South Africa’s commercial apples, pears, plums, peaches and nectarines all come from trees shaped by a very different world: the cold winters of the northern hemisphere.
These are trees built for winter shutdown. They survive freezing conditions by switching off, and they only return to the season properly once they have had enough cold.
South Africa’s main fruit regions sit at about 30 degrees south, far from the colder zones where these crops evolved. That gap helps explain why winter chill and dormancy are central to where fruit can be grown and how reliably orchards perform.
Dormancy is a survival strategy. Deciduous trees from cold climates drop their leaves in autumn and enter a resting state that protects them through winter. They resume growth in spring once conditions are safe.
Researchers describe this process in three overlapping phases. In the first, often called para-dormancy, growth is held back by signals from other parts of the plant, such as actively growing shoot tips. In the second phase, the true rest phase known as endo-dormancy, the buds themselves are locked down and will not grow even if the weather turns warm. Winter chill releases them from this state. In the third phase, eco-dormancy, the buds are ready to grow and are simply waiting for enough warmth to push out.
The amount of cold a tree needs to move through this rest phase is known as its chilling requirement, and it varies by crop and cultivar. High-chill apples need long, cold winters. Low-chill peaches can manage with far less. When the requirement is met, trees tend to wake evenly, flower well and set a manageable crop.
When winter chill falls short, the orchard shows it quickly. Bud-break becomes delayed and uneven, so a single tree can carry flowers and young shoots at several stages at once.
Flowering becomes erratic and sparse. Fruit set suffers, and the fruit that does form is often uneven in size and maturity. That complicates everything from thinning to harvest to packing.
Research published through Hortgro Science makes the point plainly: deciduous trees in South Africa can survive without adequate winter chill, but commercial production becomes unviable unless growers intervene. In a warm year, in a marginal area, a poor chill season can undo much of a grower’s investment.
Poor dormancy release can also follow the fruit beyond the orchard. Uneven ripening often carries through to the packhouse and cold store, showing up as variable quality and shorter shelf life.
Measuring something invisible
For all its importance, chill is remarkably difficult to measure. Several models exist to convert winter temperatures into an estimate of accumulated chill. The simplest, the chilling hours model, counts the hours below a set temperature.
The widely used Utah model, developed in the 1970s, is more sophisticated. It assigns different weightings to different temperatures and allows warm spells to cancel out chill already banked. The dynamic model expresses chill in units called chill portions. It tries to capture the way moderate cold can be undone or locked in by the temperatures that follow.
The catch is that a model calibrated for one climate does not necessarily hold in another. South African researchers found that the standard Utah model performs poorly in the warmer local regions, where winter daytime temperatures above 20 degrees Celsius are common. Under those conditions, the model records large negative totals as warm afternoons cancel out cold nights, and the result drifts away from what the trees actually experience.
In response, Linsley-Noakes, Louw and Allan developed the daily positive Utah chill unit model, which sums chill over each 24-hour period and, crucially, never lets a day’s total go below zero. A day that comes out negative simply counts as nothing, rather than erasing chill banked earlier. This locally adapted approach has become a practical standard for estimating chill in South African conditions.
The deeper problem with chill models
Even the best chill model rests on an assumption that can break down in a warm South African winter. As pome and stone fruit specialist Dr Nigel Cook has noted, dormancy has both an entrance and an exit. In cold climates, trees enter deep rest quickly, making it easier to decide when to start counting chill. In warmer regions, that entry is slower and less clear-cut, which means the final figure can mislead if counting starts at the wrong time. That is why local researchers treat chill models as guides rather than precision instruments, especially in the marginal areas where growers need certainty most.
Chilling requirement varies widely across the deciduous crops grown locally, which is why some thrive in one district and fail a short distance away. Among pome fruit, many of the apple cultivars that anchor the South African industry, including Granny Smith, Golden Delicious and the Cripps selections marketed as Pink Lady, carry relatively high chill requirements and are most at home in the colder districts.
Pears such as Packham’s Triumph and the popular blushed Forelle also need a solid winter. Among stone fruit, Japanese plums span a range of requirements, while many peaches and nectarines have been bred down to low chill requirements that let them be grown in warmer areas where apples would struggle. Matching the crop and the cultivar to the chill a site can reliably deliver, is the first line of defence against a disappointing orchard.
