During the day the temperature rises to 28 °C, and at night it falls to 16 °C. The daily mean is 22 °C. After subtracting the base (22 − 10), the tree gains 12 GDD10 units that day.

The daytime temperature is 13 °C and the night temperature 5 °C. The daily mean is 9 °C, which is below the base temperature of 10 °C. The gain for ripening is therefore 0 GDD10 (although in direct sunlight some assimilation in the tissues may still take place).

Heat requirement over the season (GDD10)

The length of each bar corresponds to the upper limit of the stated range, measured against a scale of 0 – 2,200 GDD10. For comparison: the warmest lowlands of Slovakia reach 1,550 to 1,850+ GDD10.

Pecans are a perfect illustration of the conflict between winter hardiness and heat requirements. Many North American varieties (the so-called northern pecans such as 'Pawnee', 'Major' or 'Posey') easily tolerate frosts below −28 °C in deep dormancy. But to split their green husk (pericarp) in October and fill the kernel with oil and dry matter, they need 1,700 to 2,100 GDD10 and 170 to 190 frost-free days.

Outside the warmest southern locations in Slovakia, these varieties therefore grow into large, healthy trees that drop unripe, darkened nuts with an empty or paper-thin kernel in autumn.

The only route for central Europe is ultra-northern selections from Iowa, northern Illinois or Ontario (for example 'Kanza', 'Warren 346', 'Snaps', 'Carlson 3' or 'Cornfield'). Their requirement drops to 1,350 to 1,500 GDD10 and 135 to 150 frost-free days.

The pawpaw is native to the eastern United States, from Florida to Canada, so its varieties differ greatly in their heat requirements. Almost all cultivars tolerate −25 to −28 °C in winter. The difference lies in the heat they need during the season.

Early selections ('Summer Delight', 'Kentucky Champion', 'Halvin', 'Prima 1216') manage with roughly 1,250 to 1,350 GDD10. In warmer areas they ripen as early as the turn of August and September.

Late varieties ('Susquehanna', 'Shenandoah', 'Potomac' and several older southern selections) need 1,550 to 1,700+ GDD10. If you plant them in a basin with cold nights or at a higher elevation, the fruits grow to full size but do not finish converting starch to sugar before the October frosts, and do not start synthesising the typical tropical esters (ethyl hexanoate and methyl octanoate).

The astringent taste is caused by soluble tannins (proanthocyanidins) in the vacuoles of the cells. During normal ripening on the tree, when there is enough heat, and during the subsequent softening of the fruit, acetaldehyde is produced. It polymerises (precipitates) the tannins into an insoluble form, and the astringency disappears.

However, if an early frost strikes a variety with a high heat requirement (for example later hybrids or pure D. kaki) while its sugar level is low and it is still unripe, the frost does rupture the cell walls and speeds up the softening of the tissue, but the tannins do not precipitate completely. The result is a watery, astringent and inedible mass.

That is why in areas with a shorter FFD (below 155 days), the ultra-early American D. virginiana varieties (for example 'Prok', 'J-59', 'H-118' / 'Early Jewel', 'Meader') and early varieties of the Ukrainian hybrids ('Sosnovskaya', 'Dar Sofiyivky') succeed reliably. Later large-fruited hybrids need the full heat budget of the southern regions or a sheltered wall.

A shortage of GDD10 and FFD has one more critical consequence, which shows up only in winter: insufficient lignification (hardening) of the shoots. After the harvest (or while a late crop is still ripening), the tree needs a certain amount of heat and sunshine to store reserve starch in the shoot tissues, reduce the amount of free water in the cells and build up protective substances against frost (sugars and specific proteins that prevent ice from forming inside the cells).

If a late variety of peach, apricot or hybrid stone fruit holds its fruit on the tree until the first frost at the limit of its GDD10 capacity, the tree puts all of its assimilates into the fruit at the expense of ripening the wood. In January, at −15 °C, even a variety whose listed hardiness is −26 °C will then freeze.

During these phases the fruit accumulates organic acids (malic, citric, tartaric) and starch.

Only when the fruit has almost stopped growing in volume do the breakdown of acids (their respiration in the Krebs cycle) and a massive influx of sucrose from the leaves get fully under way at temperatures of 20 to 28 °C. At the same time, terpenes, lactones and esters are formed, which make an apricot smell like an apricot and a peach taste like a peach.

The red blush on the skin (anthocyanin formation) is stimulated mainly by sunlight (UV-B) and cool nights, not by internal ripeness.

Plant heat-loving specialities right up against the south or southwest wall of a building, a concrete fence or a stone wall (a gabion). During the day the wall absorbs short-wave solar radiation and at night releases it as long-wave infrared radiation back towards the crown of the tree. This raises night minima (Tmin) by 2 to 4 °C. Over a whole growing season, such a heat-storing wall can add 150 to 250 GDD10 and extend the frost-free period (FFD) by 10 to 20 days.

On clear, windless spring and autumn nights the ground surface cools by radiation. Cold air is denser (heavier) and flows down a slope just like water. If your garden is on the middle of a slope, you have a great advantage: the cold air drains away into the valley and your FFD can be 3 to 4 weeks longer than that of a neighbour living 50 metres lower down by the stream.

A thick layer of light-coloured organic mulch (straw, wood chips) holds moisture well in summer, but in spring it slows the warming of the soil, and in autumn it acts as insulation that stops heat from the deeper soil layers radiating towards the tree during the first frosty nights. Clean, dark, slightly moist soil under the crown can release a considerable amount of stored heat during a clear October night and protect the leaves from the first borderline frost (−1 to −2 °C). The tree thereby gains another two weeks of assimilation.

The rootstock influences not only vigour but also the length of the growing season and the time at which shoot growth stops. Vigorous seedlings (for example in stone fruit or nut trees) with a deep root system keep the scion in active growth long into autumn. Less vigorous or specific clonal rootstocks (or interspecific rootstocks with a shorter growing season) force the grafted variety to set its terminal bud earlier and ripen its wood earlier. They also slightly bring forward the start of fruit ripening.