Start from seed: April-July
Optimum soil temperature for germination: 18-27°C
Plant spacing: 10-15 cm
Days to maturity: 92-240 days
Introduction
Onions (Allium cepa) belong to the Alliaceae family, also known as Alliums, which also includes garlic, leeks, and chives. With their distinctive flavour and versatility in cooking, onions are one of the most widely used vegetables in cuisines around the world.
Onions have been cultivated for over 5,000 years, with evidence of their use dating back to ancient Egypt, where they were considered symbols of eternity due to their circular layers. They were also used medicinally in ancient Greece and Rome. Onions were introduced to Australia with European settlement and have since become a staple crop in home gardens and commercial production.
The onion plant forms a bulb at the base of its hollow, tubular leaves. Depending on the variety, the bulbs can vary in colour from white to yellow to red, and in flavour from mild and sweet to sharp and pungent. Onions are nutritionally rich in vitamins C and B6, folate, potassium, and antioxidants, making them not only tasty but beneficial for health.
The strong taste in onions is due to a chemical called cysteine sulphoxide, produced to deter pests. Once the cells are damaged an enzyme, allinase, reacts with cysteine sulphoxide to produced sufenic acid which is responsible for the characteristic onion flavour. Another enzyme called lachrymatory factor synthase converts this acid into a volatile gas that stimulates the production of tears in humans which is why cutting onions can lead to tears.
Site and soil
Onions require a sunny position with at least 6 hours of direct sunlight daily to develop properly. Choose a location that is sheltered from strong winds, which can damage the maturing plants.
Soil preparation is crucial for successful onion growing. The soil should be well-drained and fertile with a pH between 6.0 and 6.5. Loosen and aerate the soil ideally using a broad fork to a depth of at least 15-20 cm. Incorporate well-rotted compost or aged manure and complete organic fertiliser well before planting time. Avoid using fresh manure close to planting, as this can encourage thick necks and bulb rot.
For container growing, use pots at least 30 cm deep with good drainage holes. Fill with premium potting mix enhanced with compost. While container-grown onions won’t produce as large a crop as those in garden beds, they can still yield a satisfactory harvest for home use.
For details on preparing soil and pots, including the use of lime, dolomite, gypsum and fertilizers, please refer to the COGS guide to garden bed preparation.
Planting Seeds and Seedlings
In Canberra, the timing of planting is critical for successful onion crops. Different types of onions respond to varying day length to form bulbs, so selecting the right variety for your planting time is essential. The saying is plant on the shortest day and harvest on the longest!
For planting in April, May and June, choose early or short-day varieties like Early Barletta or Early Flat White. These varieties begin forming bulbs during shorter days and mature in late spring and are generally for immediate use. For planting in June and July select keeping varieties. Either long day varieties such as Creamgold or Pukekohe or intermediate-day varieties such as Hunter River Brown or Early Creamgold, which mature in summer.
Onion seeds can be sown directly into the garden in rows 30-40 cm apart, with seeds scattered thinly and covered with about 6 mm of fine soil. Alternatively, start seeds in cell trays or trays and transplant when seedlings are about 10-15 cm tall, usually 6-8 weeks after sowing. When sowing seed in cell trays you can plant the seed in groups of 4 seeds then plant the seedlings out as a bunch. When transplanting, some people trim both the tops and roots slightly to help establishment. Plant spacing can be 5cm for small onions and 10-15cm apart for larger ones.
Ensure the soil remains consistently moist until germination, which typically occurs within 7-14 days. After thinning or transplanting, apply a light layer of mulch around the plants to suppress weeds and retain moisture, but keep the mulch away from the developing bulbs to prevent rot.
Cultivation
Onions are monocots with relatively shallow root systems. Their tall strappy leaves do not shade out or compete well with weeds. Therefore, regular weeding is essential, especially in the early stages of growth. Be careful when weeding to avoid disturbing the onion roots.
If fertiliser has not been used in bed preparation a light side dressing of complete organic fertiliser can be used about 3-4 weeks after transplanting. Avoid high-nitrogen fertilisers, which can promote excessive leaf growth at the expense of bulb development. A monthly foliar spray with seaweed solution; a mix of worm juice, seaweed and compost tea; or trace elements will help boost growth and resilience to pests and diseases.
Water regularly to maintain consistent soil moisture, particularly during dry periods. However, reduce watering as the bulbs near maturity (when some tops begin to yellow and fall over) to encourage proper curing. Irregular watering can lead to split bulbs and reduced storage life.
In Canberra’s climate, onions typically grow slowly through winter, with bulb formation accelerating in spring as days become longer and warmer. Stress (drying out or heat) can cause onions to skip bulbing and bolt. During very cold periods, a light covering of straw can help protect young plants, but remove this in spring to allow the soil to warm up.
As the bulbs mature and approach harvest time, gently brush soil away from the top third of the bulb to expose it to air and sunlight, which helps in the curing process.
Harvesting
Onions are ready to harvest when necks become soft and 50-80% of the tops are falling over. Check the days to maturity for the variety you have planted to know when to expect this to happen. In Canberra, this is typically from late November to December for autumn-planted crops, and December-March for winter-planted crops.
For storage onions, wait until a significant number of tops have fallen over, then bend over any remaining upright tops. After about 7-10 days, when the necks have dried and withered, the onions are ready to be lifted. Choose a dry, sunny day for harvesting. Gently lift the bulbs with a garden fork, being careful not to bruise them.
