Species Description: Dicerandra immaculata is a rare and endangered species of mint that grows to a height of approximately 50 cm (1.6 feet). Young plants and those growing in open sunlight have an erect growth habit and may form dense mats, while older plants and ones growing in shaded areas are arching or sprawling. Leaves are opposite (but may appear to be whorled) and linear to narrowly oblong in shape, with inrolled margins and rounded tips (FNAI 2006; Nelson 1996). Leaves measure 1.5 - 3 cm (0.6 - 1.2 inches) in length and 2 - 4 mm (0.07 - 0.16 inches) in width. Both the upper and lower surfaces of the leaves are covered with fine dots. Flowers are two-lipped and tubular in shape, measuring 1.5 - 2 cm (0.6 - 0.8 inches) in length. Color ranges from pink to rose or lavender, and there are no spots on the petals, a trait that differs from most other Dicerandra species. Anthers are white, and extend beyond the flower. Stamens are spurred. The inflorescence is 15-25 cm (5.9 - 9.8 inches) long and has overlapping cymes, each bearing 1,3 or 5 flowers (Kral 1982) in leaf axils. Fruit is a pale-colored, rounded nutlet approximately 1mm (0.04 inches) in length.
Potentially Misidentified Species: Dicerandra immaculata is distinguished from the other 4 Dicerandra species that occur in Florida by their flowers, which, as suggested by the scientific name, are immaculate, or lacking in spots.
Regional Occurrence: Dicerandra immaculata is extremely rare and endemic to the Atlantic Coastal Ridge of Florida. It was first described in 1963 (Lakela 1963), and known to occur naturally only in coastal scrub and pine scrub areas in Indian River and St. Lucie Counties, with most known plants in an area of old dune with an elevation of 45 feet. This area is approximately 1/2 mile wide and 3 miles long, lying between Vero Beach and Ft. Pierce (USFWS 1999). An experimental population was introduced into Hobe Sound National Wildlife Refuge in Martin County in 1991 and 1992 (Race 1994).
IRL Distribution: The bulk of the Dicerandra immaculata population lies on remnant Pleistocene dunes between Vero Beach and Ft. Pierce, FL. Until recently, no populations occurred in protected areas (USFWS 1999).
Age, Size, Lifespan: Lakela's mint grows to approximately 50 cm (1.6 feet) in height. Leaves measure 1.5 - 3 cm (0.6 - 1.2 inches) in length and 2 - 4 mm (0.07 - 0.16 inches) in width. Flowers are two-lipped and tubular in shape, measuring 1.5 - 2 cm (0.6 - 0.8 inches) in length.
Abundance: Dicerandra immaculata is extremely rare and endemic to a tiny range that extends over 2 Florida counties (USFWS 1999; Nelson 1996).
Reproduction: New leaves emerge from February through August. Lakela's mint is documented to bloom sporadically throughout the year, with peaks in September - November (FNAI 2006). It only reproduces through seedlings and requires insect pollinators, though which specific species of insects actually pollinate this plant is unknown. However, it produces spurred anthers, which require an insect to trigger them before pollen is dispersed (USFWS 1999; Nelson 1996). Fruiting occurs from October to December, and sporadically through the year (Austin et al 1980).
Dicerandra immaculata apparently has limited seed dispersal. Race (1994) reported that the experimental population, introduced to two sites in Hobe Sound National Wildlife Refuge in 1991 and 1992, had spread no more than 2m (6.6 feet) from the original parent population. As of 1994, 1/3 of the original plants survived at first site, and 1/2 survived at the second. Both sites contained new seedlings. As of 1997, the first site continued to produce new seedlings; plants at the second site were flowering; and most newer plants had become well established (USFWS 1999). Based on monitoring of this population, Race (1994) suggested that plants grew best when planted in the cooler spring months, and when not irrigated, as irrigation promoted the growth of competing species.
Competitors: Dicerandra immaculata competes for space with other scrub species, most likely saw palmetto (Serenoa repens) and other understory plants that colonize open gaps in scrub areas (USFWS 1999).
Habitats: Dicerandra immaculata is found in open scrub, sand pine scrub, and sandhills on remnants of old coastal dunes. Optimum habitats are open "gap" areas in scrub having varying degrees of cover, from bare sands to areas littered with other species growing nearby. Growth is most vigorous in sun or lightly shaded areas, but plants become weak and sprawling as woody species and shrubs such as saw palmetto (Serenoa repens) become established in open areas (Nelson 1996; USFWS 1999).
Specific substrate types are white or yellow sands (St. Lucie, Paola, and Astatula) that are deep and acidic, occurring on high dune-like ridges (USFWS 1999).
Associated Species: The essential oils of Lakela's mint protect it from most herbivory by insects (USFWS 1999). It is subject to mildew that grows on nectary glands. Mildew infection destroy fruits and the viability of seeds before seeds can be dispersed (Austin et al. 1980).
