Pollination of apples
Even the few apple varieties that are reasonably self-fertile will give a heavier crop when cross-pollinated. Finding suitable pollinators, however, is not always quite as simple as it might appear. The first consideration is obviously that the majority of the blossom of the pollinator should be open at the same time as most of the blossom on the tree to be pollinated. In a, where you have not the space for many trees, it is useful if the pollination can be a two-way business. It can, in fact, occur on a single tree—the so-called ‘family tree’ where two or more suitably chosen varieties have been grafted onto the same rootstock.
As the blossom on a tree does not all open on the same day, and as only a proportion of the flower has to be fertilized to secure a reasonable crop, there can obviously be some spread of blossoming time between two varieties within which satisfactory cross-pollination may occur.
However, once a flower opens it does not remain capable of being fertilized for long, and after about four days the possibility of pollination declines rapidly. Furthermore, the flowering date of any given variety, compared with its fellows, may vary by up to 17 days. Other trees of the same variety may flower later or earlier due to a difference in position and variations inand nutrients. Possibly, too, one rootstock may be different from another, so it is always safest not to rely on a single pollinator.
The more varieties of apple you can grow together, the better. In gardens where space is limited, this can be achieved by growing cordons, pyramids or bush trees on dwarfing rootstocks.
Even if two varieties of apple flower at the same time they may still be unable to pollinate each other. Most varieties are what the geneticists call diploids, that is the plant cell resulting from the fusion of a male and female cell has two sets of chromosomes (or one complete unit) in it, one from each cell. The chromosones are minute objects which carry the genes, smaller particles still, which are responsible for the appearance and growth of a plant.
Some varieties have a different chromosome arrangement in their cells: they are what are known as triploids and each sex cell carries 1-½ sets of chromosomes. The pollen from such varieties is much less fertile, if at all. A triploid, therefore, will not pollinate other varieties and so if you wish to grow one you must have one diploid to pollinate it and yet another diploid to pollinate the other—three varieties in all, with overlapping flowering periods.
Even then, your troubles are not over. Some varieties produce pollen of poor germinating power and it is probable that these are really triploids.
A few varieties, too, are incompatible with certain others. Cox’s Orange Pippin and Kidd’s Orange Red will not pollinate each other and nor will a Cox pollinate Holstein, which is a triploid. Golden Delicious will not pollinate Crispin, another triploid.
Sports will not pollinate the varieties from which they originate. Crimson Cox and Queen Cox, for example, are incompatible with Cox’s Orange Pippin, and Red Miller will not cross-pollinate with Miller’s Seedling.
Finally, in choosing possible pollinators you need to avoid those unreliable varieties which make a habit of flowering only in alternate years. Some of the worst offenders in this category are Christmas Pearmain, Emneth Early (Early Victoria), Laxton’s Superb, Miller’s Seedling, Newton Wonder, Rej. W. Wilks and Wagener.
Some other varieties show some tendency towards irregular or biennial bearing of fruit and therefore it would be unwise to rely too heavily on these for pollination. In this category are D’Arcy Spice, Devonshire Quarrenden, Dumelow’s Seedling (Wellington), Ellison’s Orange, King of the Pippins, Melba, Ontario, Owen Thomas and Rival.
Nevertheless, if we grow any of these doubtful pollinators, we have to consider their flowering times just the same, as they themselves have to be pollinated.
Eating varieties of apple can also be fertilized by certain crab-apples and as these can be most ornamental as well as providing the raw material for delicious jelly, you should consider growing them in your garden.
The crab-apples Malus floribunda and Winter Gold should pollinate the earliest flowering varieties of apples. Golden Hornet flowers two days later (about equivalent to Group 1-½ in the list above) and makes attractive jelly. Aldenhamens is later, about Group VII, and Hilleieri yet later (Group VIII) and would probably pollinate Edivard VII if it was hard pruned immediately after petal fall, thus encouraging flowering on one-year-old wood.