The Vegetable Garden

Heat Treated Onions

Heat Treated Onions
The heat treating process takes several months, but the process means they establish and grow very rapidly, giving higher yields - and with less risk of prematurely running to seed or 'bolting'.

The process is totally natural and not usually completed until late March/early April. At this point your sets will be dispatched ready for immediate planting.

During the heat treatment, the onion sets start the process by being gradually heated up over a number of days from their 3-5°C storage temperature to 30-32°C and, depending on size, are kept at this temperature for 30-60 days. During this time they are heated up to 32-35°C for 10 days and then gradually cooled to 10-14°C ready for planting in April/May.

All of our bulbs are certified as disease and virus free.

The Heat Treating Process

The onion is a biennial plant, growing from a seed into a plant and then into a dormant bulb within the first year. The following spring, following a period of being exposed to cold, bulb growth resumes to soon produce a flower spike. It is these fertilised seeds in which completes the lifecycle. However, in the cultivation of the second year onion sets, when considering heat treating onions there is maximum focus on the development of the bulb itself prior to the lifecycle being completed. During bulb growth, flower stalks may form. This, when wanting to harvest bulbs for consumption is undesirable, since it leads to less yield. In addition, it adversely affects the storage-life of the onion.

This is why we recommend to remove any stems that go to seed, they won’t store and are best used in the kitchen asap… If there is an early, prolonged heat spell during growth this can occur… they are perfectly edible but they won’t store well.

Our specialist growers use a natural process of heat and humidity, killing the sprout stalk of the onion sets. Because of this the growing power is concentrated in the bulb, preventing the onion from forming flower stalks after planting the onion. Every variety is subject to different requirements with regards to the duration of the treatment to achieve the desired effect.

Preparation is not required for all varieties, because with many varieties, the percentage of flower stalks in second year onion sets is minimal. However, white and red varieties are very sensitive to flower stalk formation in general. Therefore, we recommend the prepared variants for growers who wish to benefit from the reduced risk of bolting and the shorter growing season involved, meaning the ground and beds can be utilised for other crops.

The Heat Treating Process

  • Shorter growing season compared to seeds, sets and seedlings
  • Extends the growth period of the bulb
  • Less prone to bolting
  • Good to store
  • Quicker to get going that starting from seed
  • Less prone to pests and diseases

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