Chill Portions & Break Dormancy on Time

Chill portions are the unit of the Fishman–Erez Dynamic Model, the most accurate way to measure winter chill for fruit and nut trees. The model works in two steps: cool temperatures first build a labile intermediate product, and only once enough has built up is it irreversibly fixed into one chill portion. Crucially, warm temperatures destroy the not-yet-fixed intermediate, so the model correctly discounts chill in warm, fluctuating winters where chilling-hours and Utah units overestimate.

Compare the chill portions accumulated against your cultivar's requirement. Enter your average daily minimum and maximum temperatures and the number of days since dormancy began; the tool integrates an hourly sinusoid through the Dynamic Model and reports the chill portions banked, the percent of the cultivar requirement met, and a projected fulfilment date. If accrual has stalled because afternoons are warm, it flags a warm-winter risk.

Chilling hours simply count hours between 0 and 7.2 °C and Utah units weight them, but both keep adding chill on warm afternoons and neither subtracts it. The Dynamic Model does subtract — a warm spell after cool hours wipes out the not-yet-fixed intermediate. In warm and variable climates such as subtropical India, southern Spain, California's Central Valley or South Africa, chilling hours can overstate chill by a wide margin, leading growers to plant cultivars that never break dormancy properly.

The Dynamic Model's effectiveness peaks around 6–8 °C. Below freezing it is near zero, and above roughly 14 °C it falls toward zero and can turn negative when warm hours follow cool ones. So a steady 6 °C accumulates chill efficiently — about 0.7 to 0.8 chill portions per day — while a day that swings from a cool night to a warm afternoon can net almost no chill at all.

It varies widely by cultivar. Low-chill peaches and apples bred for warm climates need around 25–35 chill portions, mid-chill cultivars 40–55, and high-chill cultivars such as 'Bing' cherry, 'Granny Smith' apple, walnut and pistachio 62–70 or more. The tool lists published Dynamic-Model requirements for about thirty stone-fruit, pome-fruit, grape, nut and berry cultivars so you can match the variety to your winter.

Insufficient chilling delays and prolongs bud break, gives erratic and extended bloom, drops fewer and weaker flowers, and reduces fruit set and yield. In severe cases buds simply fail to open. The Dynamic Model is the best early warning: if portions stall well below the requirement as the season runs out, you can plan a rest-breaking agent or accept reduced cropping rather than be surprised at bloom.

You cannot add cold, but you can manage around a chill shortfall. Rest-breaking agents such as hydrogen cyanamide or oils can partly substitute for missing chill and synchronise bud break, and overhead cooling or evaporative cooling can shave afternoon highs in some systems. The most reliable fix is to plant cultivars whose chill-portion requirement matches your typical winter — which is exactly what this tool helps you check.

The tool builds an hourly temperature curve for each day as a sinusoid between your average daily minimum and maximum, with the minimum near dawn and the maximum mid-afternoon. It then steps that hourly series through the Dynamic Model's intermediate-product equations (the chillR parameter set). Each hour either builds, fixes or destroys intermediate, and fixed intermediate is counted as chill portions. The result is the total portions over the days you entered.

By convention the Dynamic Model chill season is often counted from around 1 September or 1 October in the Northern Hemisphere — but because the model self-corrects by destroying intermediate during warm spells, the exact start matters less than for chilling hours. Enter the days since dormancy began for your site; in practice growers count from when autumn temperatures drop and bud set completes.

Yes. The Dynamic Model applies to all temperate deciduous perennials with a chill requirement — stone fruit, pome fruit, grapevines, almonds, walnuts, pistachios, blueberries and kiwifruit are all included in the cultivar list. Each has its own chill-portion requirement; pick the closest match and the same accumulation model applies.

It extrapolates from the average daily accrual rate so far, so it is a planning estimate, not a forecast. If the coming weeks are colder than the period entered, chill will land sooner; if warmer, later, and a run of warm afternoons can stall it entirely. Re-run the tool with updated temperatures through the winter to track progress toward the requirement.

It is increasingly the recommended standard for warm and variable climates and is the basis of UC Davis chill reports and the widely used chillR software. Chilling hours remain in use for cold, stable climates out of habit and simplicity, but where winters are mild or erratic the Dynamic Model's chill portions are the more reliable measure of whether trees will break dormancy properly.