Growing onions in a greenhouse
Why grow onions under cover?
Protected growth, not an artificial summer
A good onion develops layer by layer long before a bulb is visible. First, the plant needs enough healthy leaves before it reaches the day length appropriate to its variety.
What glass changes
A greenhouse softens the difficult side of spring: persistent cold rain, saturated soil, crusted seedbeds and abrupt weather shifts. Seedlings can be raised more reliably, a prepared bed remains workable earlier, and the crop can cure under cover. Watering, ventilation and hygiene also become daily controls—useful for shallow-rooted onions that dislike both drought stress and warm, stagnant moisture.
What glass does not change
Bulb formation is driven mainly by photoperiod. A long-day cultivar still waits for long days under cover; a short-day cultivar may start bulbing before it has built enough foliage if local day length reaches its trigger early. Extra lighting is generally unnecessary for home crops. More reliable is a cultivar recommended for your New Zealand district and sowing season.
The practical role
For most home gardens, the greenhouse is chiefly an autumn, winter and spring growing space. Sow suitable varieties in late autumn or winter, or plant sturdy seedlings and sound sets in late winter or spring when the bed is workable. A well-ventilated side bed can carry the crop through harvest. If the house later becomes hot and crowded with tomatoes or cucumbers, plan airflow, light and separate watering zones carefully.
A thoughtfully arranged Classic aluminium greenhouse suits this routine well: propagation trays begin in bright shelter, while low, accessible beds leave room for upright onion foliage, soil-level watering and a clear maintenance path.
Starting material
Seed, sets or seedlings?
All three routes can produce onions. They require different schedules, offer different levels of variety choice, and each makes its own demands on greenhouse space.
For choice and control
Seed offers the widest choice of day-length type, colour, shape, keeping quality and maturity time. It is the best option when you want a variety recommended for your part of New Zealand. Sow into clean, fine, free-draining compost. Multi-cell trays or small soil blocks support even growth and limit early competition from weeds.
Seed needs planning, but under cover it can establish a healthy, even crop before the main garden season.
For speed and an easy start
Onion sets are small onions already grown to an early stage. They are easy to plant and can produce a comparatively early harvest. Buy only firm, dry, healthy sets. Soft, spotted or mouldy sets may 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 a wide choice of varieties.
For an even early crop
Bought or home-raised seedlings shorten the growing period in the final bed. They combine deliberate variety choice with a quick start and are especially practical where greenhouse space is valuable. Use fresh, upright plants with compact, healthy root balls. Discard plants that are very soft, rotting or visibly stressed.
For household quantities that store well, seedlings combine accurate spacing with a dependable head start.
Bloomcabin recommendation. Start from seed when variety, storage quality or a particular colour matters most. Seedlings suit a predictable, even crop. Healthy sets are often the easiest choice in a first greenhouse season—but they are not a universal substitute for a correctly chosen day-length variety.
Light sets the rhythm
Choose onions by their day-length response
Day length is the quiet architect of bulb formation. The greenhouse protects the plant, but the variety’s genetic response determines when 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, free-draining propagation mix. Keep it evenly moist until emergence, never waterlogged. Then grow seedlings as brightly as possible and on the cool side. A greenhouse bench works when nights are sufficiently protected and ventilation is consistent on sunny days. Otherwise, begin sowing in a bright indoor position.
After emergence, give seedlings enough root space, individually or in small groups. High temperatures with low light produce long, weak leaves and lose the plant’s compact advantage. Before planting out, the root ball should be moist but not dripping. Acclimatise plants gradually to the bed and avoid unnecessary damage to leaf bases and roots.
Plant the final greenhouse bed well before bulb formation begins for the variety. In many areas this is possible from late winter into spring, depending on region, structure and current weather. The bed must be workable and drain freely. Very early planting gains valuable growing time, but requires early ventilation on bright days and allowance for sharp frosts.
On compact sites, a Classic aluminium greenhouse can provide an orderly propagation space for a manageable onion crop. Use sturdy shelving for sufficiently deep trays, then plant strong seedlings into a prepared soil bed rather than forcing storage onions to remain in undersized containers.
Anatomy beneath the skin
An onion is not a root
An onion consists of thickened leaf bases arranged over a short basal plate. Fine roots emerge only below that plate. This structure explains why deep planting, heavy earthing-up, compacted soil and persistently wet ground can hinder clean bulb development.
Plant seedlings at about the depth they occupied in their propagation container. The white transition into the leaf should sit only lightly in the soil. Plant sets very shallowly; their tips may remain visible or only just 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 a loose, level bed with reliable drainage. Well-matured compost applied earlier is more valuable than fresh, nitrogen-rich fertiliser. Remove hard clods and compaction without disturbing the structure unnecessarily deeply. For ongoing 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 around 25 cm is a sound starting point for onions. Within the row, 7 to 10 cm is practical for medium storage onions. For larger bulbs or a more humid greenhouse, 10 to 12 cm may be useful. A dense crop does not automatically store better: it dries more slowly and is harder to inspect. A real access path is not wasted space; it protects shallow roots from compaction.
