Nursery Chill

Chill accumulation at the weather stations serving strawberry nursery districts in California and Spain.

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Count chill hours asWhy three?

Butte Valley and the Klamath Basin.

How to read these numbers

Three chill models are in wide use for temperate fruit crops. They read the same hourly temperatures and disagree about what each hour is worth. This page charts two of them side by side because we have found no validation of any of the three against California strawberry nursery outcomes, and the comparison shows how much the answer depends on the model.

Charted · left panel

Chilling Hours

One hour, one unit — in a band you choose

What it measures
Every hour whose air temperature falls in a band counts as one. Nothing is weighted and nothing is subtracted, so the curve only climbs. The band’s top, 45°F (7.2°C), is agreed. Its bottom is not, and it moves the total a great deal, so the buttons above the tabs let you pick it:
  • 32–45°F, shown first: the Chilling Hours Model as Luedeling and colleagues define it, hours “between 0 and 7.2°C”, which they call the model “predominantly used in California”. Voth and Bringhurst’s California nursery table gives the same band as its “0 to 7.2” column.34
  • 33–45°F: the column Lassen Canyon Nursery’s Macdoel ledger keeps, which UC ANR’s posts report as the chill total. The ledger’s 2026 figures match our station read in whole degrees, so an hour at 45.6°F lands in that column; this page counts exact readings.1011
  • At or below 45°F: every hour at or below 7.2°C, frost included — the “total below 7.2” in Voth and Bringhurst’s table, the usual method in 1974 by Richardson’s account, and how the transplant target below is worded.425
Where it came from
Deciduous-fruit rest studies of the 1940s. Bennett put the rest-breaking range for leaf buds at “between freezing and about 45°F”.1 For California strawberry nurseries, Voth and Bringhurst tabulated hours below 0°C (“freezing”) and from 0 to 7.2°C separately and in total, and report that plants short of chill lose earliness, quality and yield.4
What it assumes
That every hour in the band is worth the same and that hours outside it add nothing. Whether hours below freezing belong in the band is the part nobody has settled for strawberry: we have found no measurement of whether they advance a strawberry plant’s dormancy, the peach-based Utah model gives them no credit, and a station’s air reading is not the temperature of a crown at the soil surface. At Macdoel, hours below 32°F are 19 to 39 percent of the no-floor October 31 total across its ten complete seasons. Each verdict says how many there were.

The 150–400 hour target shaded on the California high-elevation charts is the chill high-elevation transplants need “prior to digging/harvest”: below it, low plant vigor; above it, excessive vegetative growth. Holmes gives it as hours below 7.2°C, citing Strand 2008, without naming a lower edge. On the 32–45°F and 33–45°F counts, Macdoel’s October 31 totals run about 330 to 500 hours, across the top of the range; on the no-floor count they run 540 to 660, above it in every complete season. Voth and Bringhurst’s 1986 figures put Macdoel at 766 hours from 0 to 7.2°C over September and October, and in their trial plants from Macdoel, the coldest nursery, yielded most. Treat the range as a reference, not a pass/fail line.54 It is not drawn on the Spanish charts, because it was published for California transplants. Ariza and colleagues, citing López-Aranda et al. 2003, describe plants from Castilla y León’s high-elevation nurseries as having had at least 150 to 200 hours below 7°C.6

Charted · right panel

Utah model

A running balance from the season start

What it measures
Each hour is weighted by temperature. Hours from 2.5 to 9.1°C (about 36.5 to 48.4°F) earn a full unit, the shoulders on either side (1.4 to 2.4°C and 9.2 to 12.4°C) earn half, and cold below 1.4°C (34.5°F) and 12.5 to 15.9°C earn nothing. Warm hours debit the total: −0.5 above 15.9°C (60.6°F) and −1 above 18°C (64.4°F).
Where it came from
An empirical temperature-response table fitted to rest completion in ‘Redhaven’ and ‘Elberta’ peach in Utah. The paper does not propose a mechanism, and neither do we.2
What it assumes
Fixed exchange rates between cold and warm hours. Richardson’s own protocol starts the count the day after the fall’s most negative point, because he was predicting when peach rest completes. This page instead carries the raw balance from September 1, the start Lassen Canyon’s Macdoel ledger uses and UC ANR relays when it reports Macdoel chill hours “offset by” hours above 60°F, so the two can be read together. Neither start rule has been validated for strawberry. That choice is why the panel can sit below zero for weeks, and it is labelled on the chart.11

