Successfully Growing Bulb Onions in your Home Garden

I love to eat and cook onions. However, I never realized how good an onion can taste until I started growing onions in my garden. In addition, I love the fact that in my zone 6a garden, I can plant both a spring and fall onion crop. All you need is patience because growing bulb onions takes between 95 to 110 days.

This post will focus on growing bulb onions (dry or common onions) from seed. A dry onion produces a single large bulb. In contrast, green onions are harvested while the tops are still green and before a large bulb develops.

Fun facts about Vidalia sweet white onions. By law, ‘Vidalia’ onions grow in a 20-county region in US state of Georgia. Further, Vidalia onions are the state vegetable of Georgia where farmers grow approximately 125 million pounds each year.

Bulb Onion Characteristics

Before we discuss growing bulb onions, let’s take a look at the characteristics of these onions.

1. Color – Bulb onions are classified as being one of three colors: yellow, white, or red.
2. Taste – Depending upon the color, bulb onions have different tastes: mild (white), sweet (red), strong (yellow).
3. Growth – Onion varieties are classified based upon the length of the day to grow. Day Length refers to how much daylight is required for the bulb of the onion to form.
   • Short-day varieties – require 10-12 hours of daylight. In the US, these short-day varieties are planted in the south (zone 7 and warmer). Plant short-day varieties in the fall.
   • Intermediate-day varieties (or day-neutral) – these onions do not depend upon the hours of daylight and can thus be grown in both the northern and southern zones. However, they do best in zones 5 and 6. Plant in the fall in southern climates and in spring in northern climates.
   • Long-day varieties – require 14-15 hours of daylight. In the US, these long-day varieties are planted in the north (zone 6 and colder). Plant long-day varieties in either fall or early spring.
4. Storage
   • Storage Onions are onion varieties that have proven to have a long shelf life (8-12 months). These are usually ‘long-day’ variety onions and are best suited to be grown in northern states. These onions have a higher sulfur content and thicker outer skins. You will recognize a storage onion when you cut into the onion. The higher sulfur produces the tearing action in your eyes.
   • Freshly eaten are onions that do not keep as long, and you need to consume more quickly

Growing Bulb Onions from Seeds

As I mentioned earlier, this post will focus on growing bulb onions from seed. While you can direct sow onion seeds, my recommendation is to start onion seeds indoors and then transplant the resulting seedlings. Transplants generally result in larger, drier, and more attractive onions.

However, there are two additional ways to plant onions:

1. Purchase onions sets. Sets are small bulbs that were grown the previous year. These can be purchased at garden centers and from seed catalogs.
2. Purchase transplants. A transplant is an onion seedling that has been started by a grower and is sold at garden centers.

Growing Requirements

Timing: Because onions are a cool-season crop, spring and fall are the best time to sow seeds or transplant seedlings. Start onions from seed 6-10 weeks before transplant.

1. In colder zones (6 and colder), there are two timing options:
   • Fall planting from mid-September to early October with a harvest in early June.
   • Spring planting from March to April with a harvest mid to late summer.
2. In warmer zones (7 and warmer), onions can be planted in late fall or winter.

Light: Onions prefer full sun.

Soil: Onions need soil that is well-drained, moist, and fertile. A pH of 6-7 is best (to determine your soil pH, perform a soil test by contacting your local Cooperative Extension). Harvests will be better if you prepare your soil with the addition of composted organic matter and an all-purpose fertilizer (like Plant Tone). Work the organic matter and fertilizer into the top 6 inches of soil.

Spacing: Plant seedlings 4 inches apart in rows that are 8-10 inches apart. Always double-check the guidelines on the back of the seed package for planting depth and spacing. In the image below, I have used ‘diagonal’ spacing as opposed to row spacing. When planting on a diagonal, space onions 4-6 inches apart in all directions.

