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Species Name:    Citrus spp.
Common Name:        Feral Citrus

 

I.  TAXONOMY

Kingdom Phylum/Division: Class: Order: Family: Genus:
Plantae Magnoliophyta Magnoliopsida Sapindales Rutaceae Citrus



Intentionally introduced non-native citrus, Citrus sinensis (varietal) is widely grown under cultivation in Florida. Photo courtesy UF/IFAS Center for Aquatic and Invasive Plants. Photographer Vic Ramey.

  

C. sinensis can escape cultivation and become a nuisance plant in Florida natural habitats, as in this hammock. Photo courtesy UF/IFAS Center for Aquatic and Invasive Plants. Photographer Ann Murray.

Species Name: 
Citrus spp. L.

Common Name(s):
Feral Citrus, Citrus

Species Description:
Citrus is a genus of flowering plants of the family Rutaceae, strongly associated with Florida but actually intentionally introduced and not native to North America.

Citrus species grow as large shrubs or small trees bearing alternate evergreen leaves, solitary white (usually), typically aromatic flowers, and large, conspicuously colored (usually orange, orange-red, yellow or green), edible and aromatic fruits. The fruit is a hesperidium, a modified berry that is globose, with a tough, leathery external rind and a fleshy interior comprised of several seed-bearing, fluid-filled sections called carpels.


Potentially Misidentified Species:
Several centuries of domestic cultivation, including the standard practice of grafting the buds of favored fruit-producing cultivars to the hardy rootstock of congeners (most often Citrus aurantium, the sour orange), combined with an equally impressive span of time in which various species, hybrids, and grafted stock have escaped cultivation has allowed a bewildering variety of feral Citrus plants to exist in Florida.


II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Widely cultivated throughout the worldwide tropics and subtropics, the original native range of the genus Citrus can be traced to southeast Asia and India. Intentionally introduced to the worldwide tropics and subtropics over a span of more than 1,000 years, Citrus was brought to the Americas by 15th/16th century Spanish explorers.

IRL Distribution:
Various Citrus varieties are grown commercially throughout the IRL region. Particularly noteworthy is the commercial production in the Indian River Citrus District, a narrow strip of land extending more than 300 km from the Daytona Beach area to West Palm Beach that is the exclusive production area for fruit sold as "Indian River" citrus.


III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Young Citrus trees typically begin to bear fruit within 2-5 years (Ferguson 1995). Citrus trees are known to regularly produce fruit for well over 50 years.

Abundance:
Feral commercial Citrus groves occur in all commercial citrus-producing counties in Florida. Groves are abandoned or allowed to go feral at a greater rate in the wake of various economic blows to the industry, including weather-related crop and stock damage and loss to disease.

Reproduction:
Reproduction in the genus Citrus occurs via two routes. Sexual reproduction occurs through pollination and most commercial cultivars are self-pollinated. Self-pollination is facilitated by citrus flowers having both sexes present on the same blossom. Cross-pollination, required only by some cultivars, occurs in tangerines and tangerine hybrids, mandarins. Where cross-pollination is required, honeybees are typically the most important insect pollinators (Sanford 1992).

In addition to sexual reproduction, Citrus spp. exhibits the unusual trait of nucellar embryony, which is the formation of maternal cloanl embryos in addition to the zygotic embryos produced through normal fertilization. This trait is exploited by commercial nurseries to propagate clonal seeds from desirable cultivars (Rieger 2006).

Embryology:
Citrus seeds are contained within the fleshy portion (endocarp) of the fruit which is a leathery-rinded berry called a hesperidium. The occurrence of nucellar embryony (see above) typically leads to polyembryony, or multiple viable embryos within each seed. Polyembryony is conducive to high germination rates that may exceed 100% (Rieger 2006).


IV.  PHYSICAL TOLERANCES

Temperature:
Yelenosky and Guy 1989) reported that Valencia orange trees (Citrus sinensis) allowed to naturally acclimatize to seasonal cold temperatures exhibited a LC-50 (temperature at which 50% of test plants died) of -3.8°C. The authors note that the acclimatization process involved a 20-25% decrease in plant leaf water content and osmotic potential and a doubling of soluble sugar content. Both of these responses appear to be natural mechanisms for decreasing susceptibility of plant tissues to freeze-induced cellular dehydration. The authors also note that plants cold-acclimated at 10°C in growth chambers tolerated freezing to about -6°C.