After lifting, leave the onions on the ground or place them on racks in a well-ventilated, dry area for 2-3 weeks to cure thoroughly. This drying process is crucial for successful long-term storage. When the outer skins are papery and the necks are completely dry, trim the tops to about 2-3 cm above the bulb and trim the roots.
Store cured onions in a cool, dry, well-ventilated place. They can be hung in mesh bags or arranged in a single layer on slatted shelves. Well-cured brown onions can last 6-8 months under optimal conditions (cool, dry, and dark), while white and red varieties typically have shorter storage lives.
Problems
The best approach is to have a reliable Australian reference book to identify problems. Garden Pests, Disease and Good Bugs by Denis Crawford is recommended.
A general strategy to follow with any problem is to observe the plant carefully before taking action. Identify the problem then carefully consider the course of action. For example, with pest insects, you often can see a predator nearby. If you watch and wait the predator may deal with the problem for you. Beneficial predatory insects which feed on pest insects can be purchased from suppliers such as Bugs for Bugs. If the problem needs further action the above recommended book has suggestions.
Practicing good garden hygiene, crop rotation, and maintaining plant vigour by providing adequate nutrients are key strategies for prevention.
Several pests and diseases can affect onions grown in Canberra:
Onion White Rot: is a serious fungal disease caused by Sclerotium cepivorum. Symptoms are yellowing and wilting of foliage, and white fluffy growth at the base of the bulb with black dots (sclerotia) growing on it. The sclerotia are the resting bodies of the fungus and can persist in the soil for up to 20 years. If found, practice long crop rotation away from all alliums and do not use compost that has had any alliums added to it. The presence of onion debris stimulates the fungal resting bodies to germinate. One prevention strategy is to propagate your own seedlings rather than risk bringing it in on purchased seedlings. If the pathogen is present seedlings can be protected by inoculating with beneficial fungi such as Trichoderma species. Seeds and seedlings can be dusted with beneficial Trichoderma species prior to planting.
Aphids and Thrips: These small sap-sucking insects can transmit celery mosaic virus. They often cluster on the undersides of leaves, causing distortion and stunting. Small populations can be controlled by washing them off with a strong stream of water or using insecticidal soap. Alternatively, neem oil or pyrethrin can be used. Beneficial predators like ladybugs and lacewings should be encouraged and can be purchased for release on the crop. Look for aphid mummies which are evidence that the aphid has been parasitised. Eggs can also be parasitised and will appear bronze in colour. Generally speaking, plants that are high in soluble nitrogen (nitrates and ammonia) are prone to attack by sap sucking insects. Ideally avoid using soluble nitrogen fertilisers and ensure plants have adequate trace elements. This can be achieved by using foliar sprays.
Downy Mildew: A fungal disease appearing as pale green-yellow patches on leaves with a purplish-grey fungal growth in humid, cool conditions. Improve air circulation, avoid overhead watering, and apply copper-based fungicides for control if severe.
Neck Rot: Caused by Botrytis species, this fungus often doesn’t show symptoms until storage, when the neck becomes soft and rotten. Ensure proper curing after harvest and store onions in cool, dry conditions to minimize damage.
Recommended Varieties
Several onion varieties perform well in Canberra’s climate:
Brown/Yellow Onions:
- Creamgold: A popular long-day variety with excellent storage properties. Light brown, globe-shaped bulbs with creamy white flesh. Best suited for planting in late winter.
- Hunter River Brown: An intermediate-day variety suited for autumn to early winter planting. Good keeper with strong flavour.
- Pukekohe: has brown skin with white flesh. It is known for its resistance to bolting and ability to be stored for a long time.
- Australian Brown: A reliable variety with excellent storage life. Brown skin and white flesh with a strong flavour.
White Onions:
- Early Barletta: a small, white, heirloom Italian onion prized for pickling and also suitable for salads and cooking. It’s known for its mild flavour and ability to be harvested early for pickling (around 7 weeks) or left to mature fully (around 16 weeks).
- Early White Globe: Earlier maturing than brown varieties with milder flavour. Good for fresh use but doesn’t store as long.
- White Spanish: A long-day variety with large, globe-shaped bulbs. Sweet and mild flavour, making it excellent for salads.
Red Onions:
- Red Shine: A beautiful red onion suited to Canberra’s climate. Milder than brown onions, making it ideal for fresh consumption in salads.
- Red Brunswick: An older variety that performs well in cooler climates. Medium-sized bulbs with good flavour.
References
- Agriculture Victoria. (2024). Onion storage diseases. https://agriculture.vic.gov.au/biosecurity/plant-diseases/vegetable-diseases/onion-storage-diseases
- Bubel, Nancy. The New Seed-Starters Handbook. Rodale Books, 2018.
- Crawford D. Garden Pests, Disease and Good Bugs. Australia: ABC Books; 2015.
- Dove, Helena. Botany of the Kitchen Garden, Kew Publishing, Royal Botanic Gardens Kew, UK, 2023.
- Fanton, Michel and Jude. The Seed Savers Handbook. The Seed Savers Network, Australia, 1993.
- Yates Australia. (2024). How to Grow Onions. https://www.yates.com.au/how-to-grow/onions/
