How to grow large onions

If you like large onions, this information is especially for you.


Although large onions are often grown and used on the show bench they are also a useful addition to the vegetable plot. They might grow into large specimens but have a sweet mild flavour, ideal for slicing in a salad or adding as whole onion rings to a freshly cooked burger.


Large onions require a long growing season to achieve a good size so are one of the first crops to be sown under glass, which usually scares on Christmas Day or Boxing Day but can be delayed until mid-January and still get good results. Sow seeds into seed trays filled with a damp seed-sowing compost. Them solid in using a level wooden tamper to gently compress the compost. This leaves a small rim making a space to sow the seeds. Before sawing the seed the trap should be watered with tap water using a fine rose in your watering can and left to drain for about half an hour. Place the seed trays in a heated propagator at 18-20’C 165-68‘F). Germination will start between seven to 14 days.

cream onion


Once the seedlings start to germinate gradually acclimatise them to the greenhouse temperature they’ll be growing in, which at this time of the year is the absolute minimum of 10-13’C (50-55F). The best way of acclimatising seedlings is to remove the top of the once the seedlings have germinated. This way the seedlings will get used to having the cooler air temperature from the greenhouse, but also benefit from the warmth from the bottom of the propagator. Leave the seedlings two or three days like this then move them to the greenhouse benching for a day before pricking them out (transplanting them) into trays.

The best time to start pricking out is when the seedlings are just past the loop stage. If you leave them until the seedling is standing upright- the root can often be too long, and you may damage this when pricking out which will then cause a check to the seedlings’ growth. It is not uncommon to find that some of the seedlings still have the seed case attached to the top of the seedling. Do not remove this as you may damage the seedling – it is believed than the seedling is still taking some goodness from the seed which is why it is still attached. This seed coat will naturally fall off once the seedling has finished with it.


Prick out into cell trays filled with a good multi-purpose compost. Water the cell trays before the seedlings are pricked out into them as this will help the seedlings get away better, and there is less chance of disturbance which can occur when pricking out into dry compost and watering afterwards.

Grow the seedling on in the greenhouse throughout the winter and early spring, maintaining a minimum temperature of 10-13C (50-55F).

From mid-March or early April, depending on the weather, move the young onion plants into the cold frame to gradually harden them off before planting. By early May the plants are ready for planting into the vegetable plot and are usually about the thickness of a pencil by now.

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