Strawberry Chill Hours – What Are Strawberry Chilling Requirements
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By: Amy Grant
Many plants require a specific number of chilling hours to break dormancy and begin to grow and fruit again. Strawberries are no exception and chilling of strawberry plants is a common practice amongst commercial growers. The number of strawberry chill hours depends on whether the plants are being grown outside and then stored or are being forced in a greenhouse. The following article discusses the relationship between strawberries and cold, and the chilling requirements for strawberries.
About Strawberry Chill Hours
Strawberry chilling is important. If the plants don’t get enough chill hours, the flower buds may not open in the spring or they may open unevenly, resulting in a reduction in yield. The production of leaves may be delayed as well.
The traditional definition of a chill hour is any hour under 45 F. (7 C.). That said, academics quibble over the actual temperature. In the case of chilling requirements for strawberries, the period is defined as the number of accumulated hours between 28-45 F. (-2 to 7 C.).
Strawberries and Cold
Strawberries planted and cultivated outside generally get enough chill hours naturally through the change of seasons. Commercial growers sometimes grow berries outside where they begin to accumulate chill hours and are then stored with supplemental chill.
Too much or too little supplemental chill affects how the plants will produce. So chilling strawberry plants has been studied to see exactly how many hours are needed for a particular variety. For instance, the day neutral ‘Albion’ needs 10-18 days of supplemental chill while the short day cultivar ‘Chandler’ needs less than 7 days of supplemental chill.
Other growers cultivate strawberries in greenhouses. Fruit is forced by providing heat and long-day illumination. But before the berries can be forced, the dormancy of the plants must be broken with adequate strawberry chilling.
In lieu of enough chill hours, plant vigor, to a certain extent, can be controlled by early season flower management. That is, removing flowers early in the season allows the plants to develop vegetatively, making up for a lack in chill hours.
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Read more about Strawberry Plants
How to Put Strawberry Plants to Bed for the Winter
Strawberries, Fragaria × ananassa, are low-profile, fruit-bearing perennials suited to cultivation in USDA Hardiness Zones 3 to 10.
Sometimes considered a challenging crop, folks in cold regions with freezing winters often grow plants as annuals and discard them at season’s end.
However, when you select the hardiest cultivars, and provide a little pre-winter TLC, your favorite berry-makers can yield sweet and tasty treats year after year.
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In our comprehensive guide to growing strawberries, we cover all you need to know to plant, care for, and harvest strawberries.
In this article, we focus on a few easy steps to take to winterize plants so they return with vigor each spring.
Day Neutral Varieties
Day-neutral strawberries will continue to set and ripen fruit all summer long until a hard frost puts them into dormancy. Day-neutral refers to the light sensitivity of the variety. Day-neutral strawberries will blossom and set fruit no matter how long or short the days are. Today there are several excellent varieties of day-neutral strawberries and they are a wonderful choice for the gardener who wants a steady supply of fruit instead of having it all ripen at the same time.
Sweet Ann – LCN
Sweet Ann is a vigorous plant with high productivity and exceptional flavor.
The attractive fruit is large in size, with a beautiful long conical shape. It has medium firmness with a good acid-sugar balance and very sweet taste. Sweet Ann is a glossy red color both inside and out. The variety produces few runners in the fruiting field.
Sweet Ann seems quite disease resistant to everything but fusarium wilt. It is an excellent choice for the organic production systems.
Albion – UC
The gold standard of the industry in terms of flavor, Albion is a day-neutral (ever-bearing) cultivar. “Albion” is resistant to Verticillium wilt (Verticillium dahliae) and Phytophthora crown rot (Phytophthora cactorum), and relatively resistant to Anthracnose crown rot (Colletotrichum acutatum).
When treated properly, Albion has tolerance to two-spotted spider mites (Tetranychus urticae). Fruit from “Albion” is typically long, conical, and very symmetrical. Albion fruit is firm and is dark red inside and out.
Albion fruits consistently throughout the season. One downside of this variety is that it produces a lot of runners that must be cut in order to maintain high production.
Cabrillo – UC
The newest Day Neutral cultivar released from UCDavis California. This is a moderately strong flowering variety that is adaptable to both winter & summer plantings. Fruit size and firmness is comparable to that of Albion, as well as San Andreas with greater individual plant yields and offers outstanding flavor.
Disease resistant to powdery mildew, verticillium wilt, phytophthora crown rot, as well as common leaf spot, and with proper treatment can be tolerant to two-spotted spider mite. But, is susceptible to Anthracnose Crown Rot.
Keoki is similar to wild strawberry varieties but doesn’t grow from seed, giving it a substantial advantage in production. This variety is perfect for the specialty garden grower.
