Flowering of the cover crop sunn hemp, Crotalaria juncea L.

Authors: Meagher, Robert - Rob; Nagoshi, Rodney; BROWN, JAMES; FLEISCHER, SHELBY - PENNSYLVANIA STATE UNIVERSITY; WESTBROOK, JOHN; CHASE, CARLENE - UNIVERSITY OF FLORIDA

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2017
Publication Date: 7/1/2017
Citation: Meagher Jr, R.L., Nagoshi, R.N., Brown, J.T., Fleischer, S.J., Westbrook, J.K., Chase, C.A. 2017. Flowering of the cover crop sunn hemp, Crotalaria juncea L. HortScience. 52(7):986-990.
DOI: https://doi.org/10.21273/HORTSCI11981-17

Interpretive Summary: Cover crops are planted before or after a vegetable cash crop to add nutrients and organic matter to the soils, reduce weeds, and suppress populations of plant-parasitic nematodes. The cover crop sunn hemp has been tested to help control populations of fall armyworm, but this plant also produces flowers that attract and provide food for pollinators and other beneficial insects. The older varieties of sunn hemp could only produce flowers late in the season when many bee species were not active. Experiments were conducted by scientists at the USDA, Agricultural Research Service, Center for Medical, Agricultural and Veterinary Entomology, Gainesville, Florida, in cooperation with a horticulturalist from the University of Florida, Gainesville, Florida, to compare flowering and seed production of domestic and foreign sunn hemp lines across different planting densities and planting dates. Our results showed that the domestically-bred cultivar ‘AU Golden’ produced flowers early in the season, and by planting at a lower seed density, promoted more flower production. Collection of bees frequenting the sunn hemp showed that carpenter bees and leafcutter bees were the most common pollinators in north-central Florida. Flowering cover crops such as sunn hemp can assist growers in promoting pollinators and beneficial insects in agricultural landscapes.

Technical Abstract: Sunn hemp, Crotalaria juncea L., is a warm-season legume that is planted before or after a vegetable cash crop to add nutrients and organic matter to the soils, for weed growth prevention, and to suppress nematode populations. Sunn hemp flowers may also provide nectar and pollen for pollinators and enhance biological control by furnishing habitat for natural enemies. Experiments were conducted in north-central Florida to compare flowering and seed production of domestic and foreign sunn hemp lines across different planting densities and planting dates. Our objectives were to test whether low seeding density would result in the production of higher numbers of flowers, and to test whether planting earlier in the season would also result in higher numbers of flowers. Our results showed that the domestically-bred cultivar ‘AU Golden’ can aid in providing substantial numbers of flowers for several species of pollinators. Planting this cultivar earlier in the season and at a lower seed density promoted more branching and flower production. Bees in the genera Xylocopa and Megachile were collected and appeared to be the most common pollinators in north-central Florida. Flowering cover crops such as sunn hemp can assist growers in promoting pollinators and beneficial insects in agricultural landscapes.