What Cover Crop Should I Grow Before Planting Brassicas?

Brassicas — cabbages, kale, broccoli, Brussels sprouts, kohlrabi, turnips, and swede — are among the most nitrogen-demanding vegetables in the kitchen garden. They also suffer from club root, a soil-borne disease that is worsened by anything that brings more Brassicaceae-family plants into the same soil. Choosing the right cover crop for a bed destined for brassicas addresses both requirements simultaneously: high nitrogen from a legume, and no risk of adding to the club-root burden.

The Best Choice: Field Beans

Field beans are the most practical winter cover crop before brassicas. They are winter-hardy, fix nitrogen reliably, and produce bulk material when incorporated. Sow in October or early November — which fits neatly with the fact that brassica beds are typically cleared of their current crop in late September or October. By the time spring brassica transplants are ready to go in, field beans sown in October will have been growing for five to six months, maximising nitrogen fixation.

Winter Tares as a Second Option

Winter tares are also excellent before brassicas and slightly easier to incorporate than field beans due to their softer stems. They are a strong nitrogen fixer and very winter-hardy. Sow in September or October and incorporate in February or March, allowing four weeks before transplanting brassica seedlings.

Red Clover for Longer-Term Cover

If a bed is available from early August onwards and the brassica crop does not go in until May or June, red clover sown in August and incorporated in April provides an entire overwinter nitrogen-fixing period. This longer growing time maximises nitrogen contribution but requires the bed to remain out of use from August through spring — practical for some rotation schemes, not for others.

The Mustard Warning

Do not use mustard as a cover crop before brassicas. White mustard is in the Brassicaceae family and can harbour and potentially spread club root. Even if your garden does not currently have club root, introducing mustard into the brassica rotation slot is an unnecessary risk. This is the most important species-choice rule in cover-crop management.

Phacelia Before Brassicas — Is It Safe?

Yes. Phacelia is unrelated to brassicas and perfectly safe in this rotation position. It does not fix nitrogen, but it is the best rotation-neutral choice if the bed was cleared too late for legumes to establish, or if you want to combine fast weed suppression with modest organic matter improvement before the brassica bed.

Liming and Cover Crops

Brassica beds typically need lime to maintain a pH above 7.0 — this is the most effective club-root control. Apply lime after incorporating the cover crop and before transplanting. Do not apply lime simultaneously with legume incorporation, as alkaline conditions temporarily reduce nitrogen availability from decomposing organic matter.