Special Status: Dicerandra immaculata has been Federally listed as an Endangered Species since July 23, 1984. It was originally listed due to its extremely limited range and because of the rate in which its primary habitat along the Atlantic Coastal Ridge was being rapidly developed for commercial and residential uses.
The State of Florida lists Dicerandra immaculata as Endangered. However, under Florida law, endangered plants on privately held land are considered the property of the owner, and are thus subject to removal when land is sold for development. Until recently, the only protected population of this species was experimentally introduced in 1991 and 1992 to two sites in Hobe Sound National Wildlife Refuge in Martin County, an area outside the species' original, limited, range.
The Florida Natural Areas Inventory (FNAI) considers Dicerandra immaculata to be critically imperiled (5 or fewer occurrences, or a total of less than 1000 individuals), both because it is a rare endemic with a limited range, and because coastal development is encroaching on the few, highly fragmented populations in existence.
Management issues: Management efforts are ongoing to monitor and protect existing populations, to purchase environmentally sensitive lands, to enforce current protections, and to learn more about the ecology of Dicerandra immaculata.
The management plan for D. immaculata recommends prescribed burning as an important conservation measure that both rejuvenates existing plants, and also maintains the quality of the limited habitat by removing understory plants in open scrub, thus reducing competition with other species (USFWS 1999). To date, however, the optimum level of prescribed burning has not been established (USFWS 1999).
Continuation of ex situ conservation conservation efforts like those ongoing at Hobe Sound are also recommended in the USFWS (1999) management plan. The plan further recommends conservation of D. immaculata germ plasm by long-term seed stage. Additionally, more rigorous enforcement of protective measures already being undertaken, such as prohibition of offroad vehicle use in Dicerandra areas, and prohibiting cutting or transplanting of existing plants, is also recommended.
St. Lucie County has purchased environmentally sensitive lands where Lakela's mint grows in order to protect existing plants and to reintroduce Lakela's mint to suitable areas. Two such purchases were a 23-acre site of scrubland, and a 296-acre remnant of the Atlantic Coastal Ridge. Both sites were within the 3-mile historical range of Lakela's mint, and border U.S. Highway 1. These sites were surveyed and found to harbor small populations of Dicerandra immaculata. The County has been working with area botanical gardens to monitor existing plants and introduce new plants to these areas.
Austin D.E., C.E. Nauman, and B.E. Tatje. 1980. Final report: endangered and threatened plant species survey in southern Florida and the National Key Deer and Great White Heron National Wildlife Refuges, Monroe County, Florida. U.S. Fish and Wildlife Service; Atlanta, Georgia.
FNAI. 2006. Florida Natural Areas Inventory website. Statewide Tracking List and on-line field guide. Accessed online at www.fnai.org. June 28, 2006.
Kral, R. 1982. Some notes on Dicerandra (Lamiaceae). Sida 9(3):238-262.
Kral, R. 1983. A report on some rare, threatened, or endangered forest-related vascular plants of the South. Athens, GA, U.S. Dept. of Agriculture Forest Service. U.S. Dept. of Agriculture Forest Service Technical Publication R8-TP2, Vol. 1.
Lakela, O. 1963. Dicerandra immaculata Lakela, sp. nov. (Labiatiae). Sida 1(3):184-185.
McCormick, K.D., M. Deyrup, E.S. Menges, S.R. Wallace, J. Meinwald, and T. Eisner. 1993. Relevance of chemistry to conservation of isolated populations: The case of volatile leaf components of Dicerandra mints. Pages 7701-7705 in Proceedings of the National Academy of Science, Volume 90.
Menges, E. S. 1992. Habitat preferences and response to disturbance for Dicerandra frutescens, a Lake Wales Ridge (Florida) endemic plant. Bulletin of the Torrey Botanical Club 119(3): 308-313.
Nelson, G. 1996. The Shrubs and Woody vines of Florida. Pineapple Press, Sarasota, FL.
Robinson, A.F. 1981. Status review of Dicerandra immaculata (Lakela's mint). Unpublished report prepared for U.S. Fish and Wildlife Service; Jacksonville, Florida.
Race, T. 1994. Establishment of a new population of Dicerandra immaculata at the Hobe Sound National Wildlife Refuge in Florida. Unpubl. report prepared for the U.S. Fish and Wildlife Service; Jacksonville, Florida. On file at U.S. Fish and Wildlife Service, South Florida Ecosystem Office, Vero Beach, Florida.
St. Lucie County Environmental Resources Department. 2006. Environmentally Significant Lands Program. Accessed online at www. stlucieco.gov/esl/. June 28, 2006.
U.S. Fish and Wildlife Service [FWS]. 1987. Recovery plan for three Florida mints. U.S. Fish and Wildlife Service; Atlanta, Georgia.
U.S. Fish and Wildlife Service [FWS]. 1999. South Florida multi-species recovery plan. Atlanta, GA. 2172 pp.
Wunderlin, R.P. 1982. Guide to the Vascular Plants of Central Florida. University Presses of Florida, University of Florida, Gainesville. 175 pp.