Reference
Blackberry Chill Hour Requirements
Most blackberry cultivars sold in the US need somewhere between about 100 and 900 hours of winter chill, depending on the cultivar and the source you read. University of Florida IFAS puts current cultivars at 300 to 900 chill hours below 45°F. Several University of Arkansas releases, including 'Natchez' and the Prime-Ark primocane types, sit near the low end; UF/IFAS notes that the cultivars in its table were released by the University of Arkansas breeding program. Treat every number below as an estimate. Extension services disagree by 100 to 200 hours on several cultivars.
What chill hours are
Blackberries, like most temperate fruit, need a period of cold during dormancy before buds will break and flower normally. A chill hour is, roughly, one hour spent in a cool temperature range during that dormant period. The University of Arkansas Division of Agriculture defines chilling hours as the number of hours a plant has spent below a certain temperature during the dormant period, and notes that 35–45°F gives good accumulation.
The catch is that there is more than one way to count:
- Hours below 45°F. The simplest model. Every hour under 45°F counts as one. UF/IFAS uses this definition.
- Hours between 32°F and 45°F. The Southeast regional caneberry guide from NC State defines chilling requirement as hours at which temperatures are between 32 to 45°F in winter, so hours below freezing don't count.
- Utah chill units. Different temperature bands earn different credit, and warm hours subtract from the total. In the version described by UC Davis, an hour above 65°F counts as -1.0 unit.
- Chill portions (the Dynamic model). A newer model that UC Davis describes as more a research tool than a settled standard.
The choice of model matters most where winters are mild, because warm and below-freezing hours are exactly what the models treat differently. A published "300 hours" figure may not mean the same thing from one source to the next. That is one plausible reason the estimates below vary, though the sources don't say which model they used for each cultivar.
What happens without enough chill
UF/IFAS says that when plants don't get enough chill, bud break becomes poor and erratic. The Arkansas Division of Agriculture lists delayed bloom, reduced fruit set, and reduced fruit quality. In other words, the problem shows up as a weak, uneven crop.
Chill estimates by cultivar
The table lists only numbers a source actually gives. A dash means that source doesn't list the cultivar. Prime-Ark cultivars are primocane-fruiting; the rest are floricane-fruiting.
| Cultivar | NC State (Southeast guide, 2023) | UF/IFAS (2024) | Other extension sources |
|---|---|---|---|
| Prime-Ark Freedom | About 100 | 300 | — |
| Kiowa | — | — | Probably near 200 (LSU); 200 (Texas A&M) |
| Natchez | About 300 | 300 | 400–500 (Oklahoma State); 500 (Texas A&M) |
| Caddo | 400 | 300 | — |
| Prime-Ark 45 | 300 | 300 | — |
| Prime-Ark Traveler | 300–400 | 300 | — |
| Choctaw | — | — | 300–400 (LSU) |
| Osage | 350–400 | 400–500 | — |
| Sweet-Ark Ponca | 400 | — | — |
| Ouachita | 400–500 | 400–500 | 400–500 (Oklahoma State); 300 (Texas A&M); probably similar to Arapaho (LSU) |
| Arapaho | — | — | 400–500 (LSU); 500 (Texas A&M) |
| Chickasaw | — | — | 500–700 (Texas A&M) |
| Navaho | 800 | 800–900 | 800–900 (LSU); 800 (Texas A&M) |
| Apache | 800 | — | 800–900 (Oklahoma State); 800 (Texas A&M); similar to Navaho (LSU) |
| Triple Crown | 800 | — | — |
| Von | 800 | — | — |
Sources: Southeast Regional Caneberry Production Guide, cultivars chapter (Fernandez, McWhirt and Bradish, NC State Extension, 2023); Choosing the Right Blackberry Cultivar in Subtropical Florida (Agehara, Lin and Deng, UF/IFAS HS1352); Blackberry and Raspberry Culture for the Home Garden (Oklahoma State HLA-6215); Blackberry Varieties for Louisiana (LSU AgCenter); Texas Fruit and Nut Production: Blackberries (Nesbitt, Kamas and Stein, Texas AgriLife Extension).
Where the sources disagree
The spread is widest for exactly the cultivars southern gardeners ask about most.
- 'Natchez' ranges from about 300 hours (NC State, UF/IFAS) to 500 hours (Texas A&M).
- 'Ouachita' is 400–500 hours in three sources and 300 in the Texas publication.
- 'Prime-Ark Freedom' is about 100 hours in the NC State guide and 300 in the UF/IFAS table.
- 'Caddo' and 'Osage' differ by about 100 hours between NC State and UF/IFAS.
Several sources flag their own numbers as rough. NC State calls most of its figures "estimated." LSU uses "probably" for 'Kiowa' and 'Ouachita.' The Texas publication appears to date from 2012 and does not cover later releases such as 'Osage' or 'Caddo'. None of this means one source is wrong. It means chill requirement is hard to pin down, and it depends on how and where it was measured.
The practical takeaway: if your winter chill is close to a cultivar's listed number, assume it is borderline. Prefer the cultivars your own state's extension recommends, since those recommendations usually reflect local trials.
Choosing by region
A few sourced reference points:
- The NC State guide advises growers in low-chill parts of the Southeast, such as southern Georgia, Mississippi, Alabama and Louisiana, to choose a low-chilling cultivar (under 400 hours).
- UF/IFAS reports that central Florida averages only about 250 chill hours, below what many cultivars need.
- Texas AgriLife specialists say most blackberries fruit well in areas of the state that receive 300 hours or more below 45°F.
- LSU AgCenter recommends 'Navaho' (800–900 hours) for north Louisiana, and says 'Arapaho' should do well in most of the state except possibly some coastal areas.
In colder regions, chill is rarely the limit. Winter cold is. Primocane types add one more wrinkle: the NC State guide notes that the Prime-Ark primocane crop is variable where summer heat impairs flower development, which has nothing to do with chill.
To find the typical chill total for your area, and which model it was counted with, ask your county extension office.
Related
For planting, trellising, and pruning, see growing blackberries organically. For timing by region, see the berry planting calendar. The Growing Berries Organically hub covers strawberries and raspberries too.