There is something quietly satisfying about planting a crop in the fading light of October and knowing you will not touch it again until the following summer. Garlic and overwintering onions are the only vegetables in most European gardens that work this way. You put them in the ground while you are still clearing away the last of the tomatoes, and they spend the cold months doing something invisible but essential: building a root system and waiting for the cold signal that tells them to form a bulb.
Get the autumn planting right and you harvest fat, well-formed bulbs in July. Get it wrong — plant too late, plant into compacted or waterlogged soil, or skip the soil preparation entirely — and you will lift something closer to a spring onion than a garlic bulb. The difference is decided almost entirely by what happens in the next six weeks.
Why garlic has to be planted in autumn
Garlic is not simply a crop that tolerates cold. It requires it. A clove that never experiences a sustained cold period will grow leaves happily enough but will fail to divide into separate cloves, producing a single undivided “round” instead. The Royal Horticultural Society puts the requirement at one to two months between 0 and 10 °C for good bulb development, and the University of Georgia Cooperative Extension gives a comparable figure of six to eight weeks of cool conditions after planting. In most of Europe, planting from the middle of autumn through to early winter delivers that cold period naturally.
The second reason for autumn planting is less discussed but just as important. Soil in September and October is still warm from summer, often several degrees warmer than the air above it. Roots grow readily in warm soil even when nothing is happening above ground. A clove planted now will have anchored itself thoroughly before the first hard frost, while one planted in February starts from nothing and spends its short season playing catch-up.
Preparing the bed: what alliums actually need
Garlic and onions are unforgiving about two things: drainage and compaction. They will sit through weeks of cold without complaint, but a winter spent in waterlogged soil rots the basal plate and the crop simply disappears. If your soil is heavy, plant on a slightly raised ridge or bed rather than trying to improve the whole plot at once.
Work in well-rotted organic matter rather than anything fresh. Raw manure applied in autumn releases nitrogen at exactly the wrong moment, pushing soft leaf growth that frost will damage. Garden compost that has finished working is ideal — and if your own heap is not yet ready, a Hortulani BioComposter treatment through autumn keeps decomposition running as temperatures fall, so there is mature compost waiting in spring rather than a cold, stalled heap.
Aim for a pH somewhere between 6 and 7. Alliums dislike strongly acid ground, and on soils below pH 6 a light liming in autumn is worth doing well ahead of planting.
The root system that barely exists
Here is the fact that explains most of what follows. Onions and garlic have an unusually poor root system — shallow, sparse, and with very few root hairs compared with almost any other vegetable. They are, in effect, badly equipped to find their own phosphorus.
Which is why they lean so heavily on soil microorganisms. A review of the genus published in Plants by Golubkina and colleagues (2020) notes that Allium crops respond strongly to arbuscular mycorrhizal fungi precisely because their root surface area is so limited compared with other species, with inoculation improving nitrogen, phosphorus and potassium uptake and, in commercial preparations, roughly doubling onion yields against untreated controls. Field work by Tawaraya and colleagues (2012) in Biology and Fertility of Soils found inoculated Welsh onions matching the yield of uninoculated plants grown at more than three times the soil phosphate level.
In a home garden this translates into a simple principle: feed the soil biology at planting, not the plant. Treating the bed with Hortulani MicroBiological Fertiliser for Vegetables before the cloves go in puts beneficial organisms into the root zone while the soil is still warm enough for them to establish. A dip or drench of Hortulani Multipurpose Vermicompost Extract at planting gives the cloves an immediate biologically active environment to root into.
Planting: depth, spacing and the one thing people get wrong
Break the bulb into cloves only on the day you plant — the basal plate dries out quickly once exposed. Use the largest cloves; small ones produce small bulbs, and the little inner cloves are better in the kitchen than in the ground.
Plant each clove flat end down, pointed end up. This sounds obvious, and it is the single most common mistake made by first-time growers planting in a hurry. Set them roughly 15 cm apart with 30 cm between rows, with the tip about 2.5 cm below the surface on heavier ground and rather deeper on light, free-draining soil, where deeper planting tends to give larger bulbs.
Overwintering onions and shallots
Japanese-type onion sets and autumn shallots follow the same logic with a slightly earlier window — generally September into October, so they establish before growth slows. They do not have garlic’s absolute cold requirement, but they do exploit the same long root-building season, and they mature several weeks ahead of spring-planted onions, which is genuinely useful in a year when the storage crop runs out early.
Through the winter and into spring
Once planted, leave them alone. A light mulch of leaf mould or compost protects against frost heave, which lifts shallow cloves clear of the soil during hard freeze-thaw cycles — worth checking after a cold snap and firming any that have popped up. Keep the bed weed-free, because alliums compete badly with weeds all season.
The real feeding moment comes in early spring, when leaves start moving. That is when a top-up of Hortulani Vermicompost Extract for Vegetables pays for itself, supporting the leaf growth that will ultimately size the bulb. Stop feeding once the bulbs begin to swell in early summer, and stop watering in the final few weeks so the skins cure properly for storage.
In short
Autumn-planted garlic and onions reward a small amount of work now with a crop that needs almost no attention for eight months. Plant in mid-autumn to early winter so the cold requirement is met naturally; choose free-draining ground and improve it with mature compost rather than fresh manure; plant the biggest cloves, flat end down, 15 cm apart; and remember that these are plants with almost no root hairs, which makes the biological condition of the soil far more important than any quick-release feed.
If you are preparing an allium bed this month, the microbiological fertiliser for vegetables and the multipurpose vermicompost extract from hortulani.eu are the two products that do the most for a crop with this particular weakness. Plant well now, and next July will take care of itself.