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Prepared on Monday, 1 September 2025, by Frank van der Linden (Senior agronomist/storage specialist at Omnivent)
At the start of the storage season, we publish our current advice for storing onions. Please note: this is general advice and based on the current conditions in Northwestern Europe. The content has been carefully compiled. However, results may vary depending on the specific conditions of your product and/or shortcomings in storage techniques. If in doubt, please contact us.
Check your storage computer before putting your onions in storage
Before loading, check that your storage computer is still functioning properly. It’s best to leave your Omnivent storage computer on at all times, but older models like the ACT-20 can sometimes display a timeout alarm after extended periods of inactivity.
In that case, go to section 00 ACT20 Terminal, page 2 Installation, line 15 Automatic clear timeout alarm, set value to on.
Using outside air for cooling and drying
It should be possible to cool and dry with outside air at night, and depending on the product temperature, also during the day. The minimum dew point difference can be set at -1°C, the minimum difference between outside and product at -1°C. The lowest dew point difference that is still drying is -0.5°C.
In recent days, the relative humidity (RH) and dew point outside have been such that drying opportunities are sometimes limited. Consider using a heater to lower the RH, allowing you to dry faster and longer.
An additional advantage is that the product temperature does not become too low, which means it is possible to dry longer with outside air.
If you don’t have mechanical cooling, try to lower the product temperature slowly so you don’t reach the final temperature too early. This should not be reached until early December.
Dry your onions as quickly as possible.
For onions, it’s important to dry them as quickly as possible. Any remaining moisture should be removed as quickly as possible. Damp soil and rotten tubers/bulbs should also be dried as quickly as possible to ensure long and trouble-free storage. Drying with outside air should be done with 100% ventilation capacity.
If it’s not possible to ventilate with outside air, ventilate internally. Keep in mind that this will only have a limited drying effect. If possible, choose a ventilation capacity of 60%.
What are the harvesting conditions for storing onions?
With an expected above-average harvest, the quality of seed onions will have a relatively large impact on market prospects in the coming marketing season. In sunny weather, watch out for sunburn.
Harvesting
Harvest the onions when 50% of the foliage has died back and cut the neck to 10 cm to reduce the risk of Neck rot (Botrytis) and still allow for quick drying. Only leave the onions in the field for drying if no precipitation is expected and there is no dew overnight. Be careful of sunburn in bright sunlight. Do not harvest when temperatures exceed 28 degrees Celsius.
Drying costs
The sunny weather may tempt growers to dry their onions in the field. However, this is discouraged because, in practice, the drying effect is very limited, while the risk of sun damage and fungal infections increases significantly.
Heating and drying at 30°C requires the same amount of energy as a treatment at 20°C. This is because a temperature of 30°C only needs to be maintained for three days, while at 20°C this process takes three weeks.
Head rot fungus
If you’ve detected Neck rot in your onion batch, you need to take special measures. Neck rot grows fastest between 22-25°C. This temperature range must be overcome quickly. This can be achieved by first drying at an inlet temperature of up to 22°C. After a few days, you can then continue drying at a temperature of 30°C.
Settings per storage phase for onions
| Loading Manual control | Adhering moisture must be removed as quickly as possible by means of ventilation. Try to bring the product indoors as much as possible on 1 day to prevent temperature variations between batches. Immediately start ventilating with fresh air and don’t turn off the fan. |
| Warm up | Heat to 20 or 30°C until the product reaches the desired temperature. Set the final product temperature to 20 or 30°C and the maximum duct temperature to 22 or 32°C. Heating will be faster if the onions are already slightly dried. This can be done with external air, but it will be faster with additional heaters. Limit heating to a temperature increase of 2°C/day. Then continue with “Drying” if the onions are not completely dry yet, see below. |
| Drying/wound healing (ACT) or Intensive drying (OmniCuro) | At 20°C, keep the temperature constant at 20°C for about 3 to 4 weeks. At 30°C, keep the temperature constant at 30°C for about 3 to 4 days. At 30°C this can easily be done with the Warm up program. The onions are dry when the RH of the onions is 65-70% at 20°C. |
| After drying (no desired RH can be set here, but this is possible with Intensive drying) | The drying period is 4-5 days, with a 2 hour pause between applications and an active period of 2 hours. Continue until the product is thoroughly dry. |
| Cooling/storage | Once the product is properly dry (the neck does not roll between your fingers), you can begin to slowly cool it down. Lower the temperature by 0.5°C/day. If only outside air is available for cooling, maintain a final temperature of 6°C. Try not to reach this before mid-December. With mechanical cooling, cooling can be achieved down to 1°C, or even 0°C. Use the RV control to set the RV. Use the table below to determine the correct RV. |
Relationship between storage temperature and relative humidity
| Temperature oC | % RH |
|---|---|
| 25-30 | 65 |
| 20-25 | 65-70 |
| 10-20 | 70-75 |
| 5-10 | 75-80 |
| 0-5 | 80-85 |
Are you unsure about the conditions for storing your onions?
Don’t wait too long to ask for advice! Our storage specialist Frank van der Linden will be happy to help you. He can visit you to inspect your products and storage techniques and give you storage advice for your onions. You can receive a personal offer for a local visit (across the globe).
Please feel free to contact us by email at fvdl@omnivent.nl or call +31 36 – 521 8062.
Do you have any questions about your storage?
You may quickly find your answer at the FAQ.
Technical problem/malfunction
Are you experiencing a technical malfunction? That’s inconvenient! Fortunately, we are available by telephone 24/7 to assist you.
Cooling malfunction Tel. +31 (0)6 5342 1436
Other technical malfunction: Tel. +31 (0)6 5514 6609
Help with setting up your storage computer
Our regional managers or dealers are happy to assist you, including with questions about cooling and other technology in your storage facility:
United Kingdom
Ronnie Laing
T: +44 749 006 7458
E: rl@omnivent.com
Other regions
Contact your local representative, you find the contact details for each region on our contact page.
