Onion – Growing Guide

Overview

• Name: Onion
• Botanical Name: Allium cepa
• Plant Family: Amaryllidaceae
• Native to: Central Asia, though onions have been cultivated worldwide for thousands of years.

Description

Onions are biennial plants, typically grown as annuals, primarily for their edible bulbs and green shoots. In their first year of growth, onions form a basal rosette of long, cylindrical leaves and develop a bulb underground. The plant’s leaves are hollow, tapering, and bluish-green in color, emerging from a single point at the base. These leaves can reach up to 30–50 cm (12–20 inches) in length.

Onions have a fibrous root system that extends shallowly into the soil, supporting the plant’s upright growth. The bulb, which is the most widely harvested part of the plant, varies significantly in size, shape, and color—ranging from small, round to flattened globes, with outer layers that may be white, yellow, or red, depending on the variety.

During the second year of growth, if allowed to mature, onions will send up a tall flowering stalk, typically reaching 1.2–1.5 meters (4–5 feet) in height. The stalk is hollow, round, and slightly swollen at the base. At the top of the stalk, a spherical umbel of small, white to pinkish flowers forms, which eventually produce seeds. These flowers are arranged in a dense, rounded cluster, and each individual flower has six tepals, six stamens, and a single pistil.

As biennials, onions usually do not produce flowers and seeds until their second year of growth, but in cultivation, they are harvested at the end of the first growing season, before the flowering process begins. In their full life cycle, onions also produce small, black seeds within a capsule after flowering.

Interesting Facts

1. Onions are rich in antioxidants, particularly quercetin, a flavonoid that has been shown to reduce inflammation, lower blood pressure, and support heart health. They also provide a significant amount of vitamin C, which is essential for immune function, skin health, and wound healing. The sulfur compounds in onions, responsible for their pungent aroma and flavor, have been linked to anti-inflammatory and antibacterial effects.
2. The tearing effect that occurs when cutting onions is due to the release of a volatile gas, a thiocarbonyl compound called Propanethial S-oxide. When an onion is sliced, its cells are broken, releasing an enzyme called alliinase. This enzyme converts sulfur-containing compounds into sulfenic acid, which quickly rearranges into the gas. When this gas comes into contact with the moisture in your eyes, it forms a mild sulfuric acid, irritating the eyes and triggering tears.
3. Onions held significant cultural and religious importance in ancient Egypt. They were often used as a form of currency and were placed in the tombs of pharaohs as offerings. The Egyptians believed that the concentric rings of an onion symbolized eternal life, making onions a valuable item in both life and death rituals.
4. Onions were introduced to North America by the first European settlers, who brought them on ships as a staple food for their voyages. They were quickly adopted by Native American tribes and became a crucial part of the colonial diet, both for their culinary uses and their medicinal properties. Onions were also valued for their ability to be stored for long periods, making them an essential crop for early settlers.
5. There are hundreds of varieties of onions, categorized mainly by their color—yellow, red, and white. Yellow onions are the most commonly used and are known for their versatility in cooking. Red onions are often used raw in salads and sandwiches for their mild flavor and vibrant color, while white onions are popular in Mexican cuisine for their sharper taste.

Basic Growing Information

Onions are a cool-season crop that grows best in temperatures ranging from 13–24°C (55–75°F), with the optimal growing temperature being around 15–20°C (59–68°F). They are highly adaptable and can be planted in early spring, as soon as the soil can be worked, or in the autumn in milder climates. Onions require a long growing season to form bulbs, typically 90-120 days, depending on the variety and day length sensitivity.

Onions are categorized based on their response to daylight:

• Short-Day Onions: Short-day varieties thrive in regions with shorter daylight hours (10–12 hours) and are best suited for planting in areas with mild winters and warm climates. These onions require about 10-12 hours of daylight to begin bulb formation and are often grown in regions where winters do not experience severe frost. They are typically planted in the fall for a spring harvest. These varieties mature more quickly due to the shorter daylight requirements and milder temperatures. Typically ready to harvest in 12-16 weeks (84-112 days) from seed sowing.
• Intermediate-Day Onions: Intermediate-day varieties require about 12-14 hours of daylight to form bulbs and are adaptable to a wide range of climates, making them suitable for regions with moderate daylight hours. They are often planted in late winter or early spring and mature in late spring to early summer. These varieties strike a balance between the rapid growth of short-day onions and the longer growing periods of long-day onions. Typically ready to harvest in 14-18 weeks (98-126 days) from seed sowing.
• Long-Day Onions: Long-day varieties need about 14-16 hours of daylight to trigger bulb formation and are best suited for regions with long summer days and cooler climates. These onions are usually planted in early spring and harvested in late summer. They have the longest growing period among the three types, which allows them to produce larger bulbs with good storage potential. Typically ready to harvest in 16-20 weeks (112-140 days) from seed sowing.

Onions prefer well-drained, loose, and fertile soil rich in organic matter, with a slightly acidic to neutral pH. They require consistent moisture, especially during bulb formation, but are sensitive to waterlogged conditions, which can lead to rot. Proper spacing is essential to allow adequate bulb development, and mulching can help retain soil moisture and suppress weeds.

