Sowing date affects growth, nitrogen uptake, soil coverage and frost hardiness of winter cover crops

Sowing date strongly affects biomass accumulation, frost susceptibility, and agroecological performance of autumn-winter cover crops. Although early sowing is known to increase frost sensitivity in some species, detailed in-season measurements of growth responses to sowing date remain scarce, especially for crop modelling. In a field experiment in a temperate environment of northern Italy, we measured the growth and nitrogen accumulation of two cover crops, white mustard ( Sinapis alba L.) and black oat ( Avena strigosa Schreb.), sown on three dates from early September to late October over three site-years (2023–2025). Nutrient availability was non-limiting in all three site-years, whereas water limitation occurred in only one site-year. Cover crops sown before 20 September rapidly increased in height, Leaf Area Index, and aboveground biomass, reaching up to 5.1 Mg DM ha⁻¹ by November, with N uptake up to 215 kg N ha⁻¹. These early-sown crops were winter-killed after sub-zero temperatures. By contrast, late-sown crops had lower autumn growth and N accumulation, and remained at earlier stages when frost occurred; consequently, they survived winter and resumed growth in spring, producing more biomass in April than early-sown crops had produced in autumn. The C/N ratio of late-sown crops was higher in spring (17–34) than that of early-sown crops in autumn (9–12), with potential impacts on net N mineralisation. Our innovative results consisting of detailed measurements and modelling of cover crop growth indicate that sowing date is a key factor for balancing cover crop autumn resource capture, winter termination, and spring residue quality.