Onion roots explore mainly the upper soil layer and react poorly to swings between complete dryness and waterlogging. Water deeply at soil level during establishment, leaf growth and bulb development. Check moisture 2 to 3 cm down rather than judging only the dry surface. Drip irrigation is particularly suitable under cover because foliage stays dry. Reduce watering gradually as bulbs mature and necks naturally soften.
When onions are not cooperating
Read the cause before correcting it
Small bulbs, healthy foliage
Bulbing may have started before the plant was large enough, or the crop may have begun too late. Check day-length type and planting date first. Late fertiliser cannot correct an unsuitable photoperiod.
Thick necks, poor storage
Too much late nitrogen, plentiful water during ripening, close spacing or premature harvest can leave neck tissue slow to dry. Reduce water and nutrients in good time, then cure the crop in moving air.
Yellow, wilting seedlings
Onion fly larvae can cause tunnels and damage at the leaf base; affected plants may often pull out easily. Remove suspicious plants and inspect the crop. Open doors and vents are not a complete barrier to flying pests.
Spots, collapse or grey mould
Fungal diseases benefit from wet foliage, crowded planting and stagnant air. Increase spacing where needed, ventilate consistently and water only the soil. Remove severely affected plants. Do not grow onions and other alliums in the same space year after year.
A greenhouse is not automatically disease-free. Its advantage is close observation: condensation, crowded growth and declining foliage become visible early. Roof vents, generous doors and a thoughtful interior layout are therefore practical features to consider in a Classic aluminium greenhouse.
Neck fall, harvest and curing
Let onions ripen to a papery finish
Storage quality begins before harvest. Wait until a large proportion of necks naturally soften, bend over and begin to dry. Do not force healthy green foliage down by bending or twisting it. Forced collapse is not the same as natural maturity and may create wounds that make clean curing harder.
Harvest on a dry day where possible. Loosen onions carefully with a garden fork, remove loose soil, and avoid damaging either the outer skin or neck. Do not wash onions intended for storage.
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 sealed plastic crates. An empty greenhouse can serve as a curing place if it is very well ventilated by day and night and the onions are protected from intense sun.
Mild onions are usually used first. Varieties bred for storage generally last longer—provided necks and outer skins are completely dry and bulbs are undamaged.
After curing
Only inspected onions enter storage
Sort thoroughly after curing. Trim dry foliage to a few centimetres if needed, leaving protective outer skins intact. Store firm onions cool, dark, dry and well ventilated—in nets, shallow open crates or on slatted racks, for example. Check supplies regularly through autumn and winter. Remove and use soft, sprouting or damaged onions immediately.
Keep storage onions away from damp places with major temperature swings, and do not store them directly beside potatoes. A greenhouse is often useful for curing, but is not automatically suitable for winter storage. Dry air, cool conditions and frost protection are what matter. A suitable cellar, sheltered garage or well-ventilated utility room may be more dependable.
Best candidates Firm bulbs with dry skins, closed necks and no pressure marks.
Use first Thick-necked, scratched, soft, split or not fully dried onions.
Avoid completely Storing whole onions while damp or keeping them in sealed plastic bags without airflow.
Season finish Remove roots and crop debris, clean the growing area, and change where alliums are grown in future if disease has been a problem.
Frequently asked questions
Answers for growing onions under cover
Can onions grow entirely in a greenhouse?
Yes, provided there is a sufficiently deep, loose and free-draining soil bed, plenty of direct light and dependable ventilation. The important limit is not the roof but the right day-length variety. In summer, avoid excessive heat, stagnant air and watering conflicts with thirsty neighbouring crops.
Can artificial light reliably make every onion variety form bulbs?
Not usefully in the typical home garden. Photoperiod does control the start of bulbing, but extending light artificially does not replace a variety suited to your region. Supplemental light may help raise compact seedlings. For the later crop, clean glazing, a bright site and an appropriate variety matter far more.
Why do my greenhouse onions grow leaves but hardly any bulbs?
Common causes include an unsuitable day-length type, a late crop start or too little plant mass when the critical day length arrives. Close spacing, insufficient light, early leaf loss and water shortage during spring can also restrict size. Check these factors before applying heavy late fertiliser.
How often should onions be watered under cover?
Do not water to a fixed calendar; respond to soil moisture, weather, plant development and soil type. During establishment and bulb growth, the root zone should remain evenly fresh but never waterlogged. Check the soil 2 to 3 cm down. Deep morning watering at soil level is better than frequent light watering over foliage.
Should I bend over foliage to make onions ripen faster?
No. Let necks soften naturally and foliage fall by itself. Artificially bending green leaves does not speed dependable storage maturity and can cause damage. Lift a largely naturally ripened crop, then cure onions in a sheltered, shaded place with plenty of air movement.
Sources and further reading
Explore growing fundamentals
More from Bloomcabin: Classic aluminium greenhouse · Custom aluminium greenhouse · Premium aluminium greenhouses for New Zealand 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 edible 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 maturity guidance
- Bavarian Garden Academy – Onion timing and row spacing
