Growing onions in a greenhouse
Why grow onions under glass?
Protected growth, not an artificial summer
A good onion is built layer by layer long before its bulb appears. First, the plant needs enough healthy leaves before it reaches the day length that suits its variety.
What glass changes
A greenhouse moderates difficult weather: driving rain, saturated soil, compacted seedbeds and abrupt changes. Seedlings are easier to raise, prepared beds stay workable, and harvested bulbs can dry under cover. Watering, ventilation and hygiene can also be checked daily, which matters for shallow-rooted onions that dislike drought stress and warm stagnant moisture.
What glass does not change
Bulb formation mainly follows photoperiod. A long-day variety still waits for sufficiently long days under glass, so it may not perform as intended in Malta. Short-day varieties are generally better aligned with Mediterranean seasonal light, provided the breeder recommends them for the site and planting period. Extra lighting is usually unnecessary for home growing.
Using the greenhouse well
For most home gardens, the greenhouse is most useful in the cooler part of the season. Sow suitable varieties in late autumn or winter, or plant vigorous seedlings and healthy sets when the bed is workable. The crop can remain in a ventilated side bed until harvest. When tomatoes or cucumbers fill the house later, plan air, light and separate watering zones carefully.
A thoughtfully arranged Classic aluminium greenhouse works particularly well for this routine: propagation trays begin in bright shelter, while low accessible beds provide room for upright foliage, ground-level watering and a clean working path.
Starting material
Seed, sets or seedlings?
All three methods can produce onions. Each needs a different timetable, offers a different choice of varieties and makes its own demands on greenhouse space.
For choice and control
Seed offers the broadest choice of day-length response, colour, shape, storage potential and maturity time. It is ideal when you want a variety recommended for Malta or comparable Mediterranean conditions. Sow into clean, fine, free-draining compost. Module trays or small soil blocks help maintain an even crop and limit early weed competition.
Seed needs planning, yet it can give an even, healthy crop under cover before the main garden season.
For speed and an easy start
Sets are small onions produced in a previous season. They are simple to plant and can give an earlier harvest. Buy only firm, dry, healthy sets. Soft, spotted or mouldy sets can introduce disease to the bed and belong neither in the planting row nor in storage.
Choose sets when a quick, straightforward crop matters more than the widest variety choice.
For an even early crop
Bought or home-raised seedlings shorten the time spent in the final bed. They combine deliberate variety choice with a prompt start and make good use of valuable greenhouse space. Select fresh, upright plants with compact healthy rootballs. Discard plants that are very soft, rotting or visibly stressed.
For household quantities intended for storage, seedlings combine precise spacing with a dependable head start.
Bloomcabin recommendation. Start from seed when variety, keeping quality or colour is important. Seedlings suit a planned, even planting. Healthy sets are often the most relaxed option in a first greenhouse season, but they are not a universal substitute for selecting the right day-length variety.
Light sets the rhythm
Choose onions by day length
Day length is the quiet architect of bulb formation. The greenhouse protects the plant, but the variety's genetic response determines when its fleshy leaf bases begin to form a visible bulb.
Propagation and planting time
Build leaves first, then give the bulb room
Sow onion seed shallowly in clean, fine, well-drained propagation compost. Keep it evenly moist until emergence, never waterlogged. Then give seedlings as much light as possible and keep them relatively cool. A greenhouse bench is suitable when nights are protected and ventilation is reliable on sunny days; otherwise begin in a bright indoor position.
After emergence, give seedlings enough root room, singly or in small groups. High temperatures with weak light produce long unstable leaves and remove their compact advantage. Before planting out, the rootball should be moist but not dripping. Acclimatise plants gradually and avoid unnecessary damage to leaf bases and roots.
Plant the final greenhouse bed well before the variety starts bulbing. The bed must be workable and drain water safely. Early planting can add valuable growing time, but it also requires early ventilation on bright days and attention to occasional cold nights.
On compact plots, a Classic aluminium greenhouse can provide an orderly propagation area for a manageable onion crop. Use stable shelves for sufficiently deep trays, then move sturdy plants into a prepared soil bed instead of forcing storage onions to remain in undersized containers.
Anatomy beneath the skin
An onion bulb is not a root
An onion is made of swollen leaf bases arranged above a short basal plate. Fine roots emerge only beneath this plate. This explains why deep planting, heavy earthing-up and compacted permanently wet soil can prevent clean bulb development.
Plant seedlings at approximately the depth they had in their propagation container. The white transition to the leaves should sit only lightly in the soil. Plant sets very shallowly, leaving the tip visible or barely covered. As the bulb swells, its shoulder should develop at the surface and dry quickly after watering. Every healthy green leaf contributes to later storage capacity.
Prepare loose, level, reliably drained soil. Mature compost from an earlier application is more valuable than fresh nitrogen-rich fertiliser. Remove hard clods and compaction without disturbing the structure unnecessarily deeply. For repeated greenhouse vegetable growing, a soil test is a better basis than routine extra feeding.
The bed as a workspace
Spacing, water, nutrients and air
A row spacing of about 25 cm is a useful starting point for bulb onions. Within the row, 7 to 10 cm suits medium storage onions. For larger bulbs or a more humid greenhouse, 10 to 12 cm may be sensible. Dense planting does not automatically improve stored crops: it dries more slowly and is harder to inspect. A real path is not wasted space; it protects shallow roots from compaction.
