Plant spotlight

Short-Day, Long-Day: Why the Wrong Onion Never Makes a Bulb

5 min read

Somewhere every year, a gardener does everything right with onions and gets nothing. Good soil, steady water, no pests worth mentioning, tall healthy green tops all summer, and then they pull them up in August to find something closer to a spring onion than a storage onion. The usual conclusion is that they are bad at onions.

They're almost certainly not bad at onions. They just bought the wrong kind.

The short answer: onions bulb when day length crosses their threshold. Short-day onions trigger at 10 to 12 hours and suit roughly 25 to 35 degrees latitude, the South; intermediate at 12 to 14 hours for the middle band; long-day at 14 to 16 for the North. The wrong type gives you leaves, or a golf ball, instead of an onion.

Onions are watching the daylight

Most vegetables in the garden are keeping track of temperature. Onions aren't. An onion spends the first part of its life making leaves, and it'll keep making leaves until the day reaches a certain length. When that threshold arrives, it stops putting energy into the top and starts pushing it into the bulb. Every leaf it managed to grow before that moment becomes a ring in the onion. Every leaf it didn't grow is a ring you don't get.

That's the whole onion mechanism, and it explains almost every disappointment. An onion that switches over too early hasn't grown enough leaves yet, so it makes a small bulb out of the few it has. An onion whose threshold your garden never reaches simply keeps making leaves and never bulbs at all.

Short-day, intermediate, long-day: which onion fits your latitude

Onion varieties are sorted by the day length that triggers them, and the labels are on the packet if you know to look.

• Short-day onions start bulbing at roughly 10 to 12 hours of daylight. They suit lower latitudes, broadly the band from about 25 to 35 degrees, which is the American South, the Gulf, much of Texas and Florida, southern California and the desert Southwest.
• Intermediate-day onions want somewhere around 12 to 14 hours, suiting roughly 32 to 42 degrees. That is a wide middle band across the central and mid-Atlantic states, and they are the safest bet if you aren't sure.
• Long-day onions hold out for about 14 to 16 hours, suiting roughly 37 to 47 degrees and up. That is the northern US, most of Canada, the UK and northern Europe.

Now put those two facts together: Day length depends on latitude, and it varies more the further you get from the equator. A garden in Alberta gets long midsummer days that a garden in Georgia never sees, and a garden in Georgia has a gentler swing across the year. So a long-day onion planted in the South waits and waits for a day length that never arrives, and makes leaves until it gives up. A short-day onion planted in the North hits its trigger far too early in the spring, bulbs while it is still a small plant, and hands you a golf ball.

Neither of those is a soil problem, a watering problem, or a gardener problem. It's a plant doing precisely what it was bred to do, in the wrong place.

When to plant onions in a warm climate

This is where onions get interesting, and where most of the generic advice online is wrong for your location.

In hot climates, onions aren't a spring plant at all. Short-day onions are typically planted in fall or winter, grow slowly through the mild months while northern gardens are frozen, and bulb up in the spring as the days lengthen. Which means late summer isn't the tail end of your onion year, it's the run-up to it. August is the month to work out what you want and order it, because the good short-day varieties sell out and the garden center selection tends to be whatever the supplier had left.

It's one of the clearest cases of the northern calendar being not just unhelpful in a warm climate but backwards. The whole warm-climate year works this way. "Plant onions in spring" is good advice in Chicago and poor advice in Phoenix.

Onion sets vs. seed

Onion sets, the little dried bulbs sold in mesh bags, are the easy way in and there's nothing wrong with them. Two things to keep in mind with sets: First, they're very often sold unlabeled by day length, or labeled only by color, which is how a lot of people end up with the wrong onion without ever making a choice. If the bin does not say, it's worth asking. Second, sets have already lived through one season, and a cold spell after planting can convince them they have been through a winter and it's time to flower rather than bulb. Smaller sets, roughly under 3/4 in across, tend to be less prone to it.

Seed gives you the labeling and the variety choice, at the cost of a longer runway. Onions from seed are slow starters and need a good head start. Garlic, the other long-clock allium, is far more forgiving about it.

How to choose the right onion

The practical version is short. Find out which day-length band your garden sits in, buy onions bred for it, and most of the mystery goes away. Everything else about growing onions, the steady water, the loose soil, the not letting weeds get established, only matters once the plant is capable of bulbing in your garden at all.

This is one of the places where Verna is doing something a generic chart can't. It knows where your garden actually is, and it carries short-day, intermediate-day and long-day onions as three genuinely different plants with different timing, rather than one entry called "onion." So when it tells you a window is open, it's telling you that for the onion you are actually growing, in the daylight you actually get.

How to tell when onions are done

When onions are finished, they announce it. The tops soften at the neck and fold over on their own, usually most of the planting within a week or two of each other. That is the plant closing up shop, and it's the signal to stop watering and start thinking about curing. Most vegetables announce themselves the same way. It's a good example of the general rule: the plant will tell you, if you know what to watch for.

See what still has time in your garden

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