The mystique behind long-day, intermediate and short-day onions is often off-putting to potential onion growers, thanks to the confusion around when and where they work. Because of that, onions are probably the most underappreciated plant in our gardens, overlooked for more sculptural leeks, and faster-growing bunching varieties, but the humble bulbing onion deserves our true consideration.
In this article, I want to simplify some of the terms used to describe their growing conditions and clarify what is meant by light levels, climate boundaries and day length for growing onions. We’ll start with the simple differences, and which ones are best for what uses, but then break down into explaining when to plant long day or short-day onions, and some of the best intermediate onions for growing.
The difference between long-day, intermediate and short-day onions
It seems obvious, as long-day, short-day and intermediate onions (sometimes called day-neutral onions) pretty much explain their differences in their name, but what exactly is a short-day, and where are you most likely to have one?
First of all, it’s based on your location rather than the time of year. If you grow short-day onions in Northern states, they will start to form bulbs too early, causing weaker bulbs, and putting too much strain on the young roots.
But why exactly does it matter? Well, onions bulbs develop based on the strength of their leaves above ground. Their leaves are fed by roots, so the stronger the roots the stronger the leaves. And the more leaves above ground, the more rings in the bulb under round.
Onion leaves directly correlate to their bulbs, so an onion with 15 leaves, will grow an onion with 15 rings. Planting short day onions too far north, means they form bulbs before the leaves have finished, or even started, growing.
Day length for onions explained
In northern states, while light levels are lower and temperatures are cooler, the day length is actually longer, therefore all long-day, short-day or intermediate onions can be planted at the same time, just in different places.
North of Nevada, long-day onions are the only choice, as the days are at least 14-16 hours long from late spring to mid-autumn.
States running across the centre of the US (from Nevada to Virginia) require 14 hours of daylight to start forming bulbs. Again, this will be the same time of year as Northern states, but the day length varies slightly.
In Southern states, where days will reach 10-12 hours in length by late spring, short-day onions are perfect as they are generally better planted from sets, rather than seed, so do not require over wintering. This means that putting a young onion set in the ground in early spring, will allow a full season of growth.
Long day onions should be planted in early spring or can be overwintered if grown from seed in late autumn, or even indoors over winter. Growing them indoors over winter can lead to stronger growth in spring, but also requires more maintenance, as your onions will be more likely to become top-heavy, with too much top growth before bulbs are allowed to form.
As soon as the day length is over 14-16 hours, and your onions are planted in a good sunny spot, your onions will begin forming bulbs underground. North of Nevada, this is usually early summer.
Never try to grow long-day onions south of Missouri, as the day lengths are never long enough to trigger bulb formation.
When to plant long-day onions
Temperatures North of Nevada are too cold to reliably grow onions outdoors in winter, so it is advised to plant long-day onion sets in early spring, after the last frost. This gives them maximum growing time to form leaves while planting from sets will give your long-day onion roots a head start.
Best long day onions for growing
Ringmaster Long-Day Onions
The bright white skins on ringmaster makes them really beautiful onions for storing or hanging. They will grow reasonably close together too, so Ringmaster Onions really make the most of your growing space and will last all the way through winter in proper storage.
Red Zeppelin Long-Day Onions
Usually, red onions are terrible for storage. Their wetter centres tend to turn quicker, and need individual wrapping, even after being left to dry in the greenhouse. Red Zeppelin though, is a fairly dry onion, with a rich red skin, sometimes growing bulbs the size of squashes, and store really well.
It’s one of the sweetest long-day onions you can buy too.
Intermediate onions, or day-neutral onions, are great for the central states and are generally considered a safer option. They start to form bulbs at around 12-14 hours of light per day, again, this will be early summer, so planting onions out as soon as the frost has passed is essential for good top growth before the bulbs begin to form.
There is less variety to choose from though, so it is possible to grow long-day onions if you prefer growing for storage. If you do grow long-day onions in intermediate states, they will form smaller bulbs, but taste just as good.
When to plant intermediate onions
Plant intermediate onions out in early to mid-spring. Planting them too early can lead to overgrown leaves, which can put too much strain on the bulb. Where long day and short-day onions are well adapted to their exact conditions, intermediate onions are a little fussier.
Planting them in mid-spring is a safe bet, as they tend to be ready to harvest later anyway. This way, they’ll have enough time to establish leaves, but not enough time that they go to flower.
Best Intermediate Onions for growing
Cabernet Intermediate Onions
Cabernet onions are a true red onion, with a deep red, almost purple skin, and a soft red hew through every ring. They aren’t as sweet as short-day red onions, but they cook up really well.
Red Candy Apple Intermediate Onions
Red Candy Apple Onion are reliable in most climates, and have a wider growing area than most intermediate onions, growing a little further south, and little further north than others due to their good frost tolerance and speedy bulb growth. Just make sure to give them a head start by growing from sets rather than seed if you’re growing anywhere more north than Kansas.
Short-day onions are sweeter, with a more rounded flavour than others. If you can grow short-day onions, they will always work best for cooking and are far more versatile in the kitchen.
While they have more limited growing conditions, those of us lucky enough to grow them successfully should take every opportunity to find space on our veg plots.
Because they begin forming bulbs at just 10-12 hours of light per day, they need to be planted south of Missouri. Planted any further north, and they will begin forming bulbs before their leaves and roots have properly developed. This leads to a small bulb, with not enough layers, that is quite likely to flower in its first year, rendering the bulb inedible.
When to plant short-day onions
Plant short-day onions in winter. The further south you are, the more likely it is that you can leave your onions out. Short-day onions are reasonably frost tolerant too, but there is less need to worry about that in southern states. So planting from seed in autumn, or planting sets in autumn will give your onions time to grow stronger roots and more leaves during winter and early spring, meaning there will be plenty of energy waiting to go into the bulb in early summer when the daylight length reaches 10-12 hours.
Best short-day onions for growing
Red Creole Short-Day Onion
Red Creole are great short-day onions. Remember that the biggest bonus of short-day onions is their sweeter flavour, so choosing a brighter red onion maximises that impact and, honestly, you’ll be eating them like apples when they’re ready to harvest.
Texas Early Grano Short Day-Onion
If you ever find a better indicator of a short-day onion than it being called ‘Texas’ please let me know. It doesn’t get much more southern, and you know it’s going to be a reliable over-winterer too.
Surprisingly, these store reasonably well, despite their flat bulbs, which are usually a sign that mould can build on stored bulbs. But I’ve found these to be reasonably good for storing, at least through most of the winter.
This article is, for the most part, only relevant to readers in the US, but there are instances in Europe where understanding the difference between long-day, short-day and intermediate onions is crucial, especially in Northern Europe, where the day length is much longer than Greece or Italy in the south.
A friend once told me they brought a bag of onion sets back to Scotland from Greece, and they flowered before the end of summer. Obviously accidentally buying the wrong onions on holiday is a rare problem to have, but be careful when buying any imported onions that come from similar climates.
Gardeners in the US are fortunate enough to have long-day, short-day and intermediate onions properly labeled on the packet, so make sure you pay attention next time you’re ordering onion bulbs or seeds, as the difference between long-day and short-day onions can also be the difference between a crop and a failure!