A Guide to Growing Onions from Seed in the UK

Posted by marshalls garden on

Let's dive into a topic that's both essential and rewarding: growing onions from seed in the UK. Onions are a versatile and staple vegetable in many kitchens, and there's nothing quite like the satisfaction of cultivating them from the very beginning. So, grab your gardening gloves and let's embark on a journey to sow and grow onions with success!

When to Sow Onion Seeds in the UK

Timing is crucial when it comes to sowing onion seeds. In the UK, onions can be grown either from seeds or sets (small bulbs). If you're opting for seeds, start your indoor sowings as early as January or February. This early start allows the onions to develop strong roots before transplanting them outdoors.

Follow these steps for sowing onion seeds indoors:

  1. Choose onion varieties that are well-suited for the UK climate. Popular choices include 'Sturon,' 'Red Baron,' and 'Electric.'

  2. Fill seed trays or small pots with a quality seed compost. Moisten the compost before sowing to ensure an even distribution of water.

  3. Sow the onion seeds at a depth of around 1cm (1/2 inch) and space them evenly in rows. Cover the seeds lightly with compost.

  4. Maintain a consistent temperature of around 15-20°C (59-68°F) for germination. Place the trays in a bright location, such as a windowsill or under a grow light.

  5. Once the seedlings have reached a manageable size and the risk of frost has passed (typically around April or May), transplant them into the garden or larger pots.

onion seeds

Growing Onions from Seed Outdoors

If you prefer sowing directly into the garden, wait until late March or early April when the soil has warmed up. Follow these steps for success:

  1. Choose a well-draining soil with added organic matter. Onions thrive in fertile, moisture-retentive soil.

  2. Sow the seeds at a depth of 1cm (1/2 inch) in rows spaced 30cm (12 inches) apart. Thin the seedlings as they grow, leaving adequate space between plants.

  3. Onions love sunlight, so choose a sunny spot in your garden. Water consistently, especially during dry periods, but avoid waterlogged soil.

  4. Provide a balanced fertiliser once the onions are established, usually around six weeks after planting.

red onions

Harvesting Onions

With proper care, your onions will be ready for harvest in late summer or early autumn. Look for signs like yellowing foliage and bending stems, indicating that the bulbs are mature. Lift the onions, dry them in a well-ventilated area, and store in a cool, dry place for long-lasting freshness.


Growing onions from seed in the UK is a rewarding experience that adds a flavourful touch to your homegrown produce. Consider starting your onion-growing journey and relish the joy of cultivating these versatile vegetables from the very beginning. Here's to another year of flourishing gardens and green success!

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