Citrus is a genus of flowering plants of the family Rutaceae, strongly
associated with Florida but actually intentionally introduced and not native to
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.
HABITAT AND DISTRIBUTION
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.
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.
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.
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 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).
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).
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.
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
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.
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.
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.
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.