Citrus - Grapefruit Citrus paradisi Macfad. Rutaceae

Species description Grapefruit is a tall, erect to spreading evergreen tree with glossy, dark green leaves and white, 4- petalled flowers. The branches have short, supple thorns. The fruit is yellow-orange, flattened spheroid, mildly to strongly acidic, and the flesh may be white, pink or red in color. The tree is well suited for hot climates. Grapefruit trees can grow 20-30 feet tall, with a canopy diameter 14 feet wide, and live for more than 50 years. Citrus are generally hardy to 23ºF-26ºF, though this may depend on the cultivar.

Natural and cultural history Grapefruit originated as a hybrid from sweet orange and pummelo, perhaps in the West Indies Barbados, and was first botanically described in 1750. Grapefruit trees were grown on Caribbean islands before being transported to Florida. The first grapefruit orchard was planted in Florida in the late 1800s, and soon the fruits were shipped to the northeast as a specialty crop. By the early 1900s, the trees were being planted in Texas, Arizona and California. By 1940, The United States was a global exporter of grapefruit juice and canned grapefruit sections. Jamaica, Trinidad, Cuba, Mexico, Central America, South America, Cyprus, Morocco, and Israel have since become commercial production centers of grapefruit.

Planting considerations and propagation techniques Grapefruit trees are adapted to very hot climates and rarely produce fruit in cooler regions. The trees prefer open sunlight and wide spacing (20 feet around the tree) for full production. Citrus trees have numerous thorns, so care should be taken in their placement, especially in public spaces. Grapefruit flowers are self-fertile and require no cross-pollination, though honeybees are effective pollinators. Citrus can grow on a wide range of soil types from sand to loam to heavy clay as long as they are well drained. The ideal soil pH for citrus trees is 6-6.5, though different rootstocks prefer different soil types. Grapefruit trees may be propagated from seed or by grafting cuttings from preferred cultivars onto other citrus rootstock.

Water needs Citrus need about 4-6 inches of water per month in the summer; they are generally not drought tolerant and need careful attention to produce well-developed fruit.

Care Citrus trees should be trained to a suitable shape with an open center. There are three primary pruning objectives: increase total leaf area, improve airflow through the canopy, and increase light to the branches. Proper training keeps trees the correct size for ease of care and harvest. Citrus are prone to many diseases and pests. We recommend consulting a Cooperative Extension citrus specialist. (See Yuma Cooperative Extension, https://extension.arizona.edu/yuma; and http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1492.pdf).

Harvesting and processing Citrus should be harvested when it has full color or flavor and when it can be eaten immediately. The tree is the best place to store citrus. In Arizona, grapefruits can be harvested from early September until the end of May when they begin to fall and seeds start sprouting in the fruit. Late harvesting, however, may cause a reduction in the following year's fruit crop. Grapefruit are traditionally eaten fresh. The fruit are also made into marmalade and syrups and canned. The juice may be used fresh or fermented into vinegar or wine.

References and resources AZ Citrus. http://www.azcitrus.com Gmitter, F.G., J. Soneji, M.N. Rao, and S. Huang. Citrus spp., pp. 773-784. Eds. Janick, J. and R.E. Paull. The Encyclopedia of Fruit and Nuts. CABI Publishing, Cambridge, MA. Morton, J. 1987. Grapefruit, pp. 152–158. In: Fruits of warm climates. Julia F. Morton, Miami, FL. https://www.hort.purdue.edu/newcrop/morton/grapefruit.html. Page, M. 2008. Growing Citrus: How to Grow Citrus in Containers, Conservatories, and the Open Garden. Timber Press, Portland, London. Soost, R.K. and M. Roose. 1996. Citrus, pp. 257-324. Eds. J. Janick and J.N. Moore. Fruit Breeding, Volume I Tree and Tropical Fruits. John Wiley & Sons, New York, NY. University of Arizona Cooperative Extension. 1998. Low desert Citrus varieties, publication AZ 1001. http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1001.pdf University of Arizona Cooperative Extension. 2000. Irrigating citrus trees, publication AZ 1151. http://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1151.pdf Useful Tropical Plants. Citrus x paradisi. 2016. http://tropical.theferns.info/viewtropical.php?id=Citrus+x+paradisi

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