Home Garden & Home Grow Your Own How to Grow and Harvest Onions

How to Grow and Harvest Onions

Grow Your Own

If you would like your grow your own efforts to yield food products that you will get great use from, onions are about as good as it gets.

You can find onion seeds and onion sets (small bulbs grown from seed) at Hillier Garden Centres. Both growing methods are reasonably straightforward, although growing from set is easiest. The majority of onion sets can be planted directly into the ground from February to April for a late summer into autumn harvest, but you can also get overwintering onion sets for an early summer crop.

Onion Varieties

Some recommended varieties of onion to try are:

Centurion F1 Onion

This classic onion variety is robust and easy to grow, producing an abundant harvest of golden onions from August to September. Plant from February to April.

Karmen Onion

This variety of red onion has firm white flesh with dark red rings and a sweet taste, making it perfect for salads. The onions will have a slightly flattened globe shape at maturity. It produces a heavy crop and stores well. Plant from March to April.

Red Baron Onion

This easy to grow red onion variety has the RHS Award of Garden Merit, signifying plant excellence. It produces firm, dark-red onions and has a fantastic, strong onion flavour.

Sturon Onion

A popular white onion variety that produces slightly flattened onions with thick brown skin and a rich flavour. Plant in spring for an August-September harvest. These onions will also store very well if needed.

Stuttgarter Onion

This well-loved variety has the benefit of being able to be planted as early as January, for those itching to get going with their produce. This variety of white onion has an excellent, mild flavour.

Growing Onions from Seed

When growing onions from seed, always check the instructions for the specific variety on the packet.

In most cases, onion seeds should be sown indoors from late winter to early spring. Sow seeds into individual pots or into a seed tray filled with seed sowing compost at a depth of 6mm. Place the pots or tray into a propagator, or, if you don’t have one, seal inside a polythene bag at a temperature of 15-20°C until germination (when the seedlings are a few inches tall). Germination typically takes 10-12 days. Once germination has taken place, grow on a temperature of 10-15°C until all risk of frost has passed and plants are large enough to plant outdoors.

At this point, follow the advice given for growing onions from sets (below).

Planting Onion Sets

The best and easiest way to grow onions is to plant directly into the ground in the spring. Onions prefer a sunny spot, as that will help them mature to full size and deter pests and diseases. They should survive in partial shade, but will be smaller. If your soil is very wet, you may want to grow in raised beds.

Prepare your soil a few weeks prior to planting by removing any weeds and digging in plenty of compost.

When planting your onion sets, place them in rows leaving at least 10cm between each bulb and 30cm space between rows to give the onions room to grow. Position in the ground with the neck of the onion facing upwards. Cover with a soil and compost mixture. The top of the onion should be just visible above the surface of the soil. Water well.

Caring for Your Onions

Water your onions regularly, particularly during dry spells.

Onions growing during the heat of summer can ‘bolt’ (i.e. start producing a flower in order to set seed). If a flower bud starts to appear, just snap it off so the plant’s energy continues to go into the development of the bulb not the flower.

When to Harvest Onions

The harvest time for your onions will depend on whether they are planted in spring (which is the majority) or in autumn and overwintered. Onion varieties planted in spring typically are ready in late summer into early autumn. Those planted in autumn will be ready in early summer.

The right time to harvest is a week or two after the leaves begin to turn yellow.

Hillier tip: stopping watering your onions towards the end of summer, when they’ve reached their ultimate size, can speed up the final process of leaves yellowing and toppling over.

Onions ready to be harvested

Onions need to be left to dry for a couple of weeks after pulling them up. This toughens the outer layer and helps them store for much longer. This is done simply by leaving them in a sunny, well-ventilated place — spaced out on the garden table (out of the rain) or in a greenhouse is ideal.