Two studies were conducted to evaluate the effects of perennial and annual cover crops (CC) on weed control and growth of organic citrus. Treatments in the first experiment (Exp 1) included perennial peanut (PP) planted during either spring or summer and were contrasted with use of annual CC. Over-seeding perennial peanut with crimson clover during the winter did not affect PP growth in at 6 month but reduced it by 58% at 28 months. Use of cover crop/weed biomass ratio (CCW-index) facilitated the assessment of the effectiveness of CC to reduce weed growth. CCW values for PP were 0.06, 0.14, and 0.4 for 2002, 2003, and 2004, respectively (very poor to poor weed control). Weed biomass with sunn hemp was 0.3 Mt/ha in 2002 (CCW=32.1, outstanding weed control however since it is tall it may shade young trees) compared to 2.0 with use of Cowpea (CCW=0.4) in 2004. In the second experiment, seven summer CC (cowpea, velvet bean, sunnhemp, lab-lab bean, hairy indigo, alyce clover, and pigeonpea) and seven winter CC (crimson and subterranean clover, lupine, radish, black oats/lupin mix, and a rye/vetch mix.) were tested. Dry weights (Mt/ha) for summer CC were: cowpea 7.9, pigeon pea 7.6, hairy indigo 7.6, sunn hemp 5.3, alyce clover 2.0, velvet bean 1.3, and lab lab bean 0.8. Respective values for CCW-indices were 11.1, 3.0, 4.0, 1.9, 6.7, 0.3, and 0.2. For winter CC biomass production (Mt/ha) was best for radish 6.5, crimson clover 3.4, oats/lupine mix 3.2, and rye/vetch mix 2.2. Respective values for CCW-indices were 6.8, 1.5, 0.9, and 0.9. Poor/inconsistent performance of most winter CC may limit their effectiveness in controlling weeds. Use of 2-4 way mixes is currently being evaluated and this approach appears to be promising.
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