Summer green manuring in organic greenhouses under French Mediterranean climate

Green manures are recognized to enhance soil fertility, weeds control and ensure a better crop rotation. In the French Mediterranean area, green manures can generally be implemented during summer, after spring crops or short cash crops like melon, zucchini or cucumber, but the species choice must be adapted to the high temperature conditions and short available periods under greenhouse in summer. Several trials have been conducted during summers in 2001 to 2002 and 2013 to 2015 under greenhouses. During the first period, we have screened a large panel of families and species to evaluate their performances in terms of biomass production, weed competition and sensitivity to pests and diseases. The second period was dedicated to legume green manures to enhance nitrogen availability for the crops. Performances of legumes were assessed and their effect on the subsequent crop of lettuce was measured. Our results showed that plants from the Poacea family, like sorghum-sudan grass species and pearl millet are usually the most efficient to reduce weeds and to produce a high biomass (8-12 t DM ha^-1) in less than 2 months. Brassicacea species, like mustards or fodder radish also produce high biomass but are the most sensitive to pests and diseases. Buckwheat is a very good green manure for short periods with a cycle of 30 to 40 days. Most of the species from the Fabaceae family are usually not adapted to hot temperatures under greenhouse. Tropical species of Vigna sinensis and Dolichos lablab gave the best results, and field pea was the only temperate specie to resist particular conditions of greenhouses in summer. However, higher biomass production and weed competition were obtained in mixtures of legumes and graminaceous species. First results on lettuce as the subsequent crops did not always show better results after green manuring of legumes, but more trials are needed to validate the interest for legumes in organic greenhouses during summer.