extension
- Deadhead1,035–1,770 GDDTranslated from a guide, University of Maryland Extension
Smooth hydrangea
Hydrangea arborescens
- Prune0–50 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
Bigleaf hydrangea
Hydrangea macrophylla
- Feed0–1,717 GDDfrom Mar 1Translated from a guide, Clemson Extension
- Prune985–1,717 GDDTranslated from a guide, Clemson Extension
Northern red oak
Quercus rubra
- Prune0–7 GDDuntil Mar 31Translated from a guide, University of Minnesota Extension
Sugar maple
Acer saccharum
- Feed0–30 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
- Transplant0–69 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
- Spray58–155 GDDPublished threshold, UMass Extension
- Prune366–985 GDDTranslated from a guide, University of Minnesota Extension
Japanese tree lilac
Syringa reticulata
- Prune824–1,319 GDDTranslated from a guide, Missouri Botanical Garden
Garden peony
Paeonia lactiflora
- Feed22–69 GDDTranslated from a guide, Iowa State University Extension
- Spray22–162 GDDTranslated from a guide, Iowa State University Extension
- DivideSeptemberTranslated from a guide, Iowa State University Extension
- Cut backOct 15 – Nov 15Translated from a guide, Iowa State University Extension
Daylily
Hemerocallis fulva
- Cut back0–44 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
- Feed7–104 GDDTranslated from a guide, University of Minnesota Extension
- Divide15–162 GDDTranslated from a guide, University of Minnesota Extension
- Deadhead895–1,649 GDDTranslated from a guide, University of Minnesota Extension
Siebold's hosta
Hosta sieboldiana
- Cut back0–75 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
- Feed36–640 GDDTranslated from a guide, University of Minnesota Extension
- Divide50–193 GDDfrom Apr 1Translated from a guide, University of Minnesota Extension
- Deadhead895–1,382 GDDTranslated from a guide, University of Minnesota Extension
Feather reed grass
Calamagrostis acutiflora
- Cut back0–36 GDDfrom Mar 1Translated from a guide, UW–Madison Extension
- Divide7–162 GDDfrom Apr 1Translated from a guide, UW–Madison Extension
- Feed7–94 GDDSet by hand, UW–Madison Extension
Switchgrass
Panicum virgatum
- Cut back0–36 GDDfrom Apr 1Translated from a guide, UW–Madison Extension
- Divide7–193 GDDTranslated from a guide, UW–Madison Extension
- Feed7–75 GDDTranslated from a guide, UW–Madison Extension
Apple
Malus domestica
- Prune0–22 GDDfrom Mar 1Translated from a guide, University of Minnesota Extension
- Feed7–63 GDDSet by hand, University of Minnesota Extension
- Spray36–419 GDDTranslated from a guide, Cornell University
Colorado spruce
Picea pungens
- Prune0–30 GDDfrom Feb 1Translated from a guide, Iowa State University Extension
- Feed7–1,291 GDDTranslated from a guide, University of Minnesota Extension
- Spray192–363 GDDTranslated from a guide, UMass Extension
Anglo-Japanese yew
Taxus media
- Prune0–30 GDDfrom Apr 1Translated from a guide, Missouri Botanical Garden
Littleleaf boxwood
Buxus microphylla
- Prune63–621 GDDfrom Apr 23Translated from a guide, Missouri Botanical Garden
Weigela
Weigela florida
- Feed7–63 GDDTranslated from a guide, Clemson Extension
- Prune542–977 GDDTranslated from a guide, Missouri Botanical Garden
Bearded iris
Iris germanica
- Spray22–193 GDDTranslated from a guide, University of Minnesota Extension
- Deadhead602–1,035 GDDTranslated from a guide, Penn State Extension
- Divide985–2,064 GDDTranslated from a guide, Penn State Extension
- Cut backOct 15 – Nov 15Translated from a guide, Penn State Extension
Catmint
Nepeta faassenii
- Cut back0–36 GDDfrom Apr 1Translated from a guide, UW–Madison Extension
- Divide7–69 GDDTranslated from a guide, University of Minnesota Extension
What degree days cannot tell you
- Development, not damage. Heat says where a plant is in its year. A late frost, a drought or a hard winter can still close a window early or change what the plant needs.
- One station, many gardens. An airport station runs warmer than an estate on the edge of the metro, and a south wall warmer than a shaded bed. Windows are ranges for that reason.
- Two ways to count a day. The catalog’s windows count on the simple average above, and so do the advisories they raise. The public season tracker draws its curve with the Baskerville–Emin method, which credits part of a day that only briefly passes 50°F; over a Minnesota season it runs a few percent higher.
- An advisory is a prompt, not an order. Kloro tells the people who plan the work that a window is open. The horticulturist who knows the garden decides when the crew goes.