Onions may be grown from seeds or setts.
Onion setts are immature bulbs that are grown from seed sown in mid-summer of the previous year.
The partially-grown bulbs are lifted in Autumn and stored in a dormant state through the winter to be replanted the following spring.
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.
Bulb formation from both seeds and sets is determined by the day length, meaning, different onion varieties e.g. summer / winter varieties may require different day lengths to initiate bulbing.
Similarly high temperatures and low humidity can be 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)
A Typical Onion Growing Season
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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)
As soon as they have germinated, normally in about seven to ten 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.
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.
Watering the seedlings largely depends upon the weather, one should allow allow the pots to nearly dry out before watering!
Note: It is advisable to spray seedlings, from the second leaf stage with a fungicide at regular intervals to deter damping off.
Gradually harden off seedlings prior to planting out.
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.
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.
As an alternative to seed sown onions, onion setts can be planted and treated in the similar manner as seed sown onions.
To get off to an earlier start set up your setts in trays / pots ( circa week 11)
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.
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.
Apply a 15-20gms (½ - ¾ oz) per sq metre dressing of Sulphate of Potash to assist the ripening process.
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.
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.
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.
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.
Onions have always been a popular "Hobby Plant" among growers where competition can become quite intense at times.
Cultural instructions and varieties suited to 'Showing' are often quite different from the cultural instructions shown above, in so far as 'Exhibitors' usually sow selected seed, and start much earlier with the aid of 'hotbeds' and agriculural lighting.
This is not to say that by following the instructions given above that a degree of success can be achieved, at say village shows and shows that are not judges to NVS rules (National Vegetable Society)
If you want to grow onions to such a standard the the best approach would be to become of your local Vegetable Society where like minded people are usually only too willing to guide you along, and may even give yo a few of their selected seeds to try.