Choosing onion type for latitude: increase yields 20% in Midwest

Quick answer: To choose the right onion type, match its photoperiod requirement (short-day, intermediate-day, or long-day) to your specific latitude’s daylight hours, which can significantly increase bulb size and yield.

In the heart of the Midwest, where summer days stretch long and winters bite deep, understanding your onion’s photoperiod — its response to day length — is not just a suggestion; it is a necessity for a successful harvest. For growers across the United States, from the warm Gulf Coast to the cool Canadian border, selecting the correct onion type for your specific latitude can mean the difference between a bountiful crop of large, flavorful bulbs and a patch of green tops with underdeveloped bulbs. This guide will walk you through the science and practice of matching onion varieties to your local daylight hours, ensuring your efforts in the garden translate into delicious results.

Onions are photoperiodic plants, meaning their bulbing process is triggered by the number of hours of daylight they receive each day. Planting the wrong type for your region can cost you the bulb crop entirely, leaving green tops and nothing to store. By aligning your planting choices with the natural light cycles of your USDA zone, you can significantly improve bulb size, uniformity, and overall crop health. We will explore the three main categories — short-day, intermediate-day, and long-day onions — and provide practical advice for growers in every corner of the country, ensuring your onion patch thrives with the right selection.

Understanding onion photoperiodism

Onions are fascinating plants that use day length as a signal to transition from growing green tops to forming a bulb. This process, photoperiodism, determines whether onion plants produce harvestable bulbs or remain green stalks. The daylight hours required to trigger bulbing vary by variety, making cultivar selection important across US latitudes. For instance, a variety that thrives in Florida’s winter might not bulb at all in Minnesota’s summer, even with ideal soil conditions.

The science of light and bulbs

The plant’s internal clock, regulated by light-sensitive pigments, measures the duration of daylight. Once a specific threshold is met, the plant redirects its energy from leaf growth to bulb enlargement. This threshold is typically between 10 and 16 hours of daylight, depending on the onion type. Optimal soil pH for onion cultivation ranges from 6.0 to 7.0, supporting nutrient uptake for bulb development, as confirmed by USDA data [2]. Amending soil with compost to a depth of 12 inches improves drainage and nutrient availability for bulb formation. Without the correct light signal, even a healthy plant may only produce a small, unmarketable bulb.

• Photoperiod: The duration of light in a 24-hour cycle.
• Bulbing: The process where the onion plant forms its underground storage bulb.
• Cultivar Selection: Choosing the specific onion variety suited to your region.
• USDA Zones: Geographic areas with similar climate conditions, influencing planting times.
• Soil pH: A measure of soil acidity or alkalinity, affecting nutrient availability.

Short-day onions: growing in southern latitudes

Short-day onions are specifically bred for regions closer to the equator, typically below 35 degrees North latitude, which includes much of the southern United States. These varieties initiate bulbing when they receive approximately 10 to 12 hours of daylight each day. This makes them an excellent choice for growers below about 35 degrees North - Florida, Texas, Louisiana, and the coastal Carolinas. Match onions to latitude, not to your USDA hardiness zone: coastal Oregon is zone 8 at 45 degrees North and needs long-day types, while Georgia is zone 8 at 32 degrees North and needs short-day. Planting typically occurs in the fall, allowing the plants to establish roots during the cooler months and bulb up as day length increases in late winter and early spring.

Popular short-day varieties and planting tips

Some popular short-day varieties include ‘Granex’ (the type sold as Vidalia), ‘Texas SuperSweet’, and ‘White Bermuda’. These onions are known for their sweet, mild flavor and are often used fresh in salads or sandwiches. For optimal growth, plant short-day onion sets or transplants in well-drained soil with a pH of 6.0 to 7.0, approximately 4 to 6 inches apart in rows 12 inches apart. In West Virginia, short-day sweet onion varieties can produce marketable bulbs up to 30 days earlier when grown in high tunnels [1], demonstrating the benefit of controlled environments for early harvests. Consistent watering, about one inch per week, is necessary during the bulbing phase.