Caring for Onions

• Fertilizing: Add a side dressing of nitrogen to the soil once or twice during the first two months of the growing season. Sprinkle the nitrogen fertilizer 3-4 inches from the onion plants and work gently into the soil.
• Watering: Because onions have shallow roots, they need a consistent 1-inch of water per week to develop strong roots. Sandy soil requires more frequent watering due to the fast draining nature of this soil type. Stop watering when the bulbs are full size (push up through the soil) and half the tops have fallen over. If onions become drought-stressed, the size of the bulb and the flavor will be impacted.
• Mulch: Use organic mulches (e. g. straw, dry shredded leaves, wood chips) to ensure that the ground retains moisture and maintains soil temperature. Apply 2-3 inches around the base of the plant.
• Row Covers: When I plant fall onion transplants, I protect the crop with row cover. Row cover protects the onions during the winter and keeps the temperature more consistent in the spring. For more information on row cover, check out my post How to use Floating Row Cover in your Organic Garden
• Companions: Common companion crops include beets, broccoli, cabbage, lettuce, and carrots. Avoid planting onions with beans, peas, or sage.
• Crop rotation: Practice crop rotation to reduce disease. Plant in a bed where you have not grown onions, chives, leeks, shallots, or garlic for the past three years. For more information on how to develop a crop rotation plan for your home garden, check out my post Crop Rotation for the home gardener.
• Seeds: Use onion seeds within a year of purchase.

Potential Problems Growing Onions

The best prevention from problems is raising healthy plants and keeping them healthy:

• Plant onions at the correct time (spring/fall)
• Fertilize with nitrogen once or twice after transplanting
• Rotate crops each year. Do not plant onions in the same bed for 3 years or in beds that grew garlic, chives, leeks, or shallots.
• Do not use seeds that are older than one year.

Here are three problems that could occur:

1. Onion maggot is the most common pest. These are small cream-colored insects that infect the bulb. To protect onions, cover the crop with a light row cover.
2. Common diseases include the following. To prevent disease, plant disease-resistant varieties, and follow good cultural practices (soil, water, sun, mulch).
   • Fusarium basal rot is a fungus that lives in the soil. It lives in soil temperatures between 59° to 90°F and can be present in the soil for many years. This disease will destroy onion roots and prevent the plant from growing.
   • Bacterial soft rot is a disease that causes bulb rot. This disease can occur pre-harvest or while the onions are in storage. Symptoms include softening and/or water-soaking of the bulb tissue
   • Botrytis neck rot is a bacterial disease that usually appears during onion storage. It begins at the neck and turns the affected onion tissue water-soaked and brown.
3. Onions bolting. Bolting refers to the event when a bulb onion sends up a flower stalk. Temperature stress can make this happen. When onions bolt, harvest right away.

If you suspect that something is wrong with your onions and live in the US, check with your Cooperative Extension for help in identifying and determining appropriate controls for pests and disease.

Harvesting and Storing Onions

Harvest bulb onions approximately 95-110 days after they are transplanted (yes, patience is required). Harvest when most of the leaves (tops) have fallen over. If the onion plant has sent up a flower stalk, harvest right away. Look at the video below to see what these flower stalks look like and how to harvest onions.

Onion bulbs must go through a curing process. After harvesting the onion, let the onion dry outdoors for 2-3 days in a shaded location. Next, the onions must cure for an additional 2-3 weeks in a dry shelter on raised slats or hung up. It is important that the onions receive full air circulation during this period.

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After the bulbs are completely dry and have formed the papery skin, cut the stem to 1-2 inches above the bulb. Next, cut the roots off. Store the bulbs in a cool, dry location with good air circulation.

Preparing

Here is some great information that I came across from bonappetit.com about onions and how to use them.

• White Onions are usually the mildest of the onions. Therefore, these are what you would eat raw on salads and sandwiches. Vidalia is a variety of white onions. Use Vidalia onions when a recipe calls for ‘sweet’ onions.
• Red Onions are distinguished by their color, sweetness, the sharpness of taste, and intensity of smell. They are prime candidates for pickling and grilling. Their red hue color provides a nice color that other onions do not have.
• Yellow Onions, also known as Spanish onions, are cooked most of the time due to the sharpness of their flavor. Cooking cuts the sharpness and allows the sweetness to shine. They are the best onion for caramelizing.

In conclusion, use any one of the varieties when you are just sautéing onions in butter or olive oil.

Recommended cultivars

Long day (zone 6 and lower)

• Walla Walla Sweet (Sweet and mild)
• Cabernet Red (Red Onion)

Intermediate or day neutral (any zone, does best in 5 and 6)

• Candy Onion (Sweet and mild)

Short Day (zone 7 and higher)

• Yellow Granex (Sweet White Georgia Vidalia)

Botanical Information

• Family: Amaryllidaceae
• Genus:   Allium 
• Species: A. cepa

References

• North Carolina Cooperative Extension
• University of Minnesota Extension
• University of Missouri