Growing onions comes with a lot of choices, but the first and most important is whether to grow onions from sets, seeds or seedlings.
Growing onions from sets gives you a great head start, but depending on your climate can lead to bolting and early flowering. Similarly, growing onions from seeds is more reliable but comes with more work, and usually requires a little spare space in the house or greenhouse over winter.
Onion seedlings can give you the best of both worlds as they take a lot of work out of the process, and can be particularly useful for growers in cooler climates.
What are onion sets?
Onion sets are young onions, with fully developed root plates and are usually bought in bags with the leaves removed. They are grown from seed started in summer, and the underdeveloped bulbs have already had a half season’s growth.
The result is a small bulb, ready to grow new leaves quickly. Once roots reestablish in the soil and the leaves re-grow, the bulbs begin to grow when triggered by summer day lengths.
What are onion seedlings?
Onion seedlings are pre-grown seeds, started in autumn indoors by commercial growers or nurseries, sold exclusively in garden centres. You can buy seedlings online in early spring, but they are delicate and often suffer during transit.
They are usually more reliable than growing onions from sets as their bulbs haven’t begun to form, as they are still in their first year. Because of this, they are much less likely to bolt and flower before bulbs form.
How to grow onions from seeds, sets and seedlings
There are some simple differences between onion sets, onion seeds and onion seedlings that dictate when you plant them, but with all onions, they are harvested at the roughly same time regardless of how you started them. This is because onion bulb growth is triggered by day length, not heat, or watering, and their size is dictated by the growth of their leaves in spring, rather than summer conditions.
Growing Onions from Sets
Growing onions from sets is the easiest option for most growers, but in hotter climates onion sets are often challenged by bolting as the bulbs are already in their second year and naturally want to flower. Bolting can be slowed down by cutting back the first flush of leaves, which causes new growth early in the season.
Onion sets can be planted outside in autumn and winter so long as temperatures don’t drop below 41°F(5°C). They will continue growing new leaves and new roots throughout winter, giving them a great head start in spring.
Their winter growth will be slow, so when these young onions begin to sprout in spring, cutting back leaves to 20-inches above ground will encourage fresh growth for their new growing season. By doing this, you restart the bulbing process. New leaves will keep growing until early summer, and when temperatures begin to rise, the bulbs will start expanding underground. The healthier the top growth is, the bigger the bulbs you will get.
Growing Onions from Seeds
Growing onion from seed is the most cost-effective way to grow onions and can be carried out as a one-year plan or a two-year plan. By starting seeds in summer, you will get small bulbs that stay in the ground over winter to form plants that are ready to start growing early the following year. This method effectively creates your own onion sets for next year.
Alternatively, starting onion seeds indoors in late autumn grows seedlings (sow in modules, with 2 seeds per module, and thin out the weakest seedling). This gives you healthy onion plants, which are weaker than sets, but ready to start growing in spring, with a reduced risk of bolting.
The versatility of onion seeds makes them a great choice for gardeners on a budget, and if like me you feel a compulsion to have garden jobs in the winter months, then onion seeds are a great way to keep connected to your garden when everything else has finished.
Growing Onions from Seedlings
Growing onions from seedlings is really simple – just dib a hole in the ground, and drop the onion seedling in, with all its roots and soil intact. Make sure the soil is loose enough for the bulb to expand, and any large stones have been removed when you plant them. As they are softer than onion sets, they will deform and can grow around stones (not at all pleasant when you come to eat them).
Seedlings can be bought in garden centres from early spring, and planted outside as soon as the last frost has finished.
Onion seedlings will grow bushy leaves above ground right through spring, which help the roots to develop. Once the leaves reach their full height, they will start feeding the bulbs. If there are thirteen main leaves above ground, this will create a bigger bulb underground. More leaves over ground is a great indication of a healthy bulb in a few months’ time.
In case you’re curious, Adriana from Tiny Garden Habit has a great YouTube video comparing her harvest of onions from sets vs seeds. And while you’re there, you can subscribe if you’d like:
Are onion sets or seeds stronger against pests and diseases?
Onion sets are more disease and pest tolerant than onions grown from seed or seedlings. Onion sets have already been through one growing season so are less likely to struggle in spring if the weather takes a turn for the worst.
The issue that needs weighing up though is that onion sets, being in their second year, are more likely to bolt.
When an onion bolts it ruins the bulb underground and begins putting its energy into seed production. A bolted onion has begun attempting to flower. Rather than a mop of tall laves over ground it will have one thick stem growing bolt upright. That stem envelopes a bud called a scape, which will later form a globe allium flower (depending on the onion variety these can be really beautiful flowers).
If you slice the entire plant down the centre you’ll see a thick water-carrying stem running all the way to the centre of the bulb, ruining its texture. While bolted onions are still edible, they take longer to cook, and can cause mildly upset stomachs.
Growing onion from seed, particularly if sowing in seed trays or plugs in early autumn, will create a strong root system, which will be ready to form bulbs the following summer. Sadly, growing onions from seed means you have weaker plants in spring that are more likely to be attacked by onion pests like onion fly, or taken down by downy mildew or onion white leaves caused by Thrips.
It’s December here now, and we began our onion sets here last month in module trays (one set per seed module). We’ve found that this is usually enough to get the roots started before spring, and means we don’t have to dedicate a full bed to the onions through winter.
While we would like to save money and sow from seed, there are too many onion flies at our allotment, and as I said at the start of the article, you need to make a choice based on your location, circumstance and space, rather than the isolated benefits of each type of onion.
There is no perfect way to grow onions, whether you grow onions from sets, seedlings or seed, but hopefully, this guide helps you make a more informed decision ahead of next year’s crop.