Onions Growing Guide

Introduction

Onions are a versatile vegetable, forming the base for soups, sauces, curries, and stews. Growing your own ensures you always have fresh, organic onions on hand. This guide covers everything you need to know about growing onions in your garden, polytunnel, or even for a homegrown start-up.

How long do spring onions take to grow?

Spring onions, part of the allium family alongside garlic, leeks, and scallions, can be harvested just eight weeks after sowing. Red and white onions take longer, but spring onions are perfect for a fast-growing crop and offer an introduction to onion growing on a smaller scale.

How to plant your onions

While seeds are available, onions are generally easier and faster to grow from immature bulbs called sets (seedlings).

Growing onions from sets

Choose a sunny, sheltered site. Plant between March and April for an autumn harvest. Onions thrive in fertile, well-drained soil, enriched with compost. Plant sets gently in rows 25-30cm apart, each set at least 5cm from the next. Raised beds can provide an ideal growing environment.

Sowing onion seeds

For seeds, sow in modules in mid-to-late winter at 10–16°C. Plant 3–4 seedlings per module, about 3cm deep and spaced 5–10cm apart.

How to grow onions

Onions can produce two harvests a year. Sow seeds outdoors from late winter to mid-spring, with proper maintenance, and your crop will be ready by the following season.

Watering and feeding

Water once every two weeks, stopping mid-summer when onions have swollen. Minimal feeding with a nitrogen-rich liquid fertiliser can help, but avoid over-fertilising to ensure proper storage.

Weeding

Onions have shallow roots and do not compete well with weeds. Weed carefully by hand to avoid damaging bulbs. Check regularly, as onions are susceptible to weed competition. Watch for onion-neck rot, especially in wet summers, and harvest in dry conditions.

Protecting your onions

Cover plants to prevent pests and birds. Horticultural fleece can help, and companion-planting with parsley deters onion fly larvae. Growing in a polytunnel, cloche, or greenhouse can increase yield and protect against hazards.

Can you grow onions in a polytunnel?

Yes! Polytunnels improve yields by warming soil, promoting faster germination, and allowing earlier planting. They protect seedlings from pests and harsh weather. Overwintered onion sets, such as Radar and Senshu Yellow, can be planted from mid-September to October for a March–April harvest.

How to harvest your onions

Harvest when foliage turns yellow and leans over, usually late summer or early autumn. Lift bulbs gently and dry on a sheltered rack for two weeks before storing in a cool, dry place or cooking immediately.

Start growing onions in a polytunnel with our help

For advice on equipment and growing your best onion crop yet, contact our team on 01282 811250 or email info@premierpolytunnels.co.uk. With over 35 years of experience, we’re here to answer your questions and help you grow onions successfully in a polytunnel.