Hydrology:
Citrus spp. are considered to be only moderately flood tolerant, although the Florida citrus industry has developed flood tolerant rootstock to which fruit-prudicing upper plant portions can be grafted for planting in floodplain areas.


V.  COMMUNITY ECOLOGY

Trophic Mode:
Autotrophic (photosynthetic).

Associated Species:
Citrus canker (Xanthomonas axonopodis citri), citrus greening disease (Liberibacter asiaticus), citrus tristeza virus, citrus leaf miners (Phyllocnistis citrella) fruit flies, and other pests and diseases affecting commercial plants also occur in association with feral Citrus trees and groves.


VI. INVASION INFORMATION

Invasion History:
The first accidental escapes of citrus plants to the wild in Florida date back to their first introduction nearly 500 years ago.

Probably no other New World exotic introduction is as tied to iconic historic figures as is that of Citrus. Columbus is believed to have brought lime (C. aurantifolia, commonly known as Key limes) seeds from the Canaries to Hispaniola (now Haiti and the Dominican Republic) on his second voyage (1493-1496), planting them at La Isabela, the first Spanish settlement in the New World (Seelig 1977). Historic journals indicate that Columbus also brought seeds of sour oranges (C. aurantium), lemons (C. limon), and citrons (C. medica) on this voyage (Seelig 1966).

Early Spanish explorers and settlers are credited with introducing C. aurantium, C. aurantifolia, and other citrus varieties to continental North America. Citrus seeds were first planted on the Florida east coast some time between the arrival of Ponce de Leon in 1513 and the founding of St. Augustine in 1565. By 1763, the fruit of C. aurantium was being exported from St. Augustine to England (Morton 1987). The discovery of wild C. aurantifolia groves along the banks of the Indian River in the 1700s points to the likelihood that various Florida Indian tribes facilitated the spread of citrus within the state (Seelig 1977).

The first post-colonial commercial Florida citrus grove dates to 1823 and was owned by French Count Odet Philippe (USDA/APHIS-FDACS/DPI.2006). Expansion to large-scale production did not begun until the 1870s, however, when Florida growers realized that most or all of the 200 million oranges a year being imported to the U.S. east coast from the Mediterranean and West Indies could be grown in Florida (Seelig 1966). To meet domestic demand, citrus growers in north central Florida extensively reworked their groves, grafting sweet orange ( Citrus sinensis) buds onto most of the existing sour orange (Citrus aurantium) root stock. A major freeze event in 1895-1895 dealt a withering blow to north central Florida citrus growers and most production began to move south with a large amount of planted citrus being abandoned and left to become feral grove land in the process (Seelig 1966).

Abandonment of commercial citrus groves continues to the present time, e.g., when weather or disease events wipe out crops and leave farmers in financial straits. These abandoned groves add to the total amount of feral Citrus in the state.

Potential to Compete With Natives:
Although feral Citrus trees and shrubs are often encountered in a number of natural and altered habitats, the genus is not considered to be invasive and there is little evidence for competition with native vegetation.

Possible Economic Consequences of Invasion:
The key economic threat of feral Citrus groves is that they act as incubators and vectors for a range of serious commercial citrus pests and diseases. Growing fears that abandoned feral groves are becoming breeding grounds for citrus canker and greening bacteria may lead to an overhaul of the Florida citrus tax system which would force the owners of abandoned groves to either clean them up or remove trees to keep them from spreading diseases and pests to nearby commercial trees.


VII.  REFERENCES

Ferguson J.J. 1995. Your Florida dooryard citrus guide - Bearing trees (years 3 to 5+). UF/IFAS document HS 888. Available online.

Morton J. 1987. Fruits of Warm Climates. Florida Flair Books, FL. 505 p.

Rieger M. 2006. Introduction to Fruit Crops. Hawthorn Food Products Press, NY. 462 p.

Sanford M.T. 1992. Pollination of citrus by honey bees. UF/IFAS document RFAA092. Available online.

Seelig R.A. 1966. Fruit and Vegetable Facts and Pointers: Oranges. United Fresh Fruit and Vegetable Association. Washington, DC.

Seelig R.A. 1977. Fruit and Vegetable Facts and Pointers: Limes. United Fresh Fruit and Vegetable Association. Washington, DC.

USDA/APHIS-FDACS/DPI. 2006. Citrus Health Response Plan (CHRP) State of Florida, 2006-2007. 20 p.

Report by:  J. Masterson, Smithsonian Marine Station
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Page last updated: December 1, 2007