Brussels sprouts bloom during their second year. If the plants survive the winter without getting eaten or frozen to death, pick out at least 5 plants, typical of the sort, and allow them to flower the year after. The safest way, however, is to dig up the chosen plants in late autumn and remove the heads and leaves except the topmost ones at the growth-point and heel them in, in a greenhouse/ earth cellar or cover them in the plot.
Set the stems out in early spring with a good space between them. Read more about seed cultivation under "Growing cabbage plants".