It produces tiny white berries packed with LOTS of FLAVOR! Bird netting is important. They are that TASTY! Keoki is an excellent runner producer for great ground cover.
This variety tolerates acidic soil and requires minimal care except weeding. Use a mulch in areas where the ground freezes. It is an ever bearing variety, Best in Zones 3-9.
Monterey – UC
Monterey is moderate in day-neutrality, slightly stronger flowering than Albion with a similar production pattern. It is a vigorous plant and may require slightly more space than Albion. This variety does well in organic applications.
The fruit from Monterey is slightly larger but less firm than fruits from Albion. Post-harvest traits for Monterey are similar to those for Albion. Monterey has outstanding flavor with a distinct sweet aftertaste that is unique among California cultivars.
Monterey has a good disease resistant profile, although it is susceptible to powdery mildew (Sphaerotheca macularis).
|Everbearer||Large||Excellent||Fresh, Commercial||Moderately Firm||Susceptible|
Portola – UC
Portola is a strong day-neutral cultivar with broad adaptability. This cultivar can be used in standard winter planting systems, where it initiates fruiting slightly earlier than Albion to initiate. Due to a strong flowering response, Portola is especially well adapted to spring and summer planting systems.
Portola is a vigorous plant and may require a slightly lower plant density than Albion. The fruit for Portola is similar in size to Albion but lighter in color and somewhat shinier. Post- harvest characteristics for Portola are similar to those for Albion although it is slightly less tolerant to rain.
Fruit flavor for Portola is excellent and especially consistent throughout the fruiting season. Portola has a good disease resistance profile.
|Early Season||Large||Good||All||Moderately Firm||Tolerant|
San Andreas – UC
San Andreas is a moderate day-neutral with a production pattern very similar to Albion. Plant vigor for San Andreas is somewhat higher than for Albion early in the season but plant size throughout the fruiting season is similar to Albion due to its high and consistent productivity. This cultivar produces few runners in the fruiting field. The fruit for San Andreas is exceptional in appearance and especially superior to Albion early in the season.
The fruit color for San Andreas is slightly lighter than for Albion and it has similar post-harvest characteristics. The flavor of San Andreas is outstanding similar to Albion. San Andreas has a good disease resistance profile with no outstanding cautions. Its typically high-quality fruit early in the season, together with a low chilling requirement, make this a good candidate cultivar for Southern California. Fruit productivity for San Andreas is similar to or slightly below that for Albion.
Seascape – UC
Seascape is a dependable standard in the strawberry industry. It was bred for tolerance to many of the common viral diseases in California, but has demonstrated susceptibility to common leaf spot.
Seascape fruit is large and flavorful with an attractive glossy finish. Seascape also has flexibility in planting requirements, such that it does not require as much chill hours to set fruit, which makes this suitable for warmer climates.
Seascape produces large berries with excellent flavor over a long season, but is concentrated in late spring with high yields in the fall.
|Late Mid Season||Medium-Large||Excellent||Fresh||Medium Firm||Tolerant|
Short Day “June Bearing” Varieties
Short day or “June Bearing” strawberries produce a single, large crop per year during a 2 – 3 week period in the spring. June bearers are the traditionally grown plants, producing a single flush of flowers and many runners. They are classified into early, mid-season and late varieties. The largest fruits are generally from June bearing varieties.
Lucia – LCN
Lucia is a medium high vigor type plant with similar plant architecture to Sweet Ann. It also has a similar inflorescence and is an open plant that enables harvesters to see through the plant to harvest or pull weed.
The plant continues to grow through the harvest period and can get tall if more aggressive fertilizer programs are used.
Lucia seems to require less nutrition in order to grow compared to Albion. Lucia fruit is not as large as Sweet Ann but has similar great flavor, better firmness and better fruit uniformity than Sweet Ann.
Ruby June – LCN
Ruby June is the latest release from Lassen Canyon Nursery that is creating a lot of positive feedback in the Southeast. Between Lucia, Ruby June and Scarlet, Ruby June is the strongest short-day variety that is more compact and has tighter plant architecture and leaf structure than Lucia and Scarlet.
Ruby June has the darkest exterior and interior color of any of Lassen Canyon’s varieties. The fruit is great for recipes calling for strawberries. Plus, Ruby June has rated very high with our in-house taste panel.
Scarlet – LCN
Scarlet is a medium to high vigor plant. It has shorter plant architecture then either Lucia or Sweet Ann. The fruit firmness is greater than Sweet Ann, Lucia and Ruby June. Interior color of Scarlet is lighter than Sweet Ann, Lucia and Ruby June. Scarlet has very high yield potential.
