Heat treatment starts in November depending on the size of the bulb. During the heat treatment, the onion sets are gradually heated up over 12 days from the 3-5°C storage temperature to 30-32°C and, depending on size, are kept at this temperature for 30-60 days (during which time they are heated up to 32-35°C for 10 days) and then gradually cooled to 10-14°C for planting in April/May. So whilst they are a little more expensive, you can be more confident of harvesting onions you can be proud of!