Plant cabbage seeds outside into a prepared seedbed or in modules under glass to be transferred outside at a later date. The seeds should be sown thinly, as this reduces the amount of future thinning necessary and potential risk from pests. Sow seeds 1/2in deep and rows should be spaced 6in apart. Once the seeds have germinated, thin the seedlings to 3in between each plant. Cabbage seedlings are ready for transplanting when they're between 2.5-3in high. Water the day before moving, and keep well watered until established. Space the plants 18 inches between rows in their final positions.