The fruit is not as uniform as Lucia but still maintains high fruit quality good size and flavor throughout the harvest season.
Stella – LCN
An early heavy producer, Stella has large fruit and is dark red inside and out. It has a uniformly conic shape throughout its production cycle. The fruit is very sweet, aromatic, and firm but not crunchy.
This variety is heat & rain tolerant but does downsize late in the season. It does have a high nitrogen requirement.
Benicia – UC
This is a moderate to high vigor plant but is smaller sized than Ventana. It does have a similar production pattern to Ventana. Productivity is better than Ventana.
Benicia is a slightly larger, more firm berry. It has good flavor and a darker more even interior color. It is disease resistant, except it is susceptible to Verticillium wilt, requiring more care from the grower in order to successfully cultivate this variety.
|Early Season||Large||Good||Fresh||Moderately Firm||Susceptible|
Camarosa – UC
Camarosa is an early short day variety. This vigorous plant produces large to very large firm fruit throughout most of its fruiting cycle. Interior color of Camarosa is a brilliant red and fruit colors uniformly.
Yield potential is high to excellent.
Camino Real – UC
Camino Real is a short-day cultivar similar to Camarosa in all aspects including production pattern. It is somewhat later to initiate fruiting with most cultural treatments, however.
External and internal fruit color for Camino Real is darker than Camarosa. Subjectively, Camino Real has very good flavor. The fruit is outstanding for both fresh market and processing.
Camino Real is susceptible to common leaf spot and sensitive to powdery mildew. It is resistant to Verticillium wilt, Phytophthora crown rot and somewhat resistant to Anthracnose crown rot. When treated properly it shows tolerance to two-spotted spider mite.
Chandler – UC
Chandler continues to be one of the “go to” varieties for roadside strawberry stands, backyard gardeners everywhere and many commercial growers in the southeast.
Chandler produces semi-early delicious sweet fruit. Fruit size is medium to large and medium in firmness. Chandler performs well in the east and is well adapted to the south.
|Early Mid Season||Medium-Large||Excellent||Fresh||Very Firm||Susceptible|
Fronteras – UC
Fronteras is a short day variety, meaning it responds to day length, produces the bulk of its fruit in the shorter days of spring and fall, and is adapted to the fall planting and winter production system of the California coast.
Fronteras is a large and vigorous plant, more so than the short day variety Ventana in use for many years by growers.
|Early Season||Large||Excellent||Commercial||Very Firm||Tolerant|
Hood – USDA
Hood is a pacific northwestern variety characterized by fruit borne well above the soil on strong, upright clusters.
The berry is large, round and conic. The skin is glossy bright medium red. The berries are firm with a pleasant flavor.
The fruit ripens midseason. This is a nice variety for preserves and jams.
|Mid Season||Medium||Excellent||Very Firm||Susceptible|
Jewel is one of the best all-around varieties. It is popular in the east. It is good for u-pick, fresh shipping and long season yields.
It produces large, firm, wedge-shaped fruit of beautiful color and quality.
Jewel’s firmness and abrasive resistant skin make it ideal for shipping.
|Late Mid Season||Large||Excellent||All||Firm||Tolerant|
Radiance aka Fortuna – Univ. of FL
This strawberry cultivar has high early-season yields and good fruit size throughout the main production period in Florida and southwest Spain.
The fruit is a glossy bright to dark red, easy to harvest, and has a smooth appearance. The flavor is acceptable and can be quite good under ideal growing conditions. Radiance fruit is firm yet juicy and sports an attractive calyx.
|Early Season||Medium-Large||Fair||Fresh, Commercial||Firm||Tolerant|
Festival – Univ. of FL
Florida Strawberry Festival is a short day cultivar. Its fruit is mostly conic in shape. The external color of a mature fruit is deep red and glossy. The internal color is bright red. Strawberry Festival fruit has a very firm texture and excellent flavor.
Strawberry Festival has a fruiting pattern and yield similar to that of Camarosa.
|Early Mid Season||Medium-Large||Excellent||All||Very Firm||Tolerant|
Marys Peak – USDA
A Pacific Northwest variety from USDA-ARS (Oregon) that was collaboratively developed with OSU and WSU researchers.
High yields and an upright growth habit for easier picking for either market (Fresh or Processing).
|Late Mid Season||Medium-Large||Excellent||All||Moderately Firm|
Merced – U.C.
Merced is a short-day (June bearing) type cultivar that produces fruit over an extended period when treated appropriately in arid, sub-tropical climates.