Onion roots explore mainly the upper soil layer and react badly to a cycle of complete dryness and waterlogging. Water thoroughly at soil level during establishment, leaf growth and bulb development. Check moisture 2 to 3 cm down rather than judging the dry surface. Drip irrigation is especially suitable under glass because it keeps foliage dry. Reduce watering gradually as bulbs ripen and necks soften naturally.
When onions do not cooperate
Read the cause before correcting it
Small bulbs, healthy foliage
Bulbing may have started before plants were large enough, or the crop began too late. Check the day-length type and planting date first. Late fertiliser cannot correct an unsuitable photoperiod.
Thick necks, poor storage
Too much late nitrogen, generous watering during ripening, close spacing or premature harvest can leave neck tissue slow to dry. Reduce water and nutrients in time, then cure the crop in moving air.
Yellow, wilting seedlings
Onion fly larvae can cause tunnels and damage around the leaf base; affected plants may pull out easily. Remove suspicious plants and inspect the crop. Open doors and roof vents are not a complete barrier to flying pests.
Spots, collapse or grey mould
Fungal diseases benefit from wet foliage, close planting and stagnant air. Increase spacing if needed, ventilate consistently and water only the soil. Remove heavily affected plants. Do not grow onions and other Allium crops on the same area year after year.
A greenhouse is not automatically disease-free. Its advantage is close observation: condensation, crowded plants and fading leaves become visible early. Roof vents, generous doors and a thoughtful internal layout are therefore practical features of the Classic aluminium greenhouse.
Neck fall, harvest and curing
Let onions ripen to a papery finish
Storage quality starts before harvest. Wait until a large proportion of necks soften, bend naturally and begin to dry. Do not artificially bend or twist healthy green foliage. Forced collapse is not the same as natural maturity and may create wounds that complicate clean curing.
Harvest on a dry day if possible. Lift onions carefully with a garden fork, remove loose soil and avoid damaging the skin or neck. Onions intended for storage are not washed.
Spread onions in a single layer where possible on mesh or slatted racks. Do not stack the harvest in a damp greenhouse corner or use closed plastic crates. An empty greenhouse can be a curing space if it is very well ventilated day and night and the onions are protected from intense sun.
Mild onions are usually eaten first. Named storage varieties generally keep longer, provided their necks and outer skins are completely dry and the bulbs are uninjured.
After curing
Only inspected onions enter storage
Sort thoroughly after curing. Trim dry foliage to a few centimetres if needed and leave protective outer skins intact. Store firm onions cool, dark, dry and well ventilated, for example in nets, shallow open crates or on slatted racks. Check supplies regularly through autumn and winter. Remove soft, sprouting or damaged bulbs at once and use them first.
Keep storage onions away from damp, highly variable conditions and do not store them directly beside potatoes. A greenhouse is often useful for curing, but it is not automatically a suitable winter store. Dry air, cool conditions and frost protection matter. A suitable cellar, sheltered garage or well-ventilated utility space may be more dependable.
Best candidates Firm bulbs with dry skins, closed necks and no pressure marks.
Use first Thick-necked, cut, soft, split or incompletely cured onions.
Avoid completely Storing whole onions while damp or in sealed plastic bags without air exchange.
Season finish Remove roots and plant debris, clean the growing area and move future Allium crops if disease has been a problem.
Frequently asked questions
Answers for growing onions under glass
Can onions grow entirely in a greenhouse?
Yes, provided there is a sufficiently deep, loose, well-drained soil bed, plenty of direct light and dependable ventilation. The crucial limit is not the roof but a suitable day-length variety. In warm weather, avoid excessive heat, stagnant air and watering conflicts with thirsty neighbouring crops.
Will artificial light make every onion variety bulb reliably?
Not usefully in the typical home garden. Photoperiod controls the start of bulbing, but technically extending light does not replace a regionally suitable variety. Supplemental light can help produce compact seedlings. For later growth, clean glazing, a bright site and a suitable short-day or Mediterranean variety matter far more.
Why do my greenhouse onions make leaves but few bulbs?
Common causes include an unsuitable day-length type, a late start or too little plant mass when the critical day length arrives. Tight spacing, insufficient light, early leaf loss and water shortage can also limit size. Check these points before applying heavy late fertiliser.
How often should onions under glass be watered?
Do not water to a rigid calendar; respond to soil moisture, weather, plant development and soil type. During establishment and bulb growth, the root zone should stay evenly fresh but never saturated. Check soil 2 to 3 cm down. Thorough morning watering at soil level is better than daily small amounts over the foliage.
Should I bend the foliage to speed ripening?
No. Let necks soften naturally and foliage fall by itself. Bending green leaves does not speed dependable storage maturity and can cause wounds. Lift a largely naturally ripened crop, then cure it under cover, in shade and with plenty of air movement.
Sources and further reading
Explore horticultural basics
More from Bloomcabin: Classic aluminium greenhouse · Made-to-measure aluminium greenhouses · Premium aluminium greenhouses for Maltese gardens
- LWG Bavaria – guide to organic gardening
- Bavarian Garden Academy – sowing and planting dates at a glance
- North Rhine-Westphalia Chamber of Agriculture – onion fly on bulb onions
- Hortipendium – commercial onion production
- FiBL, Bioland, KÖN and Bio Austria – organic onion growing
- FiBL factsheet – organic onion production
- LWG Bavaria – plant health and natural ripening guidance
- Bavarian Garden Academy – onion dates and row spacing
