The difference between long day onions and short day onion varieties is the amount of daytime length they require to trigger bulb growth. Onion bulbs will begin growing when days reach 14-16 hours of sunlight for long day onions and 10-12 hours per day for short day onions.
Long day onions are generally better for storing and many varieties can be stored for 6+ months. Short day onions are generally sweeter but will not store as long.
The amount of sunshine an area receives in a day depends on the time of the year and the latitude of the area.
During the solstice, June 22-23, the Northern Hemisphere is tilted toward the sun and so has the longest daylight hours it will have during the year ranging from 12 hours a day at the southern most latitude of the Northern Hemisphere to 24 hours a day at the North Pole.
The 35th parallel is a line that divides the US north and south. Generally gardeners are recommended to grow long day onions in the north and short day onions in the south.
Intermediate onions, also called day neutral onions, do well in middle latitudes of the country and require 13-14 hours to grow onion bulbs.
Here is a map of the United States by Johnny Selected Seeds that shows the areas of the country that are best suited for each type of onion.
In the north growers are recommended to plant long day onion seeds in the early spring or start seeds indoors and plant onion seedlings when weather permits and then harvest in the late summer.
In the south growers are recommended to plant onion seeds in the late summer or onion sets/bulbs and seedling transplants in the late fall or winter and harvest onions in the spring.
Check out “How Long Do Onions Take To Grow?” for the differences on planting with onion seeds, sets/bulbs, and seedling plants.
Varieties of Long Day Onions
- Walla Walla Sweet – This is a sweeter onion in the long day category. They grow to a large size and can be 5 inches in diameter and weigh as much as 2 lbs. Walla Walla is a popular onion variety that comes from Walla Walla, WA, and is the Washington State Vegetable.
- Yellow Spanish – This is another popular onion choice. It is a mild yellow onion that stores well and produces good green onions.
- White Spanish – A popular white onion with a mild flavor that stores well.
- Red Wing – This red onion is a medium size onion with a mild/medium strength pungent characteristics that last well.
- Rossa Di Milano – This red onion has a distinct flat shaped top. It is an onion with a good flavor, sweet and pungent, and sometimes called a cooking onion.
- Yankee – Medium sized bulbs with a golden brown exterior and bright white interior. These onions hold up well in storage.
- Patterson – A long day yellow onion with a medium/large size bulb that stores very well.
Varieties of Short Day Onions
- White Castle – A short day onion with a bright white exterior and interior. Has a mild/medium pungency and flavor and stores for 3-4 months.
- Red Marble – A red onion with a cippolini shape.
- Desert Sunrise – This is a bolt resistant variety that grows into a medium size flattened bulb. It has a sweet & spicy flavor and stores for 2-3 months.
- Granex – This yellow hybrid variety is known to be exceptionally sweet and great for onion rings or baking.
- Texas Supersweet – This onion variety is sweet enough to eat raw and grow into large softball size bulbs.
- Cippolini Red – Cippolini onions are known for their flattened shape. This red cippolini produces small sized purplish/red onions with a pungent smell and taste. They have a firm texture and carmelize well.
Varieties of Intermediate Day Onion
- Flat of Italy – This is an old Italian variety that is a cippolini onion, flat shaped. It’s a bright red color and used raw or cooked.
- Cabernet – Medium to large sized red onions that store for 4-6 months.
- Sierra Blanca – This day neutral onion grows well anywhere. It has a mild flavor and white flesh and exterior. It does not store well.
- Candy – Light yellow skin and white flesh, this is a sweet onion that will grow well in most areas.
- Australian Brown – This white onion has yellow brown skin and watery flavorful flesh.
Can You Grow Short Day Onions in the North?
The problem with planting short day onions in the North is that day lengths reach 10-12 hours too early in the season and bulb development starts too early causing small bulbs to develop.
Short day onions can be grown in the north but they need to be started outdoors as soon as possible and won’t develop into big bulbs.
The best way to grow short day onions in the North is to start them indoors in a blacked out room with a grow light and timer until they develop good size foliage and then move them outside when temperatures allow.
Long day onions are normally planted in the North in the early spring so they have time to develop vegetative growth above the ground. The green growth above the ground allows the plant to capture the energy required to develop a good sized onion bulb when day lengths reach 14-16 hours a day.
Can You Grow Long Day Onions in the South?
You can’t grow long day onions in the south without artificial lighting because the days don’t get long enough to initiate bulb development. Long day onions need 14-16 hours of sunlight in a day before they start to grow bulbs.
In Atlanta, for example, the longest day lengths get is almost 14 and1/2 hours at the solstice on June 22-23.
You can use this sunrise/sunset calendar to determine the day lengths in your area. Day lengths is one of the options in the settings that you need to click for it to be displayed.
Please comment below with your own knowledge and experiences with short, intermediate, and long day onions!