When well-tended, Merced is similar to Camarosa, but has greater productivity, and higher quality fruit. The plant is less vigorous than Camarosa and does have lighter colored fruit.
Puget Crimson – WSU
Released thru WSU by Pat Moore, Puget Crimson is a cross of ‘Schwartze’ (aka: Puget Summer) & ‘Valley Red.
This is a short day variety which offers high yields of large fruit, very late in the fruiting season with excellent flavors.
Sweet Charlie – (Florida)
Developed in Florida, Sweet Charlie is an early sweet variety.
The plant is medium in size making harvest easy.
Sweet Charlie has anthracnose tolerance and does well in southern states and in China.
Tillamook – USDA
A high-yielding, large-fruited, mid-season cultivar with very high fruit quality that is suited to the fresh and processed markets. The ripe fruit are visible in the canopy and easy to pick. The fruit is very firm and has a very good fresh flavor.
Plants of Tillamook are vigorous and hold up well into the second harvest season suggesting some virus tolerance. Tillamook has good processing characteristics. The most outstanding characteristics of Tillamook are its high yield and very large attractive, high quality fruit.
Tillamook will be an excellent cultivar for local fresh market sales, pick-your-own and for processing.
|Mid Season||Large||Excellent||All||Moderately Firm||Tolerant|
Ventana – UC
Ventana is a short-day (June bearing) cultivar similar to Camarosa. Fruiting plants of Ventana are large and vigorous, similar to Camarosa, but more open than plants of Camarosa. Ventana has similar fruit size but produces greater individual-plant yields than Camarosa.
In general, Ventana is similar to Camarosa in that it initiates fruiting at the same time but produces greater quantities of early-season fruit with most cultural treatments.
A chilling unit in agriculture is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from freezing point to, depending on the model, 7 °C (45 °F) or even 16 °C (60 °F).  Stone fruit trees and certain other plants of temperate climate develop next year's buds in the summer. In the autumn the buds become dormant, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature these units are summed up for a whole season. Advanced models assign different weights to different temperature bands.
According to Fishman, chilling in trees acts in two stages. The first is reversible: chilling helps to build up the precursor to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks.  Apples have the highest chilling requirements of all fruit trees, followed by apricots and, lastly, peaches. Apple cultivars have a diverse range of permissible minimum chilling: most have been bred for temperate weather, but Gala and Fuji can be successfully grown in subtropical Bakersfield, California. 
Peach cultivars in Texas range in their requirements from 100 chilling units (Florida Grande cultivar, zoned for low chill regions) to 1,000 units (Surecrop, zoned for high chill regions).  Planting a low-chilling cultivar in a high-chill region risks loss of a year's harvest when an early bloom is hit by a spring frost.  A high-chilling cultivar planted in a low-chill region will, quite likely, never fruit at all. A four-year study of Ruston Red Alabama peach, which has a threshold of 850 chilling units, demonstrated that a seasonal chilling deficiency of less than 50 units has no effect on harvest. Deficiency of 50 to 100 units may result in loss of up to 50% of expected harvest. Deficiency of 250 hours and more is a sure loss of practically whole harvest the few fruit will be of very poor quality and have no market value.  Rest-breaking agents (e.g. hydrogen cyanamide, trade name BudPro or Dormex), applied in spring, can partially mitigate the effects of insufficient chilling. BudPro can substitute for up to 300 hours of chilling, but an excessive spraying and timing error can easily damage the buds.  Other products such as Dormex use stabilizing compounds.
Chilling of orange trees has two effects. First, it increases production of carotenoids and decreases chlorophyll content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates. 
Biennial plants like cabbage, sugar beet, celery and carrots need chilling to develop second-year flowering buds. Excessive chilling in the early stages of a sugar beet seedling, on the contrary, may trigger undesired growth of a flowering stem (bolting) in its first year. This phenomenon has been offset by breeding sugar beet cultivars with a higher minimum chilling threshold. Such cultivars can be seeded earlier than normal without the risk of bolting. 
All models require hourly recording of temperatures.  The simplest model assigns one chilling unit for every full hour at temperatures below 7 °C (45 °F). A slightly more sophisticated model excludes freezing temperatures, which do not contribute to proper dormancy cycle, and counts only hours with temperatures between 0 °C (32 °F) and 7 °C (45 °F). 
The Utah model assigns different weight to different temperature bands a full unit per hour is assigned only to temperatures between 3 °C (37 °F) and 9 °C (48 °F). Maximum effect is achieved at 7 °C (45 °F).  Temperatures between 13 °C (55 °F) and 16 °C (60 °F) (the threshold between chilling and warm weather) have zero weight, and higher temperature have negative weights: they reduce the beneficial effects of an already accumulated chilling hours. 