Not charted yet

Dynamic model

Two steps, one of them irreversible

What it measures
Cold builds up an intermediate state that warmth can destroy. Once enough of it accumulates, it converts irreversibly into a “chill portion” that no later warm spell can take back. Output is in portions, not hours, and the order of the hours matters, not just their count.3
Where it came from
Developed in Israel, and adopted by California cherry growers to measure winter chill.3
What it assumes
That a warm spell can undo recent chilling but never chill already converted. It is “often considered superior in subtropical climates”, and it is one of the two models behind the California projections of declining winter chill.3

Not charted here yet because we have not found a validated portion target for nursery transplants; a Dynamic curve would be readable as weather but not yet as a decision. It is on the list.

Where the numbers come from

  • Each card reads one weather station, named on the card: CIMIS in California, SiAR in Spain. Nothing is estimated or interpolated.78
  • High-elevation seasons start September 1, as Lassen Canyon’s Macdoel ledger does; frigo seasons start November 1. That ledger’s 2026 sheet reads our CIMIS station in whole degrees, so its 33–45°F column runs a few hours above ours.1012
  • Dropped, not patched: CIMIS readings flagged missing or faulty, and implausible values. For SiAR, which reports half-hours, an hour needs both halves; a half-hour more than 7°C off the surrounding median is dropped as a sensor fault, and so is any stretch where two of these stations report identical readings for three or more half-hours, a sign SiAR filled a gap from a neighbour. Spanish days run on UTC+1 all year.8
  • Spain’s “horas frío” are usually hours below 7°C (7.2°C in SiAR’s calculator), frost included, closest to this page’s “at or below 45°F” count.8
  • Each Spanish card stands for a nursery area in Segovia or Ávila, two of the four provinces where Castilla y León’s 35 strawberry nurseries grow, mostly fresh plants for Andalucía, the European Union and North Africa.9 No on-farm tally is published there to check them against, and they seldom agree: the highest usually banks a quarter or more above the lowest, and more than twice as much in warm autumns like 2022 and 2023.
  • The Spanish dig window is provisional. The regional government reported nurseries still finishing the lift on November 8, 2024; the chart stops October 31.9
  • A season missing more than 40 percent of its hours is left out; one missing more than 5 percent is drawn dashed and kept out of every average, as is a Spanish season that lost copied stretches, since those fall on cold nights.
  • The current season updates each morning through yesterday. Completed seasons stay fixed, except that Spanish ones are rechecked for 30 days to pick up SiAR corrections.

