Leviticus 19:23

23. Count the fruit… as uncircumcised The fruit of the first three years was to be thrown away as unclean or uneatable. Some assign as the ground of this law that the fruit of these years was little in quantity and inferior in quality, and that by breaking off the fruit blossoms the growth of the trees and vines was stimulated and the future fruitfulness greatly increased. But it seems more reasonable to suppose that this requirement rests on the same grounds as the command to offer the firstborn of the flocks and the firstfruits of the harvest as a thank offering to Jehovah for his blessing upon the fruit-trees. The trees planted by the Canaanites, before the conquest by Joshua, were treated as exempt from this rule.