Onions are a familiar staple in British kitchens and gardens, yet their growth mysteries can prompt questions for beginners and seasoned gardeners alike. Do onions grow underground, or do they form bulbs higher in the soil? The storage bulb of an onion primarily develops below the soil surface, tucked within the substrate where it can swell as it stores carbohydrates and water for the plant. However, environmental factors, soil depth, and onion variety all influence exactly where and how that bulb forms. This guide unpacks the science behind underground growth, shares practical advice for growing onions successfully in the UK, and answers common questions about the journey from seed or sets to a harvestable bulb.
Do Onions Grow Underground: The Basics
Most garden onions (Allium cepa) form a tunicated bulb that becomes the edible portion we harvest. In many cases, that bulb grows primarily in the soil, protected by layers of dry skins, which makes it well adapted to storage and transport. The exact depth at which the bulb forms depends on several factors, including soil type, moisture, temperature, and day length. When conditions are ideal, the bulb swells underground, while the green tops remain above ground and continue to feed the developing bulb.
Bulb Formation and the Underground Environment
The onion bulb is a specialised storage organ. It develops as the plant diverts photosynthates into enlarging concentric layers, drawing water and sugars from the leaves and stem. A well-drained, fertile soil prevents excessive moisture around the base, which could lead to rot or disease. In practice, you’ll find that a well-cultivated bed allows the bulb to soften, stack its tunics, and store reserves underground while the foliage continues to supply energy earlier in the season.
What Varieties Do When Growth Occurs
Long-day onions, commonly grown in the UK, push to bulb as day length extends in late spring and early summer. Short-day onions, better suited to warmer climates, may bulb earlier with shorter daylight exposure. In the British climate, long-day onions are typically chosen for storage, while some bunched or salad onions (young onions) are harvested earlier with less emphasis on a fully formed underground bulb. So, do onions grow underground? The answer depends on variety and local conditions, but the bulb’s subterranean development is a normal part of most culinary onions’ life cycle.
How Onions Form Their Storage Organs
The process of bulb formation is complex and intimately tied to light, temperature, and soil conditions. Understanding this helps gardeners predict when bulbs will swell and how to maximize yield.
Photoperiod and Sprout Timing
Onions are sensitive to day length. Long-day varieties need longer daylight to trigger bulbing, typically when days lengthen beyond about 14 hours. Short-day varieties respond to shorter days, bulbing sooner under longer periods of darkness. For gardeners in the UK, selecting long-day storage onions aligns with the late spring and summer growing season, enabling bulbs to develop robustly in the longer days of late spring and early summer.
Temperature and Vernalisation
Cool soils at early growth support steady root and leaf development, while gradually warming soils help bulbs swell. Prolonged exposure to cold can slow bulbing, whereas too-warm soil too early may prompt excessive leaf growth, delaying bulb formation. Maintaining a soil temperature range of roughly 10–18°C (50–64°F) during bulb initiation is a practical guideline for many UK gardens.
Soil, Moisture, and Nutrition
Soil structure matters. A well-drained, loamy or sandy loam soil that retains some moisture without becoming waterlogged provides the best conditions for underground bulb formation. Compacted or waterlogged soils can stunt bulbing and encourage rot. Regular, balanced nutrition supports bulb growth; excessive nitrogen in particular tends to promote lush green growth at the expense of bulb development. A well-balanced fertiliser or well-rotted compost applied prior to planting sets the stage for successful underground swelling.
The Role of Light, Temperature, and Soil in Underground Growth
The journey from seed to bulb is guided by environmental cues, and the balance among light, temperature, and soil moisture determines when and how strongly a bulb forms underground.
Light Exposure and Plant Energy
Above-ground foliage requires light to photosynthesise. The energy captured by the leaves stores as carbohydrates within the bulb. If a bed is too crowded or light is insufficient, plants may not direct enough energy into bulbing, resulting in smaller or misshapen bulbs. Adequate spacing and well-timed thinning ensure the energy produced by leaves is allocated toward bulb development.
Soil Depth and Bulb Size
Soil depth influences how much of the bulb remains underground. Shallow soils can encourage bulbs to swell nearer the surface or even push slightly above ground in heavy winds or cool, dry spells. Deep, well-structured soils with good drainage allow bulbs to form securely below the soil surface, where they benefit from stability and moisture regulation.
Moisture Management
Consistent moisture supports bulb growth, yet waterlogged soil is detrimental. Aim for steady moisture during bulbing and a gentle reduction of irrigation as bulbs approach maturity, preventing splitting or fungal issues. Mulching can help moderate moisture and temperature fluctuations and suppress weeds that compete for nutrients.
