Onions are usually grown from small bulbs called sets, planted
outdoors in autumn or spring. These grow quickly and reliably, with minimal
maintenance. They can also be grown from seed, although this takes a little
more time and care.
Onions like a warm, sunny spot in well-drained soil, with watering in dry
spells and regular weeding. Onions can also be grown in large containers.
Harvest the crop in summer or early autumn, then either use straight away or
store them for whenever needed over the following months.....
There many varieties to choose from, producing yellow, white or red onions, some spicy and tangy, others mild and sweet. Varieties mature at different times from early summer to early autumn. Some store particularly well, or are resistant to disease or bolting.
Onions are
usually grown from sets (immature bulbs) – this is the easiest and fastest
way to grow them, and will produce an earlier crop. Plants grown from sets are
also less likely to be affected by disease. However, they are more prone
to bolting (when a flower is
produced instead of a bulb), so choose heat-treated sets to reduce the risk. A
few varieties are available for autumn planting and can be harvested a month or
two ahead of spring-planted sets and most seed-raised plants. Onion sets are
often sold in quite large packs, but if you don’t plant them all, they
can’t be kept for planting the following year.
Seeds are usually cheaper to buy, more widely available, and with a
greater choice of varieties. But the seedlings need to be looked after more
carefully and the seeds don’t usually store well, so germination rates may
decline if you keep the seeds from year to year.
Preparing the Ground
Choose a
sunny planting site, with free-draining soil, and weed thoroughly. Onions have
a limited root system, so improving the soil with lots of organic matter before
planting is invaluable – dig in a bucket of garden compost or well-rotted
manure per square metre/yard. This will add nutrients, improve the soil
structure and hold moisture in the soil. Avoid using fresh manure.
You can also
apply a high potassium general fertiliser, such as Vitax Q4, at a rate of one
handful per square metre/yard.
Sowing
Although
usually grown from sets, onions can be grown from seed, sown either indoors or
outside, usually in mid- to late winter. To ensure a good crop, the resulting
plants must be growing strongly by late spring, as the lengthening days trigger
the formation of bulbs – the more leaves that plants have at this time, the
better the bulb will be. A few overwintering varieties are available as seed,
such as ‘Toughball’, which should be sown indoors in late summer.
Sowing
indoors
Most onion
seeds should be sown in mid- to late winter, although overwintering or Japanese
varieties are occasionally available as seed for sowing in late summer.
Sow into modular trays filled with peat-free seed compost.
Although one plant per module is effective, growing three to four per module
saves space, although the resulting bulbs may be smaller. Sow two seeds per
module for the first option, five or six for the second (then thin out the
seedlings later if necessary). Place the tray in a heated
propagator or on a warm sunny windowsill, at 10–16°C (50–60°F). Seedlings
should appear within a couple of weeks. Remove from the propagator, place in
bright light and keep the compost moist.
Harden off indoor-sown plants in spring (or early autumn for overwintering
onions), before planting into prepared ground. Space plants 5–10cm (2–4in)
apart, in rows 25–30cm (10–12in) apart. Don’t split up multi-seeded modules
into individual plants – just plant them as they are, to form a clump of onion
bulbs.
Onions don’t do well in acid soil (below pH 6.5), so if necessary reduce acidity by adding lime in autumn or winter.
Sowing
outdoors
You can sow
onions outdoors from late winter until mid-spring, once your soil is drying out
and beginning to warm up. Sow seeds 1.3cm (½in) deep, in rows 20cm (8in)
apart. Thin out the seedlings first to 5cm (2in) apart, and later to 10cm
(4in). Closer spacing will result in more bulbs and a larger overall crop, but
smaller individual bulbs.
Planting
Sets are usually planted in spring, from mid-March to mid-April. There are also a few varieties for autumn planting, often sold as overwintering or Japanese onions – these are less sensitive to cold, which would otherwise cause bolting. Autumn planting is not suitable in heavy soils prone to waterlogging, as the sets are likely to rot. But they can be planted on modules indoors instead, then transplanted outdoors in spring, which still gives them a head-start.
Planting outdoors
Plant sets
2cm (¾in) deep in drills or gently push them into loose soil, so the tip is
just showing at the surface. Space them 5–10cm (2–4in) apart, in rows 25–30cm
(10–12in) apart. Firm the soil around them and water well. Birds will often
peck newly planted sets out of the ground, so cover with fleece until they’ve
rooted in.
Another planting option is to cover the ground with black weed-suppressing
membrane, then plant the sets through slits. There is then no need for weeding,
which both saves time and avoids accidental damage to the bulbs when hoeing.
Onions can also be grown in large containers if you don’t have space in the
ground or your soil is too damp. Use peat-free soil-based compost
and a container that’s at least 45cm (18in) wide and deep. Plant the sets
as described above.
Planting indoors
If your soil
is very wet and cold, you can plant onion sets indoors, then transplant outside
once the soil conditions improve. Use modular trays filled with peat-free multi-purpose
compost and plant one set per module. Keep in an unheated greenhouse or on a
sunny windowsill.
Plant CWatering
Water during any prolonged dry spells every 14 days, but
stop watering once the bulbs have swollen in mid-summer. Watering
spring-planted crops after mid-summer can mean they store less successfully.
Try to avoid overhead watering, as this can encourage fungal diseases.
Feeding
In late
winter, give autumn-planted onions a nitrogen-rich fertiliser, such as sulphate of ammonia, at a rate of 35g
(1oz) per square metre/yard. This not only boosts growth but can also
suppress premature flowering. Alternatively, use dry poultry manure. Also give
an occasional feed with a general liquid fertiliser, from spring to mid-summer.
Weeding
Onions don’t
grow well if competing with other plants, so weed between them regularly. Take
care not to damage the bulbs or foliage if using a hoe – ideally, weed by hand.
As onion foliage casts little shade, weeds grow readily and can soon swamp the
crop, which would reduce the plants’ growth and subsequent bulb size.
Removing
flowers
Remove any
flower stems as soon as they start to form, otherwise the plant’s energy will
go into producing the flower rather than swelling the bulb. It also means the
bulb won’t store well.
Harvesting
Autumn-planted
sets are ready to harvest by early to mid-summer, while spring-planted sets and
seed-raised onions are ready from late summer to early autumn.
Although
it’s sometimes suggested to bend over the foliage or gently lift the bulbs to
break the roots, this is no longer recommended. Yellowing and toppling of the
foliage is a sign that the crop is reaching maturity. Harvest before the
foliage dies down completely. Carefully lift the bulbs with a fork, taking care
not to damage or bruise them, as this could cause them to rot in storage. Use
any damaged onions straight away.
Storing
Dry your
onions for about two weeks before storing – place them in full sun
outdoors on wire racks, or similar, so air can circulate beneath
them. Alternatively, if the weather is damp, dry them in a sunny
greenhouse or well-ventilated shed.
Once all the foliage is papery and dry, put the onions in net bags or
trays in a single layer, or tie them into plaits and hang up. Keep them in
a light, cool, dry and well-ventilated place – don’t store in the dark, as this
encourages sprouting.
Autumn-planted onions will store until early winter. Spring-planted or
seed-raised onions can last until well into the following spring.
Problems
Onions are
relatively trouble free, although they can be affected by several fungal
diseases, especially in damp growing conditions or if poorly stored. Flowering
(bolting), caused by low temperatures in spring, means bulbs won’t store well.
Birds also tend to pull up newly planted onion sets, so cover them with fleece
until rooted in.
