Why are Onions Heat Treated?

Why are Onions Heat Treated?

Onions are biennial plants and in their natural life-cycle they grow from seed into a plant that forms a dormant bulb in the first year. After being subjected to the cold temperatures in Winter, the bulb begins growing again in Spring, it will then produce a flower spike. When fertilised the flowers produce seeds, and thus it’s life-cycle is complete.

When gardening, we shorten this life-cycle by growing onion seedlings and harvesting the same season or by growing from onion sets (immature bulbs). Growing from sets means the onion is in its second year of growth so they would naturally try to flower and produce seed. To prevent this from happening many sets are heat treated.

What is Heat Treating?

Heat treating is a completely natural process in which the onion sets are gradually heated up and held at temperatures of 25-26°C and 85-95% humidity for up to 3 months. This process kills off the dormant embryonic flower or sprout stalk within the onion, while the onion set survives. The growing energy is then concentrated in the bulb, preventing the onion from forming flower stalks after planting.

Heat treated onions also are quick to establish and grow, so higher yields can be achieved.

Heat treatment is not required for all varieties, because with many varieties, the percentage of flower stalks in second year onion sets is minimal. However, certain varieties, like Santero and Red Baron are very sensitive to flower stalk formation in general. 

Heat treated onions arrive later than other varieties, these onion sets aren’t ready until early March. But once they are ready for shipping out, they are ready for planting.

All our onion sets are certified disease free. Browse the full range here.