• Planting Season: Fall (October-December) for spring harvest.
• Optimal Latitude: Below 35 degrees North (e.g., Georgia, Alabama).
• Daylight Requirement: 10-12 hours.
• Flavor Profile: Typically sweet and mild.
• Common Varieties: Granex, Texas SuperSweet, White Bermuda.

Intermediate-day onions for mid-latitudes

That work on short-day onions sets up what follows here.

Intermediate-day onions offer a versatile solution for growers situated in the middle latitudes of the United States, generally between 32 and 38 degrees North. This broad band includes states like Kansas, Missouri, Kentucky, Virginia, and parts of California. These varieties require 12 to 14 hours of daylight to trigger bulb formation. This makes them adaptable to a wider range of growing conditions compared to their short-day or long-day counterparts, often performing well where other types might struggle to bulb effectively.

Cultivating intermediate-day varieties

Popular intermediate-day varieties include ‘Candy’, ‘Super Star’, and ‘Red Candy Apple’. These onions are known for their good storage qualities and balanced flavor, making them suitable for various culinary uses. Planting usually occurs in early spring, around March or April, after the last hard frost, allowing the plants to grow vigorously before the longer days of summer initiate bulbing. For best results, sow seeds or plant sets 1/2 to one inch deep, with 4 to 6 inches between plants and 12 to 18 inches between rows. Ensure your soil has excellent drainage, perhaps by using a raised bed at least 10 inches deep, and maintain consistent moisture, especially when bulbs begin to swell. You can find more detailed guidance on general onion cultivation at How to grow onions: sets, seeds, and day-length.

• Planting Season: Early spring (March-April).
• Optimal Latitude: 32-38 degrees North (e.g., Kansas, Missouri).
• Daylight Requirement: 12-14 hours.
• Flavor Profile: Balanced, good for cooking and storage.
• Common Varieties: Candy, Super Star, Red Candy Apple.

Long-day onions: growing in northern climates

This builds directly on intermediate-day onions.

For growers in the northern United States, above 38 degrees North latitude, long-day onions are the dependable choice for producing substantial bulbs, with intermediate-day types worth trying in the southern part of that range. This includes states like Michigan, Wisconsin, New York, and much of New England. These varieties require 14 to 16 hours of daylight to initiate and complete bulb formation. Long summer days in northern regions provide the photoperiod needed for these onions to mature into large, firm bulbs. Without sufficient light, long-day onions will simply continue to grow greens, never forming a bulb.

Selecting and growing long-day varieties

Popular long-day varieties include ‘Walla Walla Sweet’ (though it can also be intermediate-day in some contexts), ‘Patterson’, ‘Yellow Sweet Spanish’, and ‘Red Wethersfield’. These onions are often known for their excellent storage capabilities, making them a staple for winter pantries. Planting usually occurs in early spring, from March to May, as soon as the soil can be worked and temperatures are consistently above 45°F. Space plants 4 to 6 inches apart in rows 12 to 18 inches apart, ensuring good air circulation. Long-day onions require consistent moisture, about one inch of water per week, and a fertile soil with a pH between 6.0 and 7.0 for optimal growth. In Alaska, growers contend with short seasons but benefit from long summer days, making long-day varieties necessary for crops like onions.

• Planting Season: Early spring (March-May).
• Optimal Latitude: Above 38 degrees North (e.g., Montana, North Dakota).
• Daylight Requirement: 14-16 hours.
• Flavor Profile: Varies from sweet to pungent, often good for storage.
• Common Varieties: Walla Walla Sweet, Patterson, Yellow Sweet Spanish.

Troubleshooting common onion growing issues

Even with the correct onion type selected for your latitude, various factors can impact your harvest. Poor bulb development, disease, and pest issues are common challenges for growers across all USDA zones. For instance, if your long-day onions in Michigan are producing only green tops with no bulbs, the most likely culprit is insufficient light duration, perhaps due to late planting or excessive shading from taller plants. Conversely, short-day onions in Georgia that bolt (send up a flower stalk) prematurely often indicate exposure to cold temperatures followed by warm weather, signaling the plant to set seed instead of bulbing.

