Lathyrus hirsutus. Hardy Annual. This nitrogen fixer tolerates somewhat heavier soils and can be planted slightly later in the fall than crimson clover. Use a seed inoculant on seed before planting for maximum nitrogen fixation and to kick start the nitrogen fixing process. Sow seeds 2-8cm (1-3″) deep in fairly dense concentration. Sow Winter Field Peas cover crop seeds from mid-February to the end of May, and again late August to the end of September. Winter Field Peas are hardy to Zone 6: -23°C (-10°F). 500g covers approximately 40m2 (438 ft²). This cover crop can be mingled with Winter Wheat to store even more nitrogen and to add even more organic matter to the soil.