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Growing Onions from Sets - How to Grow Onions from Sets

Onion Sets

Onion - Allium cepa

by John Harrison, author of Vegetable Growing Month by Month

Onion sets are easier for the beginner and many experienced growers prefer growing onions from sets because they are less fuss. Sets are faster to get away, more reliable and disease resistant and only slightly more expensive than seed when brought from the garden centre.

However, many varieties are not available as sets, just seed and seed grown onions do seem to store better when harvested.

Sets are just miniature onion bulbs which means they get off to a fast start, having more stored energy than a seed to work with.

Sets are planted fat end down so that the tip is just above soil level in March and April.

Autumn Planted or Japanese Sets

Japanese onion sets or autumn planted go into the ground in September or October although you can get away with being as late as December. This enables them to form a strong seedling that will over-winter and get off to the earliest start in the spring. They usually provide a harvestable crop in June some month or two earlier than spring planted sets.

Spacing Onion Sets

You can plant 6" (15cm) apart each way or spaced at 4" (10cm) in rows 8"-12" (20-30cm) apart. Closer spacing produces smaller bulbs but there is no point going over 6" (15cm) apart unless you are trying to grow giants. Incidentally, large sets are more prone to bolting so do not discard small sets in the pack in favour of them.

Scrape a small hole or a drill to plant in, then backfill the soil. Just pushing the sets into the soil, especially a heavier soil, can damage the root plate and kill the set.

The biggest problem with sets are birds who like to pull them out of the soil. Netting or even cloching until established is advisable.

Another common problem with set grown onions is bolting. This is where the onion suddenly puts up a strong central stalk which carries a seed head. Effectively, cut off the stalk as soon as you see it developing and use those onions first.

Heat treated sets are far more resistant to bolting although you have to pay a premium for them.

Feeding Onions

Sets are a little more tolerant than seed grpown onions of poor nutrient levels but add a general purpose fertiliser like fish, blood & bone or Growmore a week before you plant for best results.

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.

Cultivating Onions

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 Sets)

Onion sets bought mail order or online tend to be expensive due to transport costs but shop bought sets are pretty cheap. Unfortunately the shops usually only offer a choice of a couple or three varieites at most.

I've done well with Red Baron from sets and Sturon although Stuttgarter have also performed well for me. Hercules have a good reputation.

You can see a range of varieties in the allotment shop - Onion Sets

Growing Onions & Shallots