SHORT AND LONG DAY ONIONS EXPLAINED
The terms short and long day onions are not often used in the UK although they are referred to occasionally in the USA and Canada.
When onions grow they first concentrate their efforts on producing lots of roots and a good head of foliage. Very little effort initially goes into the production of the onion bulb itself.
The trigger for the onion plant to divert its efforts into producing the onion bulb depend on the average temperature and the daily amount of light it receives.
Short day onion varieties tend to start producing the onion bulb when the daily light level reaches between 12 to 14 hours. Long day onions will only start to produce the onion bulb when the daily light level reaches between 14 to 16 hours.
To complicate matters even further some onion varieties are intermediate. In other words they need 14 hours of daylight before they will start producing the onion bulb.
The importance of this difference in growth behaviour becomes apparent when you realise that the Shetland Islands, north of Scotland, receive about four hours more daylight in the early summer compared to London.
So, in northern parts of the UK the best onion varieties are long day onions. This is because onion plants need to produce a good root system and lots of foliage before producing the onion bulb. If short day onions were grown in the North of the UK they would start to produce onion bulbs too early when the root system and foliage were not fully established.
In southern parts of the UK the best onion varieties are short day onions because the average daily daylight becomes shorter in the summer the further south you go.
If you are wondering does this affect you in your region of the UK then consider that Dumfries in southern Scotland is roughly half way between the northern Shetland Islands and London. So in Dumfries the ideal onion variety would be an intermediate one, in London a short day variety would be best and in the Shetland isles a long day variety should be chosen.
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