Grapefruit, a popular hybrid originally resulting from a cross between sweet orange and pummelo, is a large sour fruit in the citrus family. Grapefruit trees are grown in the American South and West, and the ripe fruit is sold worldwide.
Ripening vs. Maturing
Grapefruits do not specifically ripen so much as mature. Unlike many fruits which can ripen after being picked, the grapefruit must mature entirely on the tree.
Most grapefruit crops reap initial maturity during the middle or end of the month of October. The fruit is not ready to eat and should not be picked before this time.
Grapefruit continue to grow both sweeter and larger as time goes on and can be picked much later in the season. Because the fruit do not fall from the tree the way apples do, grapefruit can be harvested anywhere between the months of October and May.