Growing Onions in the UK: Varieties, Planting Times, and Care Tips
Choosing the right varieties and timing is crucial for successful underground bulb formation in a temperate maritime climate like the UK. Below are practical guidelines to help you plan a productive onion crop, whether you’re growing from seed, sets or transplants.
Long-Day Storage Onions for UK Gardens
In the UK, long-day onions are a reliable choice for producing well-formed edible bulbs. This category includes traditional and modern cultivars bred to respond to longer daylight hours by bulbing in late spring and early summer. Popular examples include Sturon and related strains that have earned a reputation for reliable bulbs, storage potential, and disease resistance. Bedfordshire Champion is another long-standing variety known for early reliability in cooler springs. When selecting seed or sets, look for labels that indicate long-day or storage onions suitable for northern or temperate climates.
Starting Points: Sowing, Sets, and Transplants
- Sowing indoors: Start seeds indoors 6–8 weeks before the final frost date and transplant when seedlings are sturdy. This gives you control over plant health and spacing.
- Using sets or transplants: Sets are small onions planted to grow into mature bulbs. Transplants begin as young seedlings started indoors and moved outdoors after the risk of frost has passed.
- Direct sowing in mild areas: In milder coastal regions or microclimates where frost is less of a concern, direct sowing can be a viable approach to early bulbing, though long-day onions are typically started indoors or from sets in the UK.
Soil Preparation and Bed Management
To optimise underground bulb development, prepare a bed with well-drained soil and a pH around 6.0–7.0. Incorporate plenty of organic matter—compost or well-rotted manure—before planting. Create raised beds if your soil suffers from poor drainage or compaction. Clear crops thoroughly between seasons to reduce disease carryover and consider crop rotation with legumes or brassicas to maintain soil health and reduce pest pressure.
Spacing, Depth, and Supporting Growth
- Space onion plants about 10–15 cm apart in rows 25–30 cm apart for standard storage onions. Wider spacing encourages larger bulbs but reduces overall yield per square metre.
- Plant to a depth that ensures the bulb holds in the soil but keeps the top green shoot above ground for light exposure. When using sets, plant them with the tip just above the soil surface to avoid burying too deeply.
- Keep beds free from weeds, which compete for moisture and nutrients essential for bulb formation.
Common Myths and Realities about Underground Growth of Onions
There are several old wives’ tales about onions and their underground growth. Separating myth from fact helps gardeners avoid unnecessary anxiety and improve results.
Myth: Onions Must Be Grown Completely Underground to Develop
Reality: While the bulb forms underground, it is normal for the top of the plant to thrust above the soil. Bulbing is optimised when the base remains moist but not waterlogged, and when the plant receives adequate light and nutrients. The bulb’s underground location is a natural adaptation that protects it from weathering and helps with storage longevity.
Myth: If the Onion Tops Turn Yellow Too Early, the Bulb Won’t Form
Reality: Yellowing tops can signal approaching maturity, but bulbing can proceed as long as the soil remains supportive and the plant receives adequate sun and nutrients. Timing the harvest around leaf senescence ensures bulbs are mature and store well.
Myth: You Cannot Learn Much From Growing Onions in Small Containers
Reality: Container growing is perfectly viable for onions, especially in urban environments. Choose a deep container to accommodate bulb formation, use quality potting mix with proper drainage, and water consistently. The bulbs may be smaller than those grown in a garden bed, but you can still enjoy fresh, home-grown onions from a compact space.
Pests, Diseases, and How They Affect Underground Growth
Healthy bulbs depend on sound cultural practices. Pests and diseases can compromise underground growth by weakening the plant, stunting the bulb, or increasing the risk of rotting and storage issues. Here’s what to watch for and how to mitigate risks.
Common Pests
- Onion fly (Delia antiqua): Eggs laid at the base of plants hatch into larvae that feed on roots and the base of the bulb. Protect young plants with fine mesh covers and rotate crops to reduce emergence.
- Thrips and onion thrips: These tiny insects can damage foliage, reducing photosynthetic capacity and bulb development.
- Nematodes: Soil-dwelling pests can interfere with root health and bulb formation in some settings.
Key Diseases
- Downy mildew and white rot: Fungal diseases that can weaken bulbs and reduce storage life. Use disease-free planting material, rotate crops, and ensure good air circulation around beds.
- Botrytis (grey mould): Follows poor ventilation and wet conditions. Manage by spacing, pruning, and avoiding overhead irrigation late in the day.
- Leaf blight and mould: Keep foliage dry at watering times and remove infected plant material promptly.
Integrated Management and Prevention
- Practice crop rotation: Avoid planting onions in the same beds year after year; rotate with non-allium crops for several seasons.
- Use clean seed and sets: Source disease-free planting material from reputable suppliers to minimise problems from the outset.
