Sow direct into shallow drills in a well-prepared seedbed in spring, and thin seedlings to 1cm apart. When seedlings are about pencil thickness, transplant to their final growing place, leaving 15cm (6in) between plants.
For the longest white stems, dib a hole for each plant 15cm (6in) deep and drop the seedling in before filling the hole with water. To make the white shanks as long as possible, heap soil up around the stems of the plants as they grow.