Trees vary in individual size and in the size and sweetness of their fruit.
And, some trees may be more resistant to diseases. Naturally, citrus growers, to ensure the best possible crop, want to make sure all their trees are of the same high quality. To do this, each new tree they plant is grown, or propagated, not from a seed but by grafting or budding. In most citrus trees, the scion, or top of the tree, is a different variety from the roots or rootstock of the tree. Citrus growers plant trees whose tops will grow 'Washington' Navel oranges or 'Eureka' lemons on a rootstock that has special characteristics like disease resistance, quick to bear fruit or restricting tree size for easier harvesting.