The chard is a relative of the beetroot and its seeds are grown in the same way. Chards can often stay in their plots during winter, possibly with some cover. Younger plants sown in July-August cope better with winter than older plants. Eliminate the plants which are untypical of the sort. Keep at least 5 plants to eliminate inbreeding.  If it is uncertain whether the plants will survive the winter in the plot, dig them up in late autumn and heel them in in the cellar.