Sources

Research

  1. Bennett, J.P. 1949. Temperature and bud rest period. California Agriculture 3(11): 9, 12. Gives the rest-breaking range for deciduous leaf buds as “between freezing and about 45°F”.
  2. Richardson, E.A., S.D. Seeley, and D.R. Walker. 1974. A model for estimating the completion of rest for ‘Redhaven’ and ‘Elberta’ peach trees. HortScience 9(4): 331–332. The Utah model and its start rule. Table 1 scores “< 1.4” °C as 0 and “1.5 – 2.4” as 0.5, leaving exactly 1.4°C unassigned; this page scores it 0.5. Describes the usual method of the day as counting hours “below a given base temp, generally 7.2°C (45°F)”.
  3. Luedeling, E., M. Zhang, and E.H. Girvetz. 2009. Climatic changes lead to declining winter chill for fruit and nut trees in California during 1950–2099. PLoS ONE 4(7): e6166. Open access (CC BY). Defines the Chilling Hours Model as hours “between 0 and 7.2°C”, “predominantly used in California”; describes the Dynamic Model (two steps, an irreversible “Chill Portion”, “developed in Israel”, “often considered superior in subtropical climates”); maps both across the state.
  4. Voth, V., and R.S. Bringhurst. 1990. Culture and physiological manipulation of California strawberries. HortScience 25(8): 889–892. Free access. Table 1, “Comparing hours of preharvest freezing and chilling (<7.2 C) temperatures at California high-elevation strawberry nurseries for 1986”, gives hours below 0°C, from 0 to 7.2°C, and the total below 7.2°C; Macdoel: 258, 766 and 1,024 over September and October. Plants short of chill lose earliness, quality and yield; in their trial, plants from Macdoel, the coldest nursery, yielded most.
  5. Holmes, G.J. 2024. The California strawberry industry: current trends and future prospects. International Journal of Fruit Science 24(1): 115–129. Open access (CC BY-NC). “150 – 400 hours below 7.2°C … prior to digging/harvest”, citing Strand 2008 (UC ANR Publication 3351), which we have not been able to read.
  6. Ariza, M.T., C. Soria, J.J. Medina-Mínguez, and E. Martínez-Ferri. 2012. Incidence of misshapen fruits in strawberry plants grown under tunnels is affected by cultivar, planting date, pollination, and low temperatures. HortScience 47(11): 1569–1573. Plants from Castilla y León’s high-elevation nurseries, “with at least 150 to 200 h of temperatures below 7 °C”, as reported from López-Aranda et al. 2003, which we have not read.

Data and official statistics

  1. California Irrigation Management Information System (CIMIS), California Department of Water Resources — hourly station data and quality-control flag definitions.
  2. Sistema de Información Agroclimática para el Regadío (SiAR), Ministerio de Agricultura, Pesca y Alimentación — half-hourly station data. Castilla y León stations are operated by ITACyL (InfoRiego). Hourly times are UTC from April 1, 2014 and “hora solar” before, per the SiAR user manual. Its “Horas frío” calculator counts hours at or below a fixed 7.2°C.
  3. Junta de Castilla y León, Consejería de Agricultura, Ganadería y Desarrollo Rural. 2024. Castilla y León lidera el cultivo de planta de fresa en Europa … November 8, 2024. 35 nurseries on about 1,500 ha in Segovia, Ávila, Valladolid and Soria. Fresh plants, 87% of production, go mainly to Andalucía and the European Union; almost a third go outside it, among them Morocco, Algeria and Tunisia. On November 8 the nurseries were “finishing these days” the season’s lift.

Industry records

  1. Lassen Canyon Nursery. Macdoel ranch chilling hours — published seasonal ledgers, 2018 to date. Cumulative from September 1, in whole-degree bands: 0–32, 33–45, 46–60, 60–80 and 80–100°F. The 2018–2022 ledgers are headed “Ranch: Macdoel”. The 2026 ledger matches CIMIS station 236 read in whole degrees.
  2. Bolda, M. 2021. Nursery chill: Macdoel. Strawberries and Caneberries blog, UC Agriculture and Natural Resources, October 7, 2021. “245 hours … between the critical temperatures of 33 and 45 degrees … since Sept 1”, crediting Lassen Canyon Nursery’s ledger for the figure.
  3. Bolda, M. 2021. Nursery chill: Macdoel, week of October 25. Strawberries and Caneberries blog, UC Agriculture and Natural Resources, October 28, 2021.

Common questions

How many chill hours do strawberry nursery plants need?

