Lars Lund

24 Feb 2023 14:05

Growing onions

Onions are easy to plant and don’t require a lot of space.

Should you sow your onions or buy them as small onions, also called onion sets?

There is no doubt that the easiest way is to buy onion sets and plant them out when the frost is gone. If onions are exposed to frost, their biological clock will be confused and the onion will believe it has been summer, followed by winter and therefore set seeds. The premature seeding is also called bolting. So, make sure your onion supplier is aware of the risk and your onion sets have not been exposed to frost.

You are safer against frost damage if you sow the onions from seeds. However, onions only grow optimally when they come outside in the garden and if they come out late, because you try to avoid frost, they often don’t have enough time to grow big.

No matter which option you choose, I recommend sowing red onions yourself and only buy them as onion sets, if you are very busy. My experience tells me, that red onions can be a little tricky, if you buy them as onion sets. For some reason, they have a higher tendency to bolt when they are planted as onions sets. You have been warned 😉

How to sow onions

If you want to grow your onions from seeds, here is how you do it. The method is the same, no matter if you are choosing red onions or regular onions.

  • Sow the seeds in little pots with room for maximum five seeds a pot and start inside in February. If you choose to sow in your greenhouse, start in late March.
  • The temperature is best at 15 to 20°C, however after the first sprouts have emerged, reduce the temperature to around 15°C.
  • Place your pots in a bright spot, and plant them outside in May, but never in the same place as the onions were planted last year. There should be at least two years between planting onions in the same soil, otherwise you are guaranteed problems. Your onions will easier attract mould and especially onion flies, which can ruin your onions.
Plant care

Watch out for weed and remove any weed as soon as it shows up. Weed creates moisture, which can be the cause of mould. Onions need a lot of water without overwatering, so remember to water in dry periods. Grass clippings are great for holding an appropriate amount of moisture in the soil. The need for fertilizer is very small and can be handled with grass clippings and a little organic fertilizer, that has been worked in the ground before planting.

Onion sets

Onions sets are onions that already have been growing. Buy them and check them thoroughly before planting. They should not be soft or have any signs of it or have any mould. Put them out in late April and cover them with a fibre cloth if there are any signs of frost. Place them in a sunny spot and deep in the soil, so the top sticks out. The distance must be approx. 4 inches between each onion and 1 ft between each row.


The harvest

The onions are ready to be harvested in late august. If the top starts hanging, it is a sign that it’s time to harvest. Pull them up on a sunny day and leave them to dry immediately. The next day, you can hang them up in clusters in your garage or another dry and airy spot.

The onion fly

The onion fly is a small grey fly, that often lays its eggs in April. The fly lays between five to twenty eggs next to the onion plants. The eggs are hatched after a week and straight away the larvae gnaw into the onion. Keep the soil loose around the onion and the fly won’t lay eggs. Alternatively, place some grass clippings between the rows. Planting tomatoes or carrots near the onions might confuse the fly. Placing insect netting over your onions may also prevent the flies from getting to your onions.


Onion mould comes from moisture around the onion. Therefore, the onions must be in a sunny spot and not to close. Already when you buy the onion sets, make sure to check if the onions are healthy, as mould easily gets in.