Allotment Vegetable Growing
Friday 24 May 2013
Vegetable Growing Advice - Vegetable Guides
Growing Onions from Seed - How to Grow Onions from Seed
Onion - Allium cepa
by John Harrison, author of Vegetable Growing Month by Month
Although it is undoubtedly easier, especially for the new vegetable grower, to grow onions from sets, seed grown onions are not really difficult.
There are three reasons for growing onions from seed rather than sets.
The growth of onions is ruled by day length and when the day starts to shrink they bulb up so it's important to get them started fairly early if you are to get a decent crop. In an ideal world, you will have your onions started off in early March but you can leave it to mid-April and still be successful.
I'd suggest you would be better going with sets after that as they do get going faster than seed grown onions.
Much was made in old gardening books about preparing onion beds. The importance of fine soil and adding soot to darken the soil was emphasised. Of course, the darker soil would absorb sunlight better and warm up faster but since soot is hard to find now few of us have real coal fires and plastic cloches are cheap, cloching to warm the soil is the modern option.
Another trick in the old books was to scatter a couple of ounces of salt per square yard on the bed. I've not tried this myself but I doubt it would do any harm to the soil or the old boys wouldn't have done it.
Sowed as thinly as possible in rows about 9" apart, just under the surface the onions would be thinned out to the final spacing when they germinated and were 4" high. Just as with carrots, this thinning process releases the scent of the onions and like a guided missile, the onion fly's scent based radar would guide it to the crop.
The final spacing will depend on how large an onion you want to grow. Spacing at as little as 2" apart will result in small pickling size bulbs whereas 6" apart allows room for a very decent size of onion.
Starting Onions in Modules
My preferred system for seed grown onions is to sow into 15 per tray modules. The benefits here are that you are immune to bad weather and don't need to thin out.
Starting in early March or even late February sow a couple of seeds per module into a multi-purpose compost and thin to one seedling as soon as possible if both germinate. They don't need to be too warm, between 10 and 15 degrees is fine.
Once germinated, do not let the temperature rise above 15 degrees or the onions will get confused about what time of year it is and bolt later after they're planted out. Move out of the greenhouse into a cold frame and harden off before planting out, spaced as for the sets. The beauty of this method is that the onions can be held if the weather is bad until you are ready to plant.
Onions aren't greedy feeders but they will benefit from a couple of ounces per square yard of a general purpose fertilizer like fish, blood and bone or Growmore raked into the soil before planting out or sowing.
Because high potash levels reduce the sulphur uptake, which is what causes the strong flavour of onions, if you use low potash fertilizer you will get smaller but tastier onions eventually. I use fish, blood and bone as above with a small amount of pelleted chicken manure to add a little nitrogen. The organic fertilizers release more slowly and evenly than chemical fertilizers.
Apart from some weeding, there is little to do. Do be careful not to damage the forming bulbs when hoeing. Better to have a few weeds or get on your knees and weed by hand than hoe out the crop before you start!
Harvesting and Storing Onions
You can pull at any point for immediate use but for onions to store wait until the foliage is starting to bend over of its own accord. You will read in old books about bending the foliage over - do not do it because it causes damage to the neck causing problems in storage.
Just gently lever under the bulbs with a fork to break the roots and lift a few days later. Then dry them out for storage. The ideal is to place them on some sort of rack outdoors where air can flow all around them for a couple of weeks. If the weather is really wet you need to provide some sort of cover, the odd shower will not cause any harm though. Be careful about drying onions in a greenhouse. If it gets really hot then you end up starting to cook them and they won't store.
Once they have dried out, string them up and hang in a cool dark place. Check the base of the bulbs occasionally for rot starting and remove those bulbs to prevent it spreading.
Favourite Onion Varieties (from Seed)
What onions to grow will depend on what your conditions are like and what sort of flavour you are looking for. Our favourites are Ailsa Craig, often described as a show variety although I don't grow for show, and Red Baron, an excellent red onion.
For pickling the accepted onion is Paris Silverskin, a little mild for us, or Brown Pickling SY 300. Personally I prefer to close space Ailsa Craig and have a really flavoursome pickled onion.
You can see a range of varieties in the allotment shop - Onion Seed
Growing Onions & Shallots
The Essential Allotment Guide
All you need to know!