Is lablab a good cover crop option in a hot, humid climate?

Answer: Lablab (Lablab purpureus) is a vine native to Africa that has been used there and throughout Asia for many years. It is a monotypic genus, meaning that there is only one species within the genera: Lablab purpureus, formerly known as Dolichos lablab. While there is only one species, there are several sub-species, landraces, and cultivars due to local adaptation. The vines can either be annuals or short-lived perennials. The leaves can be eaten raw or cooked, but the seeds must be cooked to destroy cyanogenic glycosides that can cause vomiting, shortness of breath, debilitation and convulsions.
As a cover crop, it is quite effective at smothering weeds and fixing nitrogen. Due to its initial slow growth, weeds should be controlled during its establishment. Once it starts actively growing, it is an aggressive competitor and will crowd out and shade newly emerging weeds. Once cut and dried, Lablab biomass contains around 50 pounds of nitrates per acre.
Some of the climbing types can grow to be 25 feet long, but unsupported in field conditions, they usually attain 40 inches or so in height. Although it’s a cover crop and food source for both humans and animals, it can also be very ornamental. Its flowers are quite showy, and then they are followed by purple seed pods. The seeds are either white or black depending on the variety, and some of the wild types have mottled seeds. The seeds have a peculiar feature: an elongated white hilum. The showy flowers attract pollinators, but the vine is subject to the same complement of insects that attack beans.
Lablab prefers acidic soils of a range from pH 4.5 up to about 7. It doesn’t tolerate flooding very well and can be quite drought tolerant after establishment. It requires a minimum of about 30 inches of rainfall per year. In addition to Rhizobium, Lablab also inoculates successfully with the Vigna group of the Bradyrhizobium. Bradyrhizobium has been shown to enhance yield components of hyacinth beans, such as shoot dry weight and overall yield, without significantly influencing pod size, which means the increased yield was from a higher number of pods produced per plant.
Hyacinth bean also relieves soil compaction more quickly than grasses. Lablab roots increase interconnected pore space in compacted soils three times faster than grasses like sorghum. Lablab also reduced soil ped size, although sorghum and wheat did not. This research suggests that since Lablab increased porosity and reduced ped size in compacted soil, it would also increase the infiltration rate and reduce runoff, which would also serve to hasten the compaction repair process.
Lablab benefits from root associations with vesicular-arbuscular mycorrhiza, especially Glomus mosseae. In field studies, G. mosseae increased dry matter, rhizobial nodulation, and phosphorous uptake. The additional nodulation led to an enhanced uptake of nitrates.
Lablab requires greater amounts of heat units than crops like clover or winter peas. It should be grown in areas suitable for its production, or it will suffer yield depression. A North Carolina study showed that it wasn’t an ideal location for Lablab production. First, they couldn’t get good nodulation, which contributed to poor performance; this stresses the importance of inoculation. Next, the weedy control outperformed the Lablab, and indeed the weeds within the Lablab plot attained more than half of the biomass of the Lablab itself, thus stressing the necessity of early season weeding. Finally, North Carolina may just not get enough heat or UV intensity for the Lablab to thrive. Mucuna was also included in this study, and performed as poorly as Lablab.
Learn more on this topic in the ATTRA publication Cover Crop Options for Hot and Humid Areas. This publication discusses the characteristics of cover crops that are better suited for areas with hot, humid summers, like the southern portions of Texas and Florida and along the Gulf Coast, the Caribbean, Hawaii, and points beyond with similar climatic conditions. It includes a table that will allow you to make the best decision for your situation about which cover crops may suit your individual needs. It also includes a general inoculant guide for legume crops.