Urban Food Garden

Growing Onions

Onions are easy to grow and can be stored for up to six months.  This webpage explains how I grow my onions.  

REQUIREMENTS
  • SOIL FULL OF ORGANIC MATTER BUT NOT HEAVILY FERTILISED
  • PREFER NEUTRAL TO SLIGHTLY ALKALI SOILS (Ph 6.5 to 7.5)
  • 10+ DEGREE SOIL TEMPERATURE
    Seedlings will transplant better when the soil is cool.
  • ARE LIGHT FEEDERS
    They do not like too much nitrogen.
  • NEED FULL SUNLIGHT TO GROW WELL
  • LIKE FAIRLY DRY CONDITIONS
    Over watering encourages fungal diseases.
Varieties

There is a big range of onion varieties to choose from with varying keeping qualities and taste.  The main two varieties that I grow are:

  • CREAMGOLD: Light brown, globe-shaped bulbs with a pungent cream flesh.  It is a mid-season maturing onion which is also an excellent keeper.
  • CALIFORNIAN RED: Red skin with a milder flavour than Creamgold.  Its mild flavour makes it suitable as a salad onion, as well as for cooking.  Though it does not keep as well as Creamgold.
wHEN TO PLANT

While onions can be sown directly (typically spring onions are grown this way) the accepted practice for growing onions is to sow the seeds in seedling trays before planting out as seedlings.

Click HERE to see a higher resolution PDF of this Onion planting chart.

GROWING ONION SEEDLINGS

While a variety of  seedling trays and small pots can be used to grow onion seedlings I prefer to grow mine in styrofoam seedling boxes as they allow the seedlings to be held in their seedling state for longer.  Due to their slow growth rate (compared to other seedlings) onions can be planted in seedling trays anything up to four months before they are needed, in fact they need a minimum of six to eight weeks to grow large enough for planting out.  The ideal height for onion seedlings is about 10 to 15 cm high.

Onion seedlings growing in styrofoam seedling boxes.  I usually plant my onion seeds in late April for planting out in the garden in August.

If your seedlings are too large, then cut the leafy tops off them.  This is to reduce the leaf area  that the roots must support.  The roots will be damaged when transplanting the seedlings,  by keeping the leaf area small they will require less nutrients during the crucial reestablishment phase when roots are not functioning well.  

Preparing the bed

Add the following per square Metre, then turn the soil in and rake flat.  Though note that this is not an exact formula, I do vary it depending on the available materials and the condition of the soil.

  • SPADEFUL OF COMPOST
  • 1/2 SPADEFUL OF SHEEP OR COW MANURE
  • HANDFUL OF ROCK DUST
  • HANDFUL BLOOD AND BONE
  • HANDFUL OF DOLOMITE
    I prefer Mudgee dolomite as it is high in Magnesium, which is essential for chlorophyll production.  Dolomite also neutralises acidity in soil.

Ideally the bed should be prepared two to four weeks before planting to allow the manure to break down a bit, but mostly I just do it on the day of planting.  As onions are light feeders there is no need to add more fertiliser after the initial preparation.

Planting distances

The most commonly recommended distance is 10 cm between plants  and 30 cm between rows.  However I have found that planting using these distances produces overly large onions (at least for my tastes).  The closer the seedlings are planted in a row the smaller the mature onions.  The distances between rows can also be reduced, though always leave enough space for a hoe blade. 

The closer the onion seedlings are planted the smaller the mature onions.  These onion seedlings were left in a bunch,  doing this produces small onions that are ideal as pickling onions.

Planting out the bed

There are two ways that I plant onion seedlings, the finger method and the trench method.

THE FINGER METHOD

This method works best when planting in light, friable, soil.

  • MARK OUT THE ROWS THAT YOU WISH TO PLANT
  • PLACE THE BASE OF EACH ONION SEEDLING ON THE ROW LINE AT INTERVALS OF 5 TO 10 CM
    Distance will vary depending on how big you want the onions to grow.
  • USE YOUR INDEX FINGER TO PUSH THE BASE OF THE ONION INTO THE GROUND
    The seedlings should be deep enough so they can stand upright, usually around 3 to 4 cm.
  • WATER IN WITH A DILUTE LIQUID FERTILISER

Illustration of onion seedlings being planted out using the finger method.

THE TRENCH METHOD

This is ideal for heavier soils or when planting lots of onion seedlings.

  • MARK OUT THE ROWS THAT YOU WISH TO PLANT
  • DIG A SHALLOW TRENCH ALONG EACH ROW, PUSHING THE SOIL TO ONE SIDE OF THE TRENCH
    The excavated soil should be right next to the trench to form a continuous slope from the base of the trench to the top of the excavated soil.  The trench should be about 3 to 4 cm deep, depending on the size of the seedlings.
  • STAND THE SEEDLINGS IN THE TRENCH AT REGULAR INTERVALS SO THAT THEY ALL LEAN UP AGAINST THE EXCAVATED SOIL
  • BACKFILL THE TRENCH USING YOUR HANDS SO AS TO PUSH THE SEEDLINGS INTO AN UPRIGHT POSITION
    Note that the seedlings do not have to be completely vertical, they will straighten themselves as they grow.
  • WATER IN WITH A DILUTE LIQUID FERTILISER

Illustration of the trench method of planting onion seedlings.  The process of back filling the trench will push the seedlings into and upright position.

Onion seedlings being planted out using the trench method.

After care.
  • DETER BIRDS FROM SCRATCHING OUT THE SEEDLINGS WHEN YOUNG
  • ONLY WATER IF CONDITIONS ARE DRY
    In a normal season I rarely water my onion crop.
  • PUSH THE TOPS OVER WHEN THE ONIONS ARE FILLING OUT
    This will encourage the bulbs to grow larger.
  • AS ONIONS ARE LIGHT FEEDERS THERE IS NO NEED TO ADD ANY MORE FERTILISER

Push the leaves over when the onions are approaching maturity to encourage the bulbs to fill out.

Pests and diseases

Below are the pests and diseases I have encountered when growing onions, though it is by no means a complete list of pests and diseases that attack onions. 

  • BLACKBIRDS
    Damage seedlings by scratching them out in their search for worms.  See PEST CONTROL – Birds for information on how to protect your onion seedlings from blackbirds.
  • THRIPS AND WHITEFLY
    Relatively minor damage.  Spray with Pyrethrum.
  • EARWIGS
    Only a problem when earwigs are in large numbers.  See PEST CONTROL – Earwigs for details on how to control earwigs.
  • VARIOUS FUNGAL DISEASES
    Usually controlled by keeping the soil fairly dry and using little or no mulch on the bed.  Wet mulch in contact with onion skins is a good conductor for fungal disease.
HARvesting and storing

Onions are ready to harvest when the leaves begin to wilt, usually in 14 to 17 weeks.   Store them in a dry well-ventilated space, either in baskets or strung on sturdy string loops.  

Onions in the process of being strung onto a loop of bailing twine suspended from the rafters in my garage.

Onions strung up on bailing twine.  Onions can be heavy, so it is important that the twine be sturdy enough to hold their weight.

Note that this posting on growing onions is based on my gardening experiences.  It does not cover all aspects of growing onions, especially the types of onions that can be grown or the pests and diseases that attack them.  For a more complete overview of growing onions the reader would be wise to consult other sources as well. 

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