Sometimes the stock itself is a vegetatively propagated tree. Most trees are propagated, however, on seedling rootstocks. Several size-controlling rootstocks are available. Uniform heights facilitate spraying and harvesting. Extremely dwarfed rootstocks are popular where space is limited.
Most apple varieties require cross-pollination in order to bear satisfactorily.
Certain varieties, known as triploids, are of no value as pollinators because of their genetic makeup. Two diploid pollinating varieties must be present to pollinate each other as well as the triploid variety. The northwestern area is noted for high yields but uses extensive artificial irrigation. Of the several thousand varieties of apples, relatively few have commercial importance. Delicious, with its color mutations, is the most widely grown variety in the United States.