• Sun Requirements: Full sun
• Water Requirements: Moderate
• Soil Types and Properties: Well-draining, fertile soil rich in organic matter
• Soil pH: 6.0 – 7.5
• Pot Size for Container Growing: 20cm pot or larger

Planting

• Best Time to Plant:
  • Climates with freezing winters: Plant onion sets (small dormant bulbs) in early spring, as soon as the ground can be worked. For larger, mature bulbs, start seeds indoors 6-8 weeks before the last frost date.
  • Climates with milder winters: Onion sets can be planted in the fall for a late spring harvest.

Note: Check gardening calendars for the best planting times in your location

• Spacing: Plant onions 10-15 cm (4-6 inches) apart in rows spaced 30-40 cm (12-16 inches) apart.
• Seed Sowing Method: Sow seeds in trays, or direct sow seeds or plant sets (bulbs) into the ground.
• Seed Sowing Depth: Plant seeds 1-2 cm (0.5-1 inch) deep; sets should be planted deeper, with the tips just showing above the soil.
• Time to Harvest: Onions are ready to harvest when the tops begin to fall over and yellow, typically 25-34 weeks when planted from seed, sometimes sooner, when planted from seed, depending on the variety.
• Mature Height/Spread: Onions typically grow about 30-45 cm (12-18 inches) tall.
• Grown From: Seeds, sets (dormant bulbs), or transplants (seedlings)

Care and Maintenance

• Watering Tips: Water regularly to keep the soil evenly moist; avoid overwatering as onions do not like soggy soil. If possible, water at the base of the plants to minimize leaf wetness, which can help prevent fungal diseases.
• Pruning: Pruning is generally not necessary for onions, but removing any flower stalks (also known as scapes) that appear is recommended. Onions are biennial plants, and if they start to bolt (produce a flower stalk) in their first year, it can divert energy away from bulb formation. To ensure that the energy remains focused on developing the bulb, cut off any flower stalks as soon as they emerge. This practice helps improve the size and quality of the harvested bulbs.
• Support: Not required.

Preparing and Fertilizing Soil for High-Yield Onion Crops

Before planting onions in early spring, it’s essential to prepare the soil to create ideal growing conditions.

1. Soil Preparation:
   • Start by incorporating (digging in) well-rotted compost into the soil. This improves soil structure, making it loose, friable, and well-draining—conditions that onions prefer.
   • To further enhance soil fertility, add aged manure. This provides a slow-release source of essential nutrients, including nitrogen, phosphorus, and potassium, which are crucial for early root development and overall plant health.
2. Incorporation:
   • Dig the manure and compost into the top 15-20 cm (6-8 inches) of soil a few weeks before planting. This allows time for the materials to break down and release nutrients, ensuring they are readily available during the onions’ early growth stages.
3. Fertilization During the Growing Season – For optimum growth (optional), additional fertilizer can be applied during the growing season at the times described below::
   • Initial Fertilization: At planting time, apply a balanced organic fertilizer, such as one with an NPK ratio of 4-4-4 or 5-5-5, or Blood & Bone (NPK 5-4-1). Use around 100 grams per square meter (approximately 3.5 ounces per 10 square feet or square yard) to support root establishment and moderate leaf growth without overstimulating the foliage.
   • Early Spring: As onions begin active growth and produce leaves, apply a nitrogen-rich fertilizer, such as Blood & Bone, at a rate of 50-100 grams per square meter (1/4 to 1/2 cup per 10 square feet). This promotes strong leaf growth, vital for large, well-formed bulbs.
   • Mid-Spring: About three to four weeks after the initial fertilization, apply a second, lighter dose of Blood & Bone or another balanced organic fertilizer. This sustains growth as the plants prepare for bulb formation.
   • Late Spring: As the bulbing process begins, reduce or cease fertilization, particularly with nitrogen-rich fertilizers. This allows the plant to focus on bulb formation rather than leaf production. Maintain even soil moisture through regular watering and use organic mulch, such as straw, to suppress weeds and conserve moisture.

By incorporating these organic practices and applying fertilizers at the appropriate stages, you can achieve a healthy onion crop with large, flavorful bulbs that store well.

Growing Difficulty

• Easy to moderate, depending on the method of cultivation and local climate conditions.

Growing Tips

• Use mulch around the plants to conserve soil moisture, maintain an even soil temperature, and suppress weeds, , which can compete with onions for water and nutrients.
• Use a crop rotation system to rotate onions to a new garden location each year, in a 3-4 year cycle to minimize the risk of soil-borne diseases.

Companion Plants

• Good companion plants include: Carrots, beets, strawberries, and tomatoes. These companions can help deter onion pests.
• Avoid planting near: Peas and beans, as onions can inhibit their growth.

Common Pests and Diseases

• Pests: Onion thrips and root maggots.
  • Prevention and Control Methods: Use floating row covers or insect exclusion netting to protect crops and practice good crop rotation. Apply appropriate organic insecticides if necessary.
• Diseases: Onion blight and white rot.
  • Prevention and Control Methods: Ensure good air circulation and soil drainage. Avoid overhead watering and remove any infected plants promptly.

