Onions may be grown from setts or seeds.

Bulb formation is determined by the day length, meaning, different onion varieties e.g. summer / winter varieties may require different day lengths to initiate bulbing,

High temperatures and low humidity are advantageous during bulbing and curing.

Onions have a shallow root system and compete poorly with weeds and grasses.

Timely shallow hoeing is important, especially when the onions are small.

Some will say that when the bulbs start swelling they don’t require as much nitrogen, meaning weed removal is not as important because they (the weeds) take up any surplus nitrogen in the soil which the onions don't require at this stage.(The choice is yours)

Growing onions from setts is probably the simplest method for the amateur gardener, because the plants establish themselves quicker.

Growing from seed and transplanting the young onion seedlings is a method of growing varieties that produce large attractive onions for slicing and exhibition.


Week 2 - 4: Sow seed in either celltrays / pots / punnets or standard trays filled with evenly firmed soil-less seed compost.

Sow the seed thinly, or singly, over the compost, and cover 5mm (3/16”) deep, with similar compost, water, then cover the tray with a sheet of glass and paper.

Germinate at a minimum temperature of 16°C (60°F)

Select a clean tray


Tray filled with seed compost


Select seed


Sow 1 or 2 seeds per cell


Cover seed with compost


Water and label tray


Place tray in Propagator


Seeds Germinated

As soon as they have germinated, normally in about five to six days, take covers off, and allow plenty light to the seedlings.

Grow on in Greenhouse at this temperature, ventilate as much as possible (depending upon the weather) giving maximum light at all times.

Week 7: As soon as they reach the crook stage, and the second leaf appears, prick out seedlings into 70mm (3”) pots of any proprietary soil-less potting compost.

Grow on, giving them plenty of light, but being careful not to force them with too much heat, and too much water, 8°-10°C (45°-50°F) will do.


Ready for Pricking out


Crook stage


Pricked out


Growing on

Watering depends so much upon the weather, so always allow them to dry out between each watering.

It is advisable to spray seedlings, from the second leaf stage with a fungicide at regular intervals to deter damping off.

Week 14: Gradually harden off seedlings prior to planting out.

Week 15: Before planting out, dress the bed/s with a general fertilizer, applied at 60gms (2 oz) per sq. metre.

Lime should also be added at this time if the ph requires adjusting.

A pH reading of 6.75 to 7.25 is recommended.

Week 19: Set plants 150-200mm (6"-8") apart in rows 500mm (18")

If growing exhibition varieties plant out 400mm (15") apart in rows 500mm (18") apart.

When planting out, ensure the planting hole is sufficiently deep to avoid crowding the root ball.

Firm in the plant around the root ball but not the neck, cover the white neck of the plant with 12mm (1/2”) of soil.

Do not water in, unless it is absolutely necessary, let the plant find its own moisture.

Onion Setts

As an alternative to seed sown onions, onion setts can be planted out now, and treated in the same manner as seed sown onions.

An alternative method in order to get off to an earlier start is to set up your setts in trays / pots a few weeks earlier.


Onion Setts


Set up in cell trays


Growth Commenced

Week 21: Approximately 2-3 weeks after planting out, give the plants a top dressing of a high Nitrogen fertiliser.

Never feed during dry conditions, always ensure that the soil is quite moist, or it is about to rain before feeding.

Week 24: From mid June onwards apply at 2-3 week intervals, Nitrate of Soda at a rate of 15-20gms (½ - ¾ oz) per sq metre.

Week 32: Apply a 15-20gms (½ - ¾ oz) per sq metre dressing of Sulphate of Potash to assist the ripening process.


Various stages of growth

Week 36: When bulbs are nearly* ready, bend the neck over from approx 100-150mm above the bulb and lay horizontally along the ground, again to assist the ripening process and stop any further growth.

The correct timing is, when bending the top over, it should do so freely, if there is a resistance, then leave the bending over till later, rather than risking a check in the bulbs growth.

* Timing of this is dependant upon the season.

Week 39: As onion leaves yellow and wither, lift the bulbs and lay them out on the surface of the bed with the roots facing into the sun to dry off.

Dry off under cover if the conditions become wet.

Remove all loose outer skins and remnants of foliage before storing bulbs.


Onions drying indoors


Onions cleaned/dressed


Alternative methods of storing

Check bulbs for soundness, store unblemished ones in net or woven plastic sacks, or tie them to a rope, or store loose in trays in a dry and airy place under cover.

Store ideally at a minimum winter temperature of about 2°C (35°F)

Use thick necked or slightly damaged bulbs immediately, never be tempted to store them, as this increases the risk of them contracting a fungal infection later.

Week 40: Prepare onion beds for the following season.

Onions prefer soil which retains moisture, therefore the onion bed/s should be well prepared with plenty of well rotted farmyard manure prior to planting out.

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