Turnera diffusa ‘Luisa’

Turnera diffusa is a widespread and variable species found throughout tropical America. Commonly known as “damiana,” it has figured extensively in folk medicine and is noted in particular for its aphrodisiacal properties for which there is actually scientific support.

In 2005, the authors conducted a plant exploration in Puerto Rico for species with ornamental potential. In the coastal mogotes (limestone hills) of southern Pe<0x00F1>uelas 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 on 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 proved 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 second author’s late mother, who was an enthusiastic admirer of Puerto Rico’s native flora.

Turnera diffusa ‘Luisa’ is a multistemmed, densely branched evergreen subshrub growing to about 30 inches tall with a spread of about 12 to 18 inches. The small, evergreen leaves are grayish green and aromatic when crushed. Half-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.

It 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 a medium of five parts aged pine bark; four parts coconut coir dust; and one part 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 these 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), amended with 3 inches of 10-year-old aged compost from vegetative solid waste, incorporated into the top 6 inches of the site substrate.

In the landscape, T. diffusa ‘Luisa’ should be situated in full sun 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 one-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; thus the threat of potential weediness appears minimal, in contrast to T. subulata and T. ulmifolia, which have naturalized in many subtropical and tropical areas.

Name: Turnera diffusa ‘Luisa’

Common Name: Luisa damiana

Hardiness: 9b to 11; potential as an annual in colder zones

Mature height: About 30 inches

Mature spread: About 12 to 18 inches

Classification: Evergreen subshrub

Landscape use: Edging, ground cover, bedding or as a container plant

Ornamental characteristics: Small, yellow flowers last one day but are quickly replaced; small, grayish green, evergreen leaves are aromatic when crushed.

The rapid rate of growth, ease of propagation and long flowering season suggest that T. diffusa ‘Luisa’ could be marketed as an annual beyond its expected hardiness range, which we estimate to be zones 9b to 11. Flowering occurs for most of the year in South Florida, ceasing production only in the coldest months of winter (normally January and February). Turnera diffusa ‘Luisa’ can be used as edging, ground cover, bedding 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 T. diffusa ‘Luisa’ are available for research and further evaluation purposes by request through the USDA-ARS National Plant Germplasm System (www.ars-grin.gov/npgs/) as accession PI 659752.

Alan W. Meerow and Tom<0x00E1>s Ayala-Silva

USDA-ARS-SHRS, National Germplasm Repository Miami, Fla.

Brian M. Irish, USDA-ARS-TARS, National Germplasm Repository, Mayaguez, Puerto Rico. Alan.Meerow@ars.usda.gov