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Onions - - Key Growing Information

Onions (Allium cepa) are a staple in kitchens worldwide, known for their versatility and robust flavor. Growing your own onions can be incredibly rewarding, and with the right techniques, you can produce larger and tastier onions. This guide will explore various methods to achieve optimal onion growth, the impact of spacing and watering, and recommendations for different onion varieties. We’ll also cover post-harvest storage and provide a comparison between growing onions from seeds versus sets.

Quick Jump:

Crop Culture

Onion sets should be planted as soon as possible—ideally within one week of receiving them. If you do not plan to plant immediately, store them in a cool, dry, and dark place with good ventilation. Ensure the onions are flat if you have ordered more than one set, as storing onion sets on top of each other can cause disease to form.

Onions require fertile, well-drained soil with a pH of 6.2-6.8. Sandy loam soils are ideal, so raised beds or raised rows are recommended for heavier soils to promote soil drainage. Full sun is required for optimal growth. Onions need regular watering during the growing season, especially when temperatures start to increase at the end of spring and the start of summer months.

Planting Methods: Bunching vs. Non-Bunching

How you plant your onions can significantly affect the size of your harvest.

  • Bunching: Planting onions closely together (bunching) can result in smaller bulbs due to competition for nutrients and space. This method is often used for green onions or scallions.
  • Non-Bunching: Spacing onions further apart (non-bunching) allows each plant to have ample room to grow, resulting in larger bulbs. This method is ideal for producing sizable cooking onions.

Spacing and It's Impact on Onions

Proper spacing is crucial for maximizing onion size.

  • Close Spacing (2-3 inches apart): This spacing is suitable for smaller onions or green onions. While it saves space, it limits the size of the bulbs.
  • Wider Spacing (4-6 inches apart): Provides each onion with enough room to expand, leading to larger bulbs. This is the recommended spacing for most onion varieties intended for storage and cooking.

Watering Techniques

Watering is another critical factor in onion development.

  • Consistent Watering: Onions need regular watering to thrive. The soil should be kept moist but not waterlogged. Consistent watering promotes uniform growth.
  • Deep Watering: Deep watering encourages the roots to grow deeper into the soil, supporting larger bulbs. Water thoroughly and less frequently to achieve this.
What is Deep Watering?

Deep watering involves applying water slowly and thoroughly so that it penetrates deep into the soil. Here’s a more detailed look at how to practice deep watering:

  • Frequency: Instead of watering a little bit every day, water your onions less frequently but more thoroughly. This encourages roots to grow deeper into the soil as they search for moisture, resulting in stronger, more resilient plants.
  • Technique: Water the soil until it is moist to a depth of at least 6 inches. You can check the depth by inserting a spade or a moisture meter into the soil. If the top few inches of soil dry out between watering sessions, that’s okay, as long as the deeper soil remains moist.
  • Timing: Early morning is the best time to water, as this allows any excess moisture on the leaves to evaporate during the day, reducing the risk of fungal diseases.
  • Tools: Use soaker hoses or drip irrigation systems to deliver water directly to the base of the plants, minimizing evaporation and ensuring that water reaches deep into the soil. If using a watering can or hose, apply water slowly to give it time to seep in rather than run off.

Growing Onions from Seed vs. Sets

Onions can be grown from seeds or sets, each with its own advantages and disadvantages.

Growing Onions from Seed
  • Cost: Seeds are generally less expensive than sets.
  • Variety: A wider variety of onions is available as seeds.
  • Growing Time: Growing onions from seeds takes longer, typically 100-120 days from sowing to harvest.
  • Establishment: Onions grown from seeds can be more adaptable and develop stronger root systems.

Growing Onions from Sets

  • Cost: Sets are more expensive than seeds.
  • Ease of Use: Sets are easier to plant and establish quickly.
  • Growing Time: Onions from sets mature faster, usually within 70-90 days.
  • Size: Sets can produce larger onions more quickly but may have a limited variety compared to seeds.

