Size Matters: Sunn Hemp Cultivar Choice Influences Companion Species Biomass Production in a Cover Crop Mixture.

As part of a project with the long-term goal of promoting the expansion of organic strawberry production in the southeastern US, we began an evaluation of alternatives to sunn hemp (Crotalaria juncea) in order to increase the diversity of cover crop species available to organic strawberry producers. In response to stakeholder evaluation of our research, we expanded our study to include a 4-way mixture of sunn hemp, hairy indigo (Indigofera hirsuta), shortflower rattlebox (C. breviflora), and American jointvetch (Aeschynomene americana) in the proportions 2:2:1:1 by seed weight. Although comprising only 33% by seed weight of the mix, sunn hemp accounted for 80% of the shoot dry biomass. In a second study, our objective was to determine the optimal proportions for the components of a 4-species cover crop mixture that would provide a diverse, weed-suppressive canopy. Shortflower rattlebox was replaced by slenderleaf rattlebox (C. ochroleuca) so that all component species were sting nematode-resistant. Four mixtures were established with seed mixes containing sunn hemp at 12%, 17%, 20%, and 25% by seed weight. Sunn hemp resulted in 46% to 73% and 80% to 90% shoot biomass in the resulting mixtures in Citra and Gainesville, respectively. Because ‘Tropic Sun’ sunn hemp used in these three trials appears to be too competitive, in summer 2016, we utilized a shorter stature sunn hemp cultivar, AU Golden. The seed mix contained sunn hemp, slenderleaf rattlebox, hairy indigo, and American jointvetch in the proportions 2:2:1:1 by seed weight. By 8 weeks after planting, the mixture consisted of 49% sunn hemp, 21% slenderleaf rattlebox, 25% hairy indigo, and 5% American jointvetch. The results thus far support the premise that a smaller stature sunn hemp cultivar is necessary when used with shorter and/or slower-growing companion species in order to achieve a mixture in which all species are well represented.