The grapefruit tree (Citrus x paradisi) resulted from a hybrid cross between a pummelo and a sweet orange in the Caribbean region. The trait of thorns or spines on the branches was inherited from the pummelo, also called shaddock.
The grapefruit tree matures to a height of 15 to 20 feet -- potentially to 30 to 50 feet -- with a rather open, spreading canopy of branches. The youngest green twigs bear firm but flexible spines, often masked by the leaves.
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The spines' or thorns' sizes and spacings vary tree to tree, but range from 1 to 3 inches long.
Grapefruit cultivars usually are propagated by cuttings and grafted on disease- or cold-resistant rootstock of other citrus trees species. Grapefruit seeds also are viable and sprout to yield plants with different traits than the mother plant. Seedlings may develop more or fewer thorns on branches based on their inherited genes, and may even mutate to produce fruits that lack seeds or thorns.
Horticulturists continue to breed grapefruits with other citrus plants to obtain improved cultivars. Mandarins, sweet oranges and pummelos are used in the hybridization. Resulting plants have variable amounts of branch spines. In fact, the cultivar Cocktail is referred to as a grapefruit but resulted from crossing a pummelo with a mandarin.