Preventative measures and solutions

To prevent common problems, always start with healthy, disease-free seeds or sets from a reputable supplier. Ensure your soil is well-drained and rich in organic matter, with a pH between 6.0 and 7.0, as recommended by the USDA Natural Resources Conservation Service. Consistent watering, about one inch per week, is important while bulbs swell. Rotate your onion crops every three years to reduce the buildup of soil-borne diseases. If you notice yellowing leaves or stunted growth, perform a soil test to check for nutrient deficiencies; onions are heavy feeders and benefit from a balanced fertilizer application every three to four weeks while they are putting on leaves. For example, a 10-10-10 granular fertilizer applied at a rate of one cup per 10 linear feet of row can significantly boost vigor. Stop feeding nitrogen once the bulbs begin to swell, since late nitrogen gives thick necks and soft bulbs that will not cure or store.

• No Bulbing: Incorrect day length for the variety or too much shade.
• Bolting: Stress from temperature fluctuations or planting too early.
• Small Bulbs: Overcrowding, insufficient nutrients, or inconsistent watering.
• Disease: Poor drainage, lack of crop rotation, or contaminated soil.
• Pests: Thrips, onion maggots, or cutworms can damage plants and cut yields.

Onion Day-Length Varieties by Latitude

Southern Growers: Short-day onions like ‘Granex’ can produce marketable bulbs up to 30 days earlier in high tunnels in West Virginia [1].

Soil Health: Optimal soil pH for onion cultivation ranges from 6.0 to 7.0, a crucial factor for nutrient uptake and bulb development.

Frequently asked questions

What happens if I plant the wrong type of onion for my latitude?

If you plant the wrong onion type, your plants may not bulb at all, or they might produce very small, underdeveloped bulbs. For example, planting long-day onions in Florida will likely result in only green tops, as they won’t receive the 14-16 hours of daylight needed to trigger bulbing.

Can I grow short-day onions in a northern state like New York?

While you can grow short-day onions in a northern state, they will likely not form large bulbs. In the north, spring day length passes the 10-12 hour trigger very early, so short-day varieties start bulbing before they have built enough leaves, which is why they finish as small, immature onions despite healthy green growth.

How do I know my latitude for choosing onions?

You can easily find your latitude using a simple online search for your city or by checking a map. Generally, states below 35 degrees North are for short-day, between 32 and 38 degrees North for intermediate-day, and above 38 degrees North for long-day varieties. For instance, Denver, Colorado, sits at about 39 degrees North, making it a long-day onion region.

Are sweet onions always short-day varieties?

Many popular sweet onion varieties, like Vidalia and Texas SuperSweet, are indeed short-day types, thriving in southern climates. However, there are also intermediate-day and long-day sweet onion varieties, such as ‘Walla Walla Sweet’ (which can be intermediate or long-day depending on context), offering options for growers in various latitudes. Always check the specific variety’s day-length requirement.

When is the best time to plant onions?

The best planting time depends on your onion type and latitude. Short-day onions are typically planted in the fall (October-December) in southern regions. Intermediate and long-day onions are usually planted in early spring (March-May) in mid-to-northern latitudes, as soon as the soil can be worked and temperatures are consistently above 45°F.

What soil conditions do onions prefer?

Onions prefer well-drained, fertile soil rich in organic matter. An optimal soil pH between 6.0 and 7.0 supports nutrient uptake and healthy bulb development. Amending your soil with compost to a depth of 12 inches can significantly improve both drainage and fertility.

References

1. Evaluating Short-Day and Intermediate-Day Sweet Onions as Early-Season High Tunnel Onions in West Virginia (2026). Evaluating Short-Day and Intermediate-Day Sweet Onions as Early-Season High Tunnel Onions in West Virginia.
6. USDA Natural Resources Conservation Service (2024). USDA Natural Resources Conservation Service.