How to Grow Bulb Onion Seeds

Grow Guide #2295
Family: Amaryllidaceae
Binomial name: Allium sp.
Life Cycle: biennial (usually grown as an annual)

This 'How to Grow' guide details everything a home gardener needs to know to plant, grow and care for Onions (Allium sp.).

When to Sow Onion Seeds

Onion is a cool season crop. Use the table below to identify the best time of year to sow onion in your climate.



Onion plants are best grown in full sun. Choose a location that will receive at least 6 hours of full sun each day.

Onion plants need a well drained soil enriched with plenty of organic matter. Prepare soil by weeding it thoroughly, digging it over to loosen it and adding aged animal manure or compost. Keep the area free of weeds until planting. Learn more about preparing soil for planting here.

How to Sow Onion Seeds

Onion seeds do not require any treatment (eg soaking, stratification) before sowing.

Onion seeds can be sown directly into the garden OR seedlings can be raised in trays or other containers and transplanted to the garden once established.

Sow Direct

  1. Sow seeds directly in the garden 5mm deep and 5-10cm apart, with rows 25-40cm apart.
  2. Keep soil moist but never wet or dry.
  3. Seeds should germinate in around 7-14 days at a soil temperature of 20-25°C.
  4. Young seedlings will need protection from pests, pets and weather until they are established.

Raise Seedlings

  1. Fill trays, punnets or jiffy pots with a good quality seed-raising mix, or use soil starter pellets.
  2. Sow seeds 5mm deep.
  3. Keep soil moist but never wet or dry.
  4. Seeds should germinate in around 7-14 days at a soil temperature of 20-25°C.
  5. Transplant seedlings to the garden once they have their first true leaves and are large enough to handle (usually 5-10cm tall).
  6. Plant out, spacing plants 5-10cm apart, with rows 25-40cm apart.

Tip: Onion seeds are quite small. Handle them carefully to avoid them blowing away or being washed away. Mix seeds with sand or fine potting mix prior to sowing or use a seed dispenser, damp toothpick or tweezers to help space them evenly. Press lightly into the surface after sowing so that the seeds make good contact with the soil. Take extra care to make sure seeds and seedlings don’t dry out. Read more about sowing small seeds here.

How to Grow Onion

Onion plants may need watering during the growing season. Water when the soil is dry about 5cm below the surface (test this by scratching away a little soil with your finger). Water deeply in the early morning or late afternoon. Avoid watering the leaves of plants to avoid fungal diseases. Learn more about watering here.

If soil was well prepared no extra fertiliser should be necessary. In poor soil or to give your plants an extra boost, application of a balanced fertiliser or one formulated for fruit and vegetables can be beneficial:

  • Apply slow release fertiliser at the recommended rate when transplanting or when seedlings are 5-10cm tall.
  • Apply liquid fertiliser at the recommended rate and frequency while plants are fruiting or flowering.

Optional: To give plants room to grow, thin seedlings when they are large enough to handle. Pull out any weak or small seedlings so plants are spaced about 5-10cm apart.

How to Harvest Onions

Onions should be ready to harvest in approximately 60-230 days.

Onions are ready to harvest when the bulbs are large enough to eat. To harvest onions for storage, wait until the leaves fall over and start to turn brown. Use a fork to gently lift the entire plant from the bed and shake off any excess soil. Cure onions for storage by placing whole plants on wire racks, leaving them in a dry cool place for 2-3 weeks. Cut the dried leaves off 3-4cm above the bulb. Harvested onions can be stored in a cool, dry place. For longer term storage, onions can be frozen or pickled.

Onions can also be harvested as 'spring onions' when the stems begin to swell or the bulbs are small. Use a fork to gently lift the entire plant from the bed and shake off any excess soil. Spring onions can be stored short term in the fridge. 

Common Problems when Growing Onions

Like all plants, onion is susceptible to some pests, diseases and other problems. Below is a list of the most common problems gardeners encounter when growing onion plants:

  • Aphids
    Aphids are small (2-4mm long) sap-sucking insects that congregate on the new shoots or the undersides of leaves. They can cause leaves to wilt or become discoloured, and also excrete honeydew which can attract ants and other insect pests. To manage aphids, remove them by spraying with a garden hose, apply a soap or alcohol spray, or encourage predatory insects to your garden. Read more about aphids here.
  • garlic bulb
    Bulbs not forming can be caused by harvesting too early, inconsistent watering or weather, planting bulbs or cloves that are too small, or too much nitrogen during the growing season. Plant varieties suited to your climate, do not over fertilise, water regularly and harvest after the leaves have started to die down.
  • Rust fungus
    Rust (Puccinia sp.) is a fungal disease that causes brown to orange raised spots or patches to appear on foliage. Fungal spores are spread by wind or water to neighbouring plants, especially in temperatures of 10-20C and when humidity is high. To manage rust, space plants to avoid overcrowding, grow them in the recommended amount of light (eg full sun), do not over fertilise crops, remove dead plants and practice crop rotation. Read more about rust fungus here.
  • Leaves of an onion plant
    Onion thrips (Thrips tabaci) are flying insects less than 1.5mm long with slender pale yellow to light brown bodies. They suck sap from plants, leaving silver patches on the leaves and reducing the productivity and yield of plants. Thrips can live in the inner leaves and leaf folds of alliums, and may also infest bulbs. Wash thrips from affected plants using a garden hose, spread an organic mulch around plants, and encourage predatory mites and lacewings with companion planting.

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