White mustard

Sinapis alba

On bare ground, white mustard covers the soil in a few weeks. It's a fast green manure, a crop you grow and then dig into the soil. It smothers weeds and adds organic matter when you dig it in. Sow it from spring to late summer in gaps between crops. It belongs to the cabbage family, so don't grow it where clubroot, a root disease of cabbages, is a problem. When you rotate crops between beds, count it as a cabbage.

Looking after it

Water: It needs little water and rain is usually enough. Water the seedbed only in a dry spell, so the seed comes up evenly. Feeding: Mustard needs no feeding at all. It takes up the nutrients left in the soil and gives them back when you dig it in.

Cutting and digging in

Cut it down before it flowers, usually four to eight weeks after sowing. Chop the stems and let them wilt for a few days. Then dig them into the top layer of soil. Wait two to three weeks before sowing or planting there.

What can go wrong

Clubroot. Because it's related to cabbages, it can carry clubroot and keep the disease in the soil. Don't sow it where clubroot has appeared. Don't sow it just before cabbages, turnips or radishes either.
Going to seed. In warm weather it flowers within weeks and the seed comes up as weeds. Check it often in summer and cut it down at the first flowers.

Growing facts: RHS, UC ANR and seed-company sowing guides. Green manure: RHS. pH: University of Connecticut extension.