Large numbers of tropical legume species were evaluated for adaptation and potential usefulness as forage plants in Florida, primarily in the 1970s and 1980s. One area of focus for these evaluations was the Spodosol (flatwoods) region, which has been a major cattle production area. Additional evaluations were conducted on the central ridge where citrus crops predominated. Early evaluations were also conducted further north where winter temperatures restricted most of the germplasm evaluated. Soils throughout these areas where pasture legumes were especially needed were infertile, acid, and sandy. At least a few mild winter frosts each year characterize the entire region and greatly limit adaptation of tropical legumes. Seasonally waterlogged soils distinguish the Spodosol portion and further limit the range of species adapted.
On the well-drained, non-waterlogged soils, rhizoma peanut was identified in early evaluations as adapted and was developed largely as a high-value hay crop. Although a tropical plant, cold tolerance is sufficient for this legume to be useful into Alabama and Georgia as well as in northern Florida and west at least into Louisiana. The species is vegetatively propagated for commercial plantings, and the small amount of seed produced is below the soil surface. Thus, this plant does not present an invasion hazard. It has been commercially successful but only on a small scale. On the Spodosols, the genus Aeschynomene has been widely planted for forage with the annual A. americana predominating. Although some species of the genus have potential for weediness, plantings in Florida have more often been characterized by establishment difficulties and lack of persistence. Limited commercial use of the short-lived perennial phasey bean (Macroptilium lathyroides) has been made with seed harvested from naturalized populations on an opportunistic basis. Plantings of this species did not provide sustained pasture stands. The only relatively sustainable pasture species for the Spodosols has been carpon desmodium (Desmodium heterocarpon), which has often been difficult to establish rather than invasive. Leucaena (Leucaena leucocephala) showed promise particularly on well drained soils where both low soil pH and winter frosts were sufficient limitations to prevent commercial adoption. Seasonally wet soils, low pH, and excessive defoliation of seedlings by deer were limiting constraints despite considerable research effort with this species on the Spodosols.
In contrast to results from forage development efforts, leucaena is considered invasive on limestone outcroppings in extreme southern Florida. These sites are naturally hardwood hammock forests where leucaena invades the forest edges [Horvitz et al., 1998]. It has been proposed that this species is among a group of hardseeded, non-native leguminous trees which dominate soil seedbanks and displace native species with similar functional roles through competition among seedlings following disturbance [Horvitz et al., 1998]. Leucaena has been marketed in southern Florida for decades as a landscape plant under the name lead tree and is now on the voluntary "do not sell" list developed by the Florida Nursery Growers and Landscape Association and the Florida Exotic Pest Plant Council [Caton, 2005]. Control is difficult in natural areas where invasion problems are recognized, and available herbicide treatments have not been consistently effective [Langeland and Stocker, 2001].
Phasey bean, mentioned above as non-persistent as a forage plant, can be weedy in some situations and is sometimes considered invasive. It is an early colonizer with considerable seed production potential. In addition to functioning as an early successional plant, phasey bean is highly palatable to grazing livestock. Deer selectively browse immature pods, which limits seed production. In addition to rather sporadic occurrence in localized colonies on Spodosol soils following disturbance, phasey bean can be particularly weedy in disturbed areas of the southern Florida landscape mosaic of agricultural land, residential areas, pine savannas, and hardwood hammock forests. It is not a recognized, persistent threat to natural areas, but it can be a highly visible undesired temporary component of disturbed sites.
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