COMPARISON OF 32 COVER CROPS IN AN ORGANIC VINEYARD ON THE NORTH COAST OF CALIFORNIA

In a replicated study conducted from 1990 to 1992, cover crops and a c ontrol (resident vegetation) were evaluated in an organic wine-grape v ineyard (cv. 'Chardonnay'), located at a valley floor site in Hopland, Mendocino County, California. The purpose was to assess plant phenolo gy, stature, biomass production, competitiveness with resident vegetat ion, and second-year stand regeneration or persistence in an untilled vineyard. Legumes (Fabaceae) evaluated included nine annual and two pe rennial types of clover (Trifolium spp.), bell bean and three other ty pes of Vicia, two types of cool-season annual medic (Medicago spp.), f ield pea (Pisum sativum ssp. arvense), a mixture of biennial sweetclov ers (Melilotus alba and M. officinalis), broadleaf birdsfoot trefoil ( Lotus corniculatus). There were six types of cool-season annual grasse s (Poaceae) including three cereal grains. Native Californian perennia l grasses included one rhizomatous and five bunch types. The sole repr esentative of Brassicaceae was black mustard (Brassica nigra). Unseede d, resident vegetation plots served as a control. In the first year af ter seeding, all cover crops except purple needlegrass (Nassella pulch ra) established stands. Observations of phenological state suggested p articularly advanced seed maturity by early May for the annual grasses barley, rattail fescue, soft chess, and cereal rye, as well as for bu rr medic and 'Dalkeith' subterranean clover. By contrast, 'Austrian Wi nter' field pea. berseem clover, biennial sweetclovers, and perennial legumes had not begun flowering. Other entries were at various stages of flowering. Height was generally greatest for black mustard followed by annual grasses, perennial grasses, vetches, field pea, and was sim ilar for annual clovers and medics. Eighteen cover-crop regimes produc ed vegetational cover in excess of 90%. Particularly great percentages of vegetational cover were obtained for control (resident vegetation) , common vetch (Vicia sativa), annual ryegrass (Lolium multiflorum), o at (Avena sativa), and berseem clover(Trifolium alexandrinum), Particu larly small percentages of vegetational cover by cover crops occurred (in increasing order) for creeping red fescue (Festuca, rubra cv. 'Mol ate'), broadleaf birdsfoot trefoil, and biennial sweetclovers. Particu larly great biomass estimates were obtained (in decreasing order) for barley (Hordeum vulgare), oat, black mustard, purple vetch (Vicia begh alensis), rye, 'Koala' subterranean clover (Trifolium subterraneum ssp . brachycalycinum) soft chess (Bromus hordeacens), woollypod vetch (Vi cia villosa ssp. dasycarpa), and common vetch (Vicia sativa). Especial ly low biomass production was observed (in increasing order) for creep ing red fescue (Festuca, rubra), biennial sweetclovers, strawberry clo ver, meadow barley, and the mix of ladino and strawberry clovers. Biom ass for resident vegetation in the control plots was significantly gre ater than biomass produced by creeping red fescue. In the second May f ollowing seeding, cover crop stands were inspected visually and regene ration scored qualitatively and subjectively as none, very poor, poor. fair, good, and very good. Percentage of vegetational cover by crops was also assessed visually. Stands scored as having good or very good regeneration were assessed for biomass as in 1991. The following entri es attained good or very good regeneration: soft chess, California bro me, annual ryegrass, both forms of blue wildrye, oat, cereal rye, crim son clover, four varieties of subterranean clover, and both the mixtur e of ladino and strawberry clovers and the sole seeding of strawberry clover. Particularly great biomass values were obtained for crimson cl over and for 'Mt. Barker' and 'Trikkala' subterranean clovers. None of the three vetches attained high percentages of cover in the second ye ar; these are thought to benefit from tillage, which was not used in t his study. Biomass readings were, for the most part, lower than in 199 1. This may have been due to low soil moisture during autumn as a resu lt of a lack of irrigation and rain.