The mechanism behind all of this remains only partly understood, even after two centuries of research into the breaking of dormancy. What is clear is that the resting bud is held in check by internal signals, and that exposure to cold gradually dismantles those brakes until the bud is competent to grow once warmth arrives.
Chill models are imperfect precisely because they estimate this hidden process from air temperature alone, without being able to see inside the bud. For the grower, the practical upshot is that chill is real and consequential even though it cannot be observed directly, which places a premium on good record-keeping and local experience alongside the models.
For all their limitations, chill estimates do real work in one of the most expensive decisions a grower makes: which cultivar to plant where. Chilling requirement is a primary filter in matching a cultivar to a region. High-chill cultivars belong in the colder areas such as the Koue Bokkeveld and the higher parts of Ceres, while warmer districts suit lower-chill selections.
Planting a high-chill cultivar in a marginal area is one of the costliest mistakes in deciduous fruit growing, because the orchard will underperform for its entire productive life, and the error is only fully visible years after the trees go in. Reliable chill information, region by region, is therefore part of the foundation of a sound planting decision.
Chill is not uniform even within a district. Cold air drains into valleys and hollows on still winter nights, so low-lying blocks may bank more chill than the slopes above them, while aspect, altitude and distance from the coast all shift the local picture. Experienced growers treat chill as a site-by-site variable rather than a regional average, which is why a new orchard’s position in the landscape forms part of the chill calculation long before the first tree goes into the ground.
Helping trees along
Where natural chill is marginal, growers do not simply accept the consequences. Rest-breaking treatments, applied as sprays in late winter, are used to compensate for inadequate chilling and to pull a more even bud-break out of trees that would otherwise wake slowly and unevenly. The timing of these treatments is critical and depends on how the season has unfolded, which brings the discussion back to chill monitoring.
Many growers, and the insurers who carry some of their risk, track chill accumulation through the winter precisely so that they can judge when, and whether, to intervene. Used well, rest-breaking can rescue evenness in a poor chill year; used carelessly, it can do more harm than good, which is why it sits alongside good cultivar choice rather than replacing it.
In practice, managing chill is a winter-long exercise in reading the season rather than a single calculation. Growers combine the output of a chill model with local weather records and their own observation of the trees, watching how buds swell and how evenly the orchard is moving toward bud-break, and they weigh all of this when deciding whether a rest-breaking treatment is needed and when to apply it. A cold winter may need no intervention at all, while a mild one may call for carefully timed action across several blocks. The same reading informs the spring ahead, because an orchard that has had a marginal chill season is likely to flower unevenly and will need closer attention through pollination, fruit set and thinning. Chill therefore stays in play right through the early season, well beyond the midwinter weeks when it is measured.
A warming winter
The chill challenge is not static, and the direction of travel is unfavourable. Mean temperatures in South Africa have risen over the past century, and climate projections for the Western Cape fruit regions point to a continued decline in the winter chill that growers can count on. Work using the daily positive Utah chill unit model for apple districts has shown accumulated chill trending downward, with some areas already only marginally meeting requirements in current conditions.
As winters warm, the band of country in which a given high-chill cultivar can be grown reliably is expected to narrow, pushing growers toward lower-chill cultivars, toward cooler sites, and toward a heavier reliance on rest-breaking and careful monitoring.
Deciduous fruit still has a future in South Africa, but the margin for error is narrowing, and that raises the premium on good science. Research through Hortgro Science and Stellenbosch University continues to refine chill models, to improve the prediction of bud-break, and to support breeding and selection of cultivars better suited to a warmer climate.
For growers, the practical message is consistent: treat winter chill as a central planning variable, match cultivars honestly to the chill a site can deliver, monitor accumulation through the season, and use rest-breaking as a measured tool rather than a routine crutch. The growers who manage dormancy well are the ones best placed to keep producing quality fruit as the climate that their northern-hemisphere trees were built for slowly slips further out of reach.
Sources consulted: Hortgro Science and Fresh Quarterly articles on winter dormancy and chill models (research reported by Anna Mouton, with input from Dr Nigel Cook and Stellenbosch University); the daily positive Utah chill unit model (Linsley-Noakes, Louw and Allan, 1995; Linsley-Noakes, Allan and Matthee, 1994); peer-reviewed studies on climate change and accumulated chill at South African fruit sites.