Onions: When to plant for winter?

Question:

I would like to try planting winter onions this year. I have the ground ready and also the onion sets called Senshyn – I understand these are a good onion to grow. What I would like to know is: when to plant them? Do I leave them until the shortest day, like garlic, or is it OK to plant now in this mild weather? (R Rackham, Pulham Market)

Answer:

Onions and shallots are essential vegetables and one way to extend the onion season is to plant over-wintering or Japanese onions. These have now become very popular and provide large fleshy onions from June onwards.

To get early onions like this they need to be planted this month, so don't wait until the shortest day or you will have problems with your onions, and if they grow at all they will be ready at the same time as regular onions.


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Senshyn is certainly one of the most popular varieties and will crop reliably in our climate.

Choose an open, sunny site and welldrained soil. High humidity around the foliage and wet soils make the crop more prone to disease attack.

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Over-wintering onions are not suitable for heavy soils prone to waterlogging as the crop is more likely to succumb to rots and disease. Onions do not thrive on acid soils with a pH below 6.5. An application of lime will help to raise the pH making the soil less acid this is also a good time to do that.

Onions have a limited root system, therefore improving the soil with plentiful organic matter is invaluable. Apply two bucketfuls of well-rotted manure or other organic matter such as garden compost every square metre. This will help add nutrients, improve the soil structure and hold moisture. Avoid using fresh manure.

Plant overwintering onion sets in late September–October 5-10cm apart leaving 25-30cm between the rows. Plant about 2cm deep in drills. Take care not to merely push the sets into loose earth as this inhibits rooting and can cause them to rot. Makes sure that only the tips of sets are showing above the soil. Cover with fleece to prevent birds uprooting the sets. The fleece can be removed when the onions are established.

Over-wintering onions will ripen and are ready for lifting by early- to mid-summer, although they will not store well and need to be used by early winter. As these onions grow through the winter many of the normal onion problems are not experienced – just ensure they do not become waterlogged over winter.

•This article was first published on October 22, 2011.

•This comment was sent in as a result of the article on Japanese or winter onion sets but would also apply to spring onion sets: 'As an avid, reader of your column I have for the first time come across something with which I totally disagree. It is very true onion sets mysteriously 'unplant' themselves. The local sage was leaning on my fence watching me replant half a dozen which had moved house overnight. 'Ah,' he pronounced, 'them collared doves can't leave nothin' alone'. I pointed out that most of the sets had been 'replanted' upside down an inch or so away from their original position, so were our industrious feathered friends replanting them? The real culprit, nine times out of 10 is worms! I respectfully suggest that covering them with fleece will actually exacerbate the problem.

It will of course deter the odd inquisitive blackbird, but it makes a snug environment for the worms. Please see my book 'Beyond the Potting Shed' page 79. Vest wishes, Paul Rix'

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