Planting Guide

  • Planting Depth: Plant onions approximately 1 inch deep with the base of the bulb (the rounded root-end portion of the bulb) facing down.
  • Spacing: Space onions 4 inches apart in rows 12-18 inches apart. For green or fresh onions, space plants closer in-row and harvest every other onion as needed.
  • Timing: For spring plantings, plant as soon as the soil can be worked. For fall plantings, plant around the same time you would garlic, before the ground freezes.
  • Low Tunnel Growing: For fall plantings, use a low tunnel covered with row cover and greenhouse plastic to protect from winter lows. Vent the tunnel in spring to avoid excessive heat.

Recommended Onion Varieties

Choosing the right variety is essential for a successful harvest. Here are some recommended varieties and their characteristics:

Variety Type Sowing Time Storage Life Characteristics
Sturon Yellow Onion Early spring Up to 8 months Reliable, good storage, mild flavor
Red Baron Red Onion Early spring Up to 6 months Sweet flavor, good storage, vibrant color
Ailsa Craig Yellow Onion Late winter to early spring Short-term (2-3 months) Large, mild, and sweet, best for fresh use
Centurion Yellow Onion Early spring Up to 10 months Good storage, firm texture, pungent flavor
Hyred Red Onion Early spring Up to 6 months Crisp texture, sweet flavor, good storage
White Lisbon White Onion Spring, late summer Short-term (2-3 months) Mild flavor, good for fresh use

Grow Guides

Full growing information for onions can be read on the onion grow guide.

Diseases, Insects, and Weeds

Adequate air circulation helps reduce the risk of foliar disease, as do crop rotation and good sanitation—remove all culls and crop debris each season. Thrips are the most common allium crop pest, but certain varieties, such as 'Forum', have shown resistance.

Reduce thrips' overwintering habitat by keeping fields free of weeds and crop debris. Combat serious thrips infestations with insecticides such as spinosad, pyrethrin, or insecticidal soap. Overhead irrigation can help knock thrips off leaves, though it may increase the risk of foliar disease.

Controlling weeds is also critical to the success of your onion crop. Competition from weeds reduces yields and encourages insect pest populations.

Cultivate carefully to avoid damaging the base of the plants and shallow root systems. An application of organic mulch helps control weeds and preserve moisture.


During the growing process, hill the soil around the base of the onion plants 2-3 times. This technique helps produce longer stems and larger bulbs by encouraging the plant to focus on upward growth.

Harvesting and Drying

  • For Green Onions: Harvest once small bulbs have developed.
  • For Full-Size Onions: Wait until necks become soft and tops begin to fall over. At this point, discontinue watering. When approximately 50% of the tops have fallen over, pull the onions out of the ground and let them dry in the sun for 2-7 days. When foliage and necks are completely dry, clip roots and cut back tops to 1 inch.
  • Curing: Cure onions by drying them in a warm, well-ventilated area for 2-3 weeks. This process helps toughen their skins and improve storage life.
  • Storage: Once dried, trim the tops and roots and store onions in a cool, dry, and dark place.

Packet Size
Variety Packet Size Sowing Time Price
Red Baron 200 Seeds per packet Spring & Winter Sows £0.99
Paris Silver Skin 800 Seeds per packet Spring Sowing £0.99
Globo Onion 200 Seeds per packet Spring & Winter Sowing £0.99
Senshyu Onion 300 Seeds per packet Winter Sowing £0.99
Ailsa Craig 300 Seeds per packet Spring Sowing £0.99

Variety Packet Size Delivery Time Price
Troy F1 Onion Set 250g set. Approx 75 Bulbs Delivery Oct - Nov £3.99
Snowball Onion Set 250g set. Approx 75 Bulbs Delivery late Nov - Dec £3.49
Red Winter Onion Set 250g set. Approx 75 Bulbs Delivery Late Nov - Dec £3.49
Red Karmen Onion Set 250g set. Approx 75 Bulbs Delivery from FEB 2024 £3.49
Senshyu Onion Set 250g set. Approx 75 Bulbs Delivery Oct - Dec £3.49
Sturon (Globe type) Onion Set 250g set. Approx 75 Bulbs Delivery from FEB 2024 £3.49

Onion Storage Comparison

Onion Type Storage Life Best Use
Yellow Onions Up to 10 months General cooking, long-term storage
Red Onions Up to 6 months Fresh use, salads, grilling
White Onions 2-3 months Fresh use, mild flavor

By following these guidelines and choosing the right varieties, you can enjoy a plentiful harvest of large, flavorful onions that store well and enhance your cooking.