Southwick et al. wrote that neither of these models is accurate enough to account for application of rest-breaking agents widely used in modern farming. They advocated the use of a dynamic model tailored to the two-stage explanation of dormancy. 
Chilling Strawberry Plants: Learn About Chilling Requirements For Strawberries - garden
I had a conversation quite recently with Doug Shaw, UC plant breeder, concerning the concept of chill in the day neutral varieties and the desire of some Central Coast growers to reduce the recommended amounts significantly, even all the way down to zero days of chill:
In the way of review, please recall that chill requirement in strawberry in California is made up of two essential parts. One part is what the plant accumulates in the field before being harvested, and the other is accumulation of chill after harvest and the plant is in storage. There is a big difference between the two and one does not supplant the other. In-field chill takes place when the plant is still in the soil, out in the open and still has all its leaves. Supplemental chill takes place after harvest of the plant and occurs in a constant near freezing temperature, in the dark and the plant has none to very few leaves left (Figure 1 below).
Accumulation of chill, especially supplemental chill, in the strawberry transplant makes it stronger and better able to survive the stress of plant harvest, transplant and the growing season beyond.
As some of the chill sensitivity has been bred out of the modern UC day neutral varieties such as Albion, Monterey and especially San Andreas, it is indeed possible to grow these varieties with less than the recommended amounts (10-18 days) of supplemental chill. However, those who choose to reduce chill below these amounts should recognize the amount of risk they are taking and that this is not an exercise for those still using training wheels. Should growing conditions take a turn for the worse, these underchilled plants do not have the vigor to help them pull through and will suffer more than those adequately chilled. Moreover, while UC day neutral strawberry plants chilled significantly less than the recommended 10-18 days can produce fruit earlier than others (probably owing to the earlier planting date), these plants quite likely will not perform optimally in terms of overall yield and quality along with showing a tendency to produce smaller fruit later on in the season.
The only case where one would want to go short on supplemental chilling time would be if the plant harvest was so late that a minimum chill time of 10 days would result in a planting date so late that it would compromise plant growth and establishment.
Other than that, it is still recommended to give the UC day neutral varieties 10-18 days of supplemental chill.
The cold weather we have been experiencing over the past few days has prompted a lot of talk and even articles in the popular press over what the effect of this cold would be to local berry growers. Beyond the damage that very cold temperatures could cause tender plant parts such as flowers and emerging vegetative parts (of which we fortunately don’t have very many right now), the question worth exploring is what benefit this weather could be bringing to our berry crops.
Many of our cultivated fruiting plants originate from temperate regions, including many berry species and tree fruits, and as such go dormant in response to oncoming cold weather in the autumn. This adaptation of dormancy protects the plant buds from injury when temperatures fall below freezing and the buds stay this way until enough cold has been accumulated over time.
This accumulation of cold over time, known as chilling requirement and measured in hours as chill units, is the minimum amount of cold after which many fruit trees, caneberries and strawberries need to be exposed to in order to grow properly in the following spring. The total number of hours of chill needed to establish proper flowering and vegetative growth vary substantially for plant types and even between varieties of the same plant species.
If plants requiring a certain amount of chill hours do not receive it, they may end up blooming or leafing out late in the spring or in an spread out, uneven fashion. Additionally, they may subsequently experience reduced fruit production and quality.
Another complication of calculating chill units in California, as compared to much colder regions of the country, for example Wisconsin where temperatures can be below freezing for weeks at a time (go Badgers!), is that our region tends to have a cycling of warm and cold weather throughout the winter. How then do we as agriculturalists in California calculate chill accumulation in this back and forth between cold and warm?
To calculate chill hours, there are three common models all based on the principle that plants accumulate chill between 45 degrees F and freezing (32 degrees F and not below). One model ignores the below freezing threshold and simply calculates total number of hours under 45 degrees F, another calculates number of hours between 32 degrees and 45 degrees, and another, called the Utah model, is bounded by 34 degrees and 45 degrees but also accounts for negative chill accumulation, being the understanding that temperatures above 61 degrees detract from chill hours already accumulated. It is worth pointing out that in the Utah model, temperatures under 34 degrees do not accumulate chill, nor do they detract from it.
Yet, the fluctuating temperatures of California still are a challenge to some degree for these models, and the University of California is engaged in research to get a better handle on these conditions, and is has been testing a “Dynamic Chill Model” and a “Chill Portion Model”. Both of these are beyond the scope of this blog, but Central Coast agriculturalists seeking to further their understanding about chill and how to manage it, will find an excellent resource at :