- Maintain thorough bed hygiene: Clear plant debris after harvest to reduce overwintering pests and diseases.
- Encourage natural allies: Plant companion species that deter pests and attract beneficial insects that prey on onion pests.
Harvesting, Curing, and Storage: What to Look For
Knowing when and how to harvest is essential to ensure the onion bulbs, formed underground, store well for winter use and beyond. Harvest timing depends on the variety, sowing method, and local conditions.
Recognising Maturity
The tops of onions typically begin to yellow, bend, and fall over when bulbs are ready. Leaves that are still green can be a sign that bulbs are not yet fully mature. In the UK, late summer and early autumn harvest windows are common for storage onions when grown from sets or seeds started indoors. If you harvest too early, bulbs may be small and store poorly; too late, and bulbs may push soil and suffer from splitting or rot.
Curing for Longevity
After lifting, cure onions in a dry, airy location with good ventilation. Spread them in a single layer or in small loose piles to facilitate drying of the outer skins. Curing typically lasts 2–3 weeks, depending on weather. Once skins are dry and the necks have dried firmly, trim the tops and roots, and store in a cool, dry, dark area with good air movement. Proper curing is essential for long-term storage and flavour retention.
Storage Considerations
Store onions in a well-ventilated environment, ideally at 0–4°C (32–39°F) with around 60–70% humidity for extended shelf life. Keep them away from potatoes, as potato ethylene can hasten onion sprouting. Regularly check stored bulbs and remove any that show signs of spoilage to prevent the spread of rot.
Practical Tips for Successful Underground Bulb Growth
Whether you are starting from seed or using sets, these practical tips help ensure that your onions form strong underground bulbs and are ready for storage when harvest time arrives.
Soil Health
- Work in plenty of organic matter before planting to improve drainage and nutrient availability.
- Aim for a slightly acidic to neutral pH (6.0–7.0) to optimise nutrient uptake.
- Consider mulching to regulate temperature and moisture, especially in exposed sites.
Water Management
- Keep soils evenly moist but not waterlogged during bulb formation.
- Avoid overhead watering late in the day to reduce foliar diseases; use drip irrigation where possible.
- As bulbs mature, ease back on irrigation to encourage proper drying and curing.
Nutrition and Feeding
- Incorporate a balanced fertiliser containing nitrogen, phosphorus, and potassium, following label instructions to avoid over-fertilising.
- Supplement with trace elements if a soil test indicates a deficiency, such as boron or magnesium.
- Top-dress with compost mid-season to support steady growth.
Frequently Asked Questions: Do Onions Grow Underground and Related Queries
Q1: Do Onions Grow Underground or Do They Grow Above Ground?
A1: The bulb forms underground for protection and storage, while the green leaves extend above ground to capture light and energy. The onion’s life cycle is a balance between subterranean storage development and above-ground foliage growth.
Q2: How Deep Should Onion Bulbs Grow Underground?
A2: Bulbs typically form a few centimetres below the soil surface, but depth varies with soil type and cultivar. In shallow soils, bulbs may sit close to the surface, while in deeper, richer soils, bulbs can develop more completely underground. The aim is to have a stable base with adequate moisture and air movement around the bulb while the tops remain above ground.
Q3: Can You Grow Onions in Containers?
A3: Yes. Container growing can be highly productive for onions, particularly in urban settings. Choose a deep container, provide rich potting mix, ensure good drainage, and water consistently. Bulbs may be smaller, but fresh onions can still be harvested from containers with careful management.
Q4: When Should I Harvest Onions?
A4: Harvest timing depends on variety and growing method. For storage onions, harvest when the tops have fallen over and the necks have dried. For fresh, green onions or bunching onions, harvest when the stems are tender. In the UK, late summer to early autumn is a common window for storage onions when grown from sets or seed indoors.
Q5: How Do I Store Onions Safely?
A5: Cure onions in a dry, well-ventilated space, then store in a cool, dark, dry place. Keep bulbs away from damp or warm conditions to prevent sprouting or rot. Rotate stock to ensure older bulbs are used first.
Conclusion: Understanding Underground Growth and Growing Onions in the UK
In short, the answer to the question Do Onions Grow Underground? is yes, for the most part. The onion bulb forms primarily below the soil surface, buoyed by careful management of light, temperature, soil conditions, and moisture. By choosing appropriate long-day storage varieties for the UK climate, starting from reliable seed or sets, and maintaining good soil health and pest management, you can achieve robust underground bulb development, with bulbs that store well and taste excellent. Remember that a well-drained bed, proper spacing, and sensible irrigation are the bedrock of success, enabling the bulb to swell underground while the foliage above ground supports the plant’s energy needs. With patience and good horticultural practice, you can enjoy a plentiful harvest of onions that honour the region’s climate and seasons.