Harvesting

• Harvest Time: Onions are ready for harvest when about half of the tops (foliage) have naturally fallen over and begun to yellow. This typically indicates that the bulbs have reached maturity and have stopped growing. Depending on the variety and growing conditions, this stage usually occurs 90-120 days after planting for onions grown from seeds, or slightly earlier for those grown from sets (bulbs) or transplants (seedlings). It’s crucial not to rush the harvest, as harvesting too early can result in smaller bulbs that are not fully developed.
• Harvesting Tips: To harvest onions, gently pull or dig them from the ground, taking care not to bruise or damage the bulbs. Using a garden fork or shovel can help loosen the soil, especially in compacted or heavy soils, making it easier to lift the onions without damaging them.
• Drying: Once harvested, lay the onions in a single layer on the soil surface or on a dry, well-ventilated area to cure. Allow them to dry in the sun for several days, typically 2-3 days, until the outer skins are papery and the necks are thoroughly dried. Proper curing is essential as it reduces the moisture content of the bulbs, helping to prevent rot during storage.
  • In areas with high humidity or frequent rain, it’s advisable to cure onions in a sheltered location, such as a covered porch, barn, or garage, to ensure they dry properly. After curing, trim the roots and cut the tops back to about 2.5-5 cm (1-2 inches) above the bulb. Store the cured onions in a cool, dry, and well-ventilated space. Ideal storage conditions are around 0-5°C (32-41°F) with low humidity to prolong shelf life. Properly cured and stored onions can last several months, providing a long-lasting supply through the winter.

Propagation

• Propagated by seeds, sets (bulbs) or transplants (seedlings). Seeds offer the most varieties but take longer to mature. Sets are easier and faster but can be more prone to disease.

Culinary Uses

Onions are one of the most essential and versatile ingredients in global cuisine, contributing flavor, texture, and depth to a wide variety of dishes. Their culinary applications range from serving as a foundational element in cooking to being the star ingredient in many recipes.

• Raw Use: Onions can be eaten raw, adding a sharp, pungent flavor and crunchy texture to salads, salsas, and sandwiches. Red onions, in particular, are often used raw because of their milder flavor and vibrant color, making them a favorite in dishes like Greek salads, guacamole, and as a topping for burgers.
• Cooking and Flavor Base: Onions are foundational in many cuisines as a base for soups, stews, sauces, and braises. Sautéing onions is often the first step in building flavor for dishes like French onion soup, risottos, and curries. The Maillard reaction, which occurs when onions are cooked, creates a rich, savory flavor that enhances the overall taste of the dish.
• Caramelizing: When cooked slowly over low heat, onions caramelize, releasing their natural sugars and developing a deep, sweet flavor. Caramelized onions are used in dishes such as French onion soup, atop pizzas, in tarts, or as a rich addition to sandwiches and burgers. The process of caramelization transforms the sharpness of raw onions into a mellow, complex sweetness.
• Pickling: Onions can be pickled to add a tangy, acidic bite to various dishes. Pickled onions are commonly used in tacos, salads, and as a garnish for grilled meats. The pickling process softens the texture of the onions and balances their natural sweetness with acidity, creating a refreshing contrast in many dishes.
• Grilling and Roasting: Grilling or roasting onions concentrates their natural sugars and enhances their flavor, making them a delicious addition to grilled meats, vegetables, and kebabs. Roasted onions can also be pureed into sauces or served as a side dish.
• In Soups and Stocks: Onions are a crucial ingredient in making stocks and broths, where they contribute to the aromatic base. French onion soup is a classic example of a dish where onions are the main component, slow-cooked to develop a deep, rich flavor.
• Onion Varieties in Cooking: Different varieties of onions are used for specific culinary purposes.
  • Yellow onions are the most versatile and are commonly used for general cooking due to their balance of sweetness and astringency.
  • White onions are sharper and are often used in Mexican cuisine.
  • Red onions are popular for their color and are typically used raw in salads or as a garnish.
  • Sweet onions, such as Vidalia or Walla Walla, are ideal for raw preparations like salads and sandwiches due to their milder flavor.
• Culinary Pairings: Onions pair well with a wide range of ingredients, including garlic, tomatoes, bell peppers, meats, cheeses, and herbs like thyme and rosemary. In French cuisine, the combination of onions, carrots, and celery, known as mirepoix, serves as a flavor base for many dishes.
• Preservation: Onions can be preserved through drying or dehydrating, turning them into onion powder or flakes, which can be used as a seasoning in various dishes. Additionally, onions can be frozen or turned into condiments like onion jam or chutney, which preserves their flavor for extended use.

Overall, onions are an indispensable ingredient in the kitchen, capable of enhancing and transforming the flavor profile of dishes across different culinary traditions.

In conclusion, growing onions is a rewarding endeavor that provides a staple kitchen ingredient. With proper care and attention, onions can be grown successfully by gardeners of all skill levels, yielding a bountiful crop that can enhance a wide array of dishes.