The published range for California high-elevation nursery transplants is 150 to 400 chill hours before digging, shaded on the California high-elevation Chilling Hours charts. Below it, low plant vigor; above it, excessive vegetative growth. It does not apply to low-elevation frigo ground, where plants carry around 330 hours (32–45°F) when digging opens in December and around 890 by late February, and it is not drawn for Spain, where Ariza and colleagues, citing López-Aranda et al. 2003, describe plants from Castilla y León's high-elevation nurseries as having had at least 150 to 200 hours below 7°C. The source gives the range as hours below 7.2°C (45°F) without naming a lower edge; on a count that starts at freezing, Macdoel's season totals run across the top of the range, and on one with no floor they pass it in every complete season. Treat it as a reference range rather than a pass/fail line: we have not traced it to a strawberry trial, and we have found no validation of any chill model against California strawberry nursery outcomes.

What is the difference between Chilling Hours and Utah chill units?

Chilling Hours counts every hour in a temperature band (32–45°F by default on this page) as one unit and subtracts nothing, so the curve can only climb. The Utah model weights each hour instead: hours in the most effective range count fully, hours to either side count half or nothing, and warm hours count NEGATIVE. A Utah balance can therefore fall during a warm spell while the Chilling Hours total stands still. The two are on different scales and are not convertible — the page charts both because the answer depends on which model you pick.

Do hours below freezing count as chill hours?

It is not settled, so this page lets you choose. The Chilling Hours definition most used in California counts hours from 32 to 45°F (0 to 7.2°C) and leaves frost out; that is the page's default. Lassen Canyon Nursery's Macdoel ledger keeps a 33–45°F column, which UC ANR has reported as the chill total. The 150–400 hour transplant range is worded as hours below 7.2°C, and Spain's "horas frío" are usually hours below 7°C (7.2°C in SiAR's calculator), frost included. California's own strawberry nursery research (Voth and Bringhurst, 1990) tabulates hours below 0°C and from 0 to 7.2°C separately and in total, without ruling on whether the frost hours count. The buttons above the tabs switch between the three, and each site's verdict says how many hours fell below freezing. We have found no measurement of whether sub-freezing hours advance a strawberry plant's dormancy; the peach-based Utah model gives cold below about 34.5°F no credit.

How much chill have the California strawberry nursery districts accumulated this season?

Macdoel is at 201 chill hours (32–45°F) through Oct 1, 2026, and Tulelake is at 182 chill hours (32–45°F) through Oct 1, 2026. Manteca and Denair are between seasons; that clock starts Nov 1. Totals come from the CIMIS station named on each card and update daily, early morning Pacific.

How much chill have Spain's strawberry nursery districts accumulated this season?

El Carracillo is at 27 chill hours (32–45°F) through Oct 1, 2026, La Moraña is at 18 chill hours (32–45°F) through Oct 1, 2026, and Valle Amblés is at 31 chill hours (32–45°F) through Oct 1, 2026. Totals come from the SiAR station named on each card and update daily, early morning Pacific. Each is one weather station standing for a nursery area in Castilla y León, not a nursery-field reading, and in a warm autumn the highest of the three can bank more than twice what the lowest does.

When is the strawberry nursery dig window?

California's high-elevation districts run a season from Sep 1, with digging from about Sep 25 to Oct 31. Low-elevation frigo ground runs from Nov 1, dug from about Dec 20 into late February. Frigo plants go into frozen storage fully dormant, so their season total is a record of the winter rather than a transplant target. Spain's plateau nurseries are shown on the same Sep 1 clock with a provisional Sep 25 – Oct 31 dig window; the regional government reported the nurseries still finishing the lift on November 8, 2024, and this chart stops on October 31.

Where does this chill data come from?

Air temperature from CIMIS, the California Irrigation Management Information System, for the California sites, and from SiAR, Spain's agroclimatic station network, for the Spanish ones — read from the one station named on each site card and run through each model directly. SiAR's half-hour readings are averaged into clock hours first, and stretches SiAR filled with a neighbouring station's readings are dropped. Nothing is estimated or interpolated: an hour the station did not report, or whose reading was dropped, is an hour this page does not count. A season missing more than 5 percent of its hours is drawn dashed and left out of every average, and so is a Spanish season from which copied stretches were dropped, whatever its coverage, because those stretches fall on cold nights. A season missing more than 40 percent is not shown at all.