In the wild, apples grow quite readily from seeds. However, like most perennial fruits, apples are ordinarily propagated asexually by grafting. This is because seedling apples are an example of "Extreme heterozygotes", in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead different from their parents, sometimes radically. Triploids have an additional reproductive barrier in that the 3 sets of chromosomes cannot be divided evenly during meiosis yielding unequal segregation of the chromosomes (aneuploids). Even in the very unusual case when a triploid plant can produce a seed (apples are an example), it happens infrequently, and seedlings rarely survive. Most new apple cultivars originate as seedlings, which either arise by chance or are bred by deliberately crossing cultivars with promising characteristics. The words 'seedling', 'pippin', and 'kernel' in the name of an apple cultivar suggest that it originated as a seedling. Apples can also form bud sports (mutations on a single branch). Some bud sports turn out to be improved strains of the parent cultivar. Some differ sufficiently from the parent tree to be considered new cultivars.
Breeders can produce more rigid apples through crossing. For example, the Excelsior Experiment Station of the University of Minnesota has, since the 1930s, introduced a steady progression of important hardy apples that are widely grown, both commercially and by backyard orchardists, throughout Minnesota and Wisconsin. Its most important introductions have included 'Haralson' (which is the most widely cultivated apple in Minnesota), 'Wealthy', 'Honeygold', and Honeycrisp'.
Apples have been acclimatized in Ecuador at very high altitudes, where they provide crops twice per year because of constant temperate conditions in a whole year.
Apples are self-incompatible; they must cross-pollinate to develop fruit. During the flowering each season, apple growers usually provide pollinators to carry the pollen. Honeybee hives are most commonly used. Orchard mason bees are also used as supplemental pollinators in commercial orchards. Bumble bee queens are sometimes present in orchards, but not usually in enough quantity to be significant pollinators.
There are four to seven pollination groups in apples depending on climate:
- Group A – Early flowering, May 1 to 3 in England (Gravenstein, Red Astrachan)
- Group B – May 4 to 7 (Idared, McIntosh)
- Group C – Mid-season flowering, May 8 to 11 (Granny Smith, Cox's Orange Pippin)
- Group D – Mid/Late season flowering, May 12 to 15 (Golden Delicious, Calville blanc d'hiver)
- Group E – Late flowering, May 16 to 18 (Braeburn, Reinette d'Orléans)
- Group F – May 19 to 23 (Suntan)
- Group H – May 24 to 28 (Court-Pendu Gris) (also called Court-Pendu plat)
One cultivar can be pollinated by a compatible cultivar from the same group or close (A with A, or A with B, but not A with C or D).
Varieties are sometimes classed as to the day of peak bloom in the average 30 day blossom period, with pollinizers selected from varieties within a 6 day overlap period.
Maturation and Harvest
Cultivars vary in their yield and the ultimate size of the tree, even when grown on the same rootstock. Some cultivars, if left unpruned, will grow very large, which allows them to bear much more fruit, but makes harvesting very difficult. Mature trees typically bear 40 kg of apples each year, though productivity can be close to zero in poor years. Apples are harvested using three-point ladders that are designed to fit amongst the branches. Dwarf trees will bear about 10 kg of fruit per year.
Commercially, apples can be stored for some months in controlled-atmosphere chambers to delay ethylene-induced onset of ripening. The apples are commonly stored in chambers with higher concentrations of carbon dioxide with high air filtration. This prevents ethylene concentrations from rising to higher amounts and preventing ripening from moving too quickly. Ripening begins when the fruit is removed. For home storage, most varieties of apple can be stored for approximately two weeks, when kept at the coolest part of the refrigerator (i.e. below 5 °C). Some types of apple, including the Granny Smith and Fuji, have an even longer shelf life.
- John Lloyd and John Mitchinson. (2006). QI: The Complete First Series - QI Factoids. [DVD]. 2 entertain.
- Bob Polomski; Greg Reighard. "Apple". Clemson University. http://hgic.clemson.edu/factsheets/HGIC1350.htm. Retrieved 22 January 2008.
- "Apples". solarnavigator.net. http://www.solarnavigator.net/solar_cola/apples.htm. Retrieved 22 January 2008.
- "Controlled Atmosphere Storage (CA)". Washington State Apple Advertising Commission. http://www.bestapples.com/facts/facts_controlled.shtml. Retrieved 24 January 2008.