Plant Overwintering Onions Now, Harvest by Early Summer

Most onions go into the ground in March. But there is a second round that starts in autumn and gives you full-sized bulbs by June, at a point when spring-planted sets are still putting out leaves. What it takes: the right variety, and a planting window that is opening right now.

Why the detour through winter pays off

Overwintering onion sets put down roots in autumn, sit out the cold as small plants, and take off the moment February or March turns mild. They make use of spring soil moisture while spring-planted sets are still waiting in the bag.

The result: harvest from early June to early July instead of August, and as early as late May in very mild areas. And the bed frees up early, leaving room for bush beans, lettuce or a late green manure.

One honest caveat: overwintering onions do not store as long as summer onions. They are for fresh use in early and mid summer, not a pantry supply that lasts until Christmas. If you want both, grow both.

Not every onion set survives frost

This is where it goes wrong most often: the sets on the spring shelf are not winter hardy. They either bolt in spring or rot away over winter.

You need varieties explicitly sold as winter hardy. Common ones across German-speaking Europe include Senshyu Yellow (yellow, sturdy), Electric (red) and Radar (yellow, very frost tolerant). The packaging will say winter onion, overwintering set or autumn planting.

Worth knowing when you look things up: the plant lexicon lists onion with a March and April planting time and a summer harvest. Those are the figures for the classic summer onion. For winter varieties, go by the dates in this article.

The same principle applies to garlic, by the way, except that its planting window only opens in October and November.

Timing decides whether they bolt

For most of the German-speaking region the planting window runs from mid-September to mid-October. Go early in that range at higher, colder elevations; in mild wine-growing climates you can wait until late October.

The logic is simple. The set needs to root well before the first hard frost, which takes roughly four to six weeks of warm soil. But it must not enter winter too far advanced: plants that develop too much will produce a flower stalk instead of a bulb in spring. That is called bolting, and planting too early is its most common cause.

So if you are hesitating this week, no harm done. For most gardens the next two to four weeks are actually the better slot.

Spacing, depth and soil

Onions are light feeders. They want free-draining, rather lean soil, and definitely no fresh manure or heavy fertiliser. That produces soft foliage instead of firm bulbs and invites rot.

Spacing: about 10 cm within the row, 25 to 30 cm between rows. Depth: shallow enough that the tip just peeks out of the soil, roughly 2 cm. Set deeper, they are more likely to rot in a wet winter.

Waterlogging is the main enemy. On heavy clay it pays to raise a low ridge or use a raised bed. Once the sets have taken and the first frost arrives, add a thin layer of leaves or straw, just a few centimetres. Mulch it too thickly and the layer traps moisture, and they rot underneath.

Good neighbours and bad ones

The classics hold up in a winter bed too: carrot and onion are the best known pairing, because carrot fly and onion fly are each put off by the other plant’s scent. Beetroot, lettuce and strawberry work well alongside too.

What does not belong next to them: beans, peas and brassicas. With beans and peas the reason is the sulphur compounds alliums release: they hold back the nitrogen-fixing bacteria on legume roots, and both sides end up growing poorly.

In autumn the practical companion is a different one anyway. A strip of lamb’s lettuce beside the onion rows keeps the soil covered, and neither bothers the other. If you are planting early in the window, spinach still works too, since its sowing window closes at the end of September.

Four years off for the allium family

Onion, garlic, leek and chives all belong to the same family, the alliums. Grow them too often in one spot and soil-borne fungi build up. Onion neck rot and white rot are the usual suspects, and they persist in the soil for years.

So: leave at least four years before another allium returns to the same patch. Potato makes a good preceding crop, as the ground clears in autumn and the soil is nicely worked.

This is exactly where a planned bed saves you a lot of mental arithmetic. In Rootivo you can see what grew in that spot in previous years the moment you place a plant, and the companion-planting lines show you in green and red which neighbours fit.