Turnera diffusa is a widespread and variable species found throughout the tropical Americas. Commonly known as “damiana,” Turnera diffusa has figured extensively in folk medicine and is particularly noted for its aphrodisiacal properties for which there is scientific support.
In 2005, we conducted a plant exploration in Puerto Rico for species with ornamental potential. In the coastal mogotes (limestone hills) of southern Peñuelas Municipality, we encountered a population of T. diffusa. The population was striking for its small, densely hairy gray-green leaves that we perceived as more attractive than typical of the species elsewhere across its broad range. One individual in the population appeared particularly compact.
Cuttings of this individual were collected and placed at a mist propagation facility at the USDA-ARS Tropical Agricultural Research Station (TARS) in Mayaguez. Once rooted, these were shipped to the National Germplasm Repository at the Subtropical Horticulture Research Station (SHRS) in Miami, where we have propagated and evaluated the plant for the past five years. It has proven to be a dependable, problem-free ornamental with great potential as a perennial in warm climates or for annual use in the temperate zone. We have given this cultivar the name ‘Luisa’ in honor of the author Tomás Ayala-Silva’s late mother, who was an enthusiastic admirer of Puerto Rico’s native flora.
‘Luisa’ is a multi-stemmed, densely branched evergreen sub-shrub growing to about 2.5 feet tall. The small evergreen leaves are grayish green and aromatic when crushed. The small (1/2-inch) yellow flowers are borne singly in the axils of the more terminal leaves, and though lasting only a single day, are quickly replaced by new flowers the next day.
‘Luisa’ is readily propagated from softwood cuttings throughout the year. We have found that cuttings from actively growing plants treated with a five-second basal end dip in 1,000 ppm indolebutyric acid in 50 percent EtOH root within four to six weeks when placed under intermittent mist.
We have successfully grown the cultivar in five-parts-aged pine bark: four coconut coir dust, one coarse sand (by volume). Rooted cuttings in cell packs are placed directly into 6-inch containers. The plants grow rapidly and will be saleable in 6-inch containers in six to eight months after rooted cuttings are potted. In smaller containers (4 inch), they are ready in four to six months. Trial blocks were situated in the ground at the USDA-ARS National Germplasm Repository in Miami from plants established in 6-inch containers in October 2005, April 2007 and again in March 2009 in crushed limestone and sand-fill soil similar to urban residential lots in much of South Florida. Soil was amended with three inches of 10-year-old aged compost from vegetative solid waste, incorporated into the top six inches of the site substrate.
In The Landscape
In the landscape, ‘Luisa’ should be situated in full sun and on well-drained soils. It is tolerant of alkalinity to a pH of at least 8.5 and grows equally well in slightly acidic substrates (pH 6.5). After establishment, the plant is drought tolerant, requiring supplementary irrigation only during prolonged periods of no precipitation.
Nutritional requirements appear low. We have fertilized field plantings only once annually with no apparent nutritional deficiencies observed. Once each year the plants can be cut back to half their height. No pest or disease problems have been observed in five years of cultivation history in Miami. Fruit set has never occurred at our location, so the threat of weediness appears minimal in contrast to T. subulata and T. ulmifolia, which have naturalized in many subtropical and tropical areas. The rapid rate of growth, ease of propagation and long flowering season suggest ‘Luisa’ could be marketed as an annual beyond its expected hardiness range, which we estimate to be USDA 9B-11 (USDA, 1990). Flowering occurs for most of the year in South Florida, ceasing production only in the coldest months of winter.
‘Luisa’ can be used as an edging, groundcover, bedding plant or as a container plant. Although the flowers are small, they are produced in great profusion and contrast well with the gray-green foliage, creating a conspicuous presentation in the landscape. The dwarf growth habit and small leaves and flowers are markedly different from the coarse, rangy habit of the large-flowered T. ulmifolia and T. subulata.
Small quantities of ‘Luisa’ are available for research and further evaluation purposes by request through the USDA-ARS National Plant Germplasm System as accession PI 659752.