Using autumn mustard to convert washed-out elements into organic formsSowing mustard in late August creates a window of opportunity to improve soil structure before the first freezes. Procedure for Sowing Autumn Mustard The optimal timing for this operation is 40-50 days before expected first frosts, typically late August or early September. Sowing Distribute seeds evenly across the bed surface and incorporate them into the soil at a shallow depth. Sowing too late prevents the plant from forming a sufficient volume of biomass, which results in minimal soil improvement. During spring digging, an increase in dark organic matter serves as a visible sign of the siderate's effect.