How To Plant Onion Plants

Mastering Onion Cultivation: A Comprehensive Guide to Planting Onion Plants

Onions (Allium cepa) are a foundational ingredient in countless culinary traditions worldwide, prized for their pungent flavor, versatility, and nutritional value. Growing your own onions offers a rewarding experience, providing fresh, flavorful bulbs superior to many store-bought varieties. This comprehensive guide will walk you through every stage of planting onion plants, from selecting the right starting material to harvesting, curing, and storing your bountiful crop. By understanding the specific needs of these fascinating plants, you can ensure a successful harvest, even as a novice gardener.

The Allium Family

Onions belong to the Allium genus, a diverse family that also includes garlic, leeks, chives, and shallots. While sharing some characteristics with their relatives, bulb onions have distinct requirements for optimal growth and bulb formation. They are biennials, meaning they typically complete their life cycle over two years: producing a bulb in the first year and then flowering and setting seed in the second. However, for culinary purposes, onions are cultivated as annuals, harvested at the end of their first growing season when the bulb is fully developed.

The journey to growing robust, healthy onion plants begins with careful planning and an understanding of the fundamental principles that govern their growth. From determining the ideal planting time for your specific region to preparing the perfect soil, each step contributes significantly to the final yield and quality of your harvest. With attention to detail and consistent care, you can enjoy a steady supply of homegrown onions, elevating your cooking and gardening satisfaction.

Choosing Your Onion Starting Material

The first crucial decision when planning your onion patch is selecting the appropriate starting material. Gardeners typically choose from three main options: seeds, sets, or transplants. Each method has its own advantages and disadvantages, making the best choice dependent on your climate, available time, and desired outcomes.

Planting from Seeds

Starting onions from seed offers the widest selection of varieties, including many unique types not available as sets or transplants. It is also the most economical option, especially if you plan to grow a large quantity of onions. However, onions started from seed require a longer growing season and typically need to be started indoors 8-12 weeks before the last frost date in cooler climates. This method demands more time and attention in the initial stages, providing consistent warmth, moisture, and adequate light for delicate seedlings.

• Pros: Greatest variety selection, most economical, less prone to bolting (premature flowering) when properly started.
• Cons: Long growing season required, demands indoor starting and care, initial seedlings are delicate.
• Considerations: Requires a reliable light source (grow lights recommended) and controlled environment for germination and early growth.

Planting from Onion Sets

Onion sets are small, immature onion bulbs, typically about 1/2 to 3/4 inch in diameter. They are the easiest and fastest way to grow onions, making them an excellent choice for beginner gardeners or those with shorter growing seasons. Sets are less susceptible to early pest and disease issues and establish quickly in the garden. They are planted directly into the garden once the soil can be worked in early spring.

• Pros: Easiest and fastest method, good for beginners, quicker maturity, generally more disease-resistant.
• Cons: Limited variety availability, can be prone to bolting if large sets are used or if subjected to significant temperature fluctuations, slightly more expensive than seeds.
• Considerations: Choose sets that are not too large, as larger sets are more likely to bolt.

Planting from Transplants (Starts)

Onion transplants, also known as starts, are young, actively growing onion plants usually purchased from nurseries or garden centers. They offer a good balance between the variety selection of seeds and the convenience of sets. Transplants provide a significant head start on the growing season, bypassing the initial indoor seeding phase. They are typically planted after the danger of hard frost has passed, once the soil has warmed sufficiently.

• Pros: Offers a good range of varieties, provides a head start, less prone to bolting than large sets, eliminates indoor starting.
• Cons: More expensive than seeds or sets, availability can be limited to specific periods, can experience transplant shock if not handled carefully.
• Considerations: Select healthy, vigorous transplants with green, upright leaves. Trim leggy tops to about 4-6 inches before planting to reduce transplant shock.

Ultimately, the best starting material depends on your specific needs and gardening experience. Many gardeners experiment with a combination of methods to enjoy a diverse and continuous harvest.

Understanding Onion Varieties and Day Length

One of the most critical factors for successful onion cultivation, often overlooked by new gardeners, is understanding the concept of day length sensitivity. Onion varieties are categorized based on the number of daylight hours required to trigger bulb formation. Planting the wrong type for your geographical region will result in lush green tops but small or non-existent bulbs.

Short-Day Onions

Short-day onions are genetically programmed to begin bulbing when the day length reaches approximately 10-12 hours. These varieties thrive in southern regions (roughly USDA Zones 7 and warmer) where winters are mild, allowing for fall planting, and summers bring ample warmth but not excessively long days. They typically mature earlier in the season.

• Best for: Southern United States, Gulf Coast, and other regions with mild winters and less extreme summer day lengths.
• Characteristics: Generally milder, sweeter flavor due to lower sulfur content; tend to have a shorter storage life.
• Examples: ‘Granex’, ‘Vidalia’, ‘Grano’, ‘Georgia Sweet’.

Long-Day Onions

Long-day onions require a longer period of daylight, typically 14-16 hours, to initiate bulb development. These varieties are perfectly suited for northern regions (roughly USDA Zones 6 and colder) where summer days stretch significantly longer. They use these extended daylight hours to maximize photosynthesis, leading to larger, more robust bulbs.

• Best for: Northern United States, Canada, and other regions with distinct seasons and long summer days.
• Characteristics: Often possess a stronger, more pungent flavor and excellent storage capabilities due to higher sulfur content.
• Examples: ‘Walla Walla Sweet’, ‘Yellow Sweet Spanish’, ‘Redwing’, ‘Stuttgart’.

Intermediate-Day (Day-Neutral) Onions

Intermediate-day onions, sometimes referred to as day-neutral, are a versatile option, bulbing when day length reaches approximately 12-14 hours. This makes them suitable for transitional zones, typically the mid-latitudes (USDA Zones 5-7), where they perform well without the extreme day length requirements of short-day or long-day types.

• Best for: Middle regions of the United States and similar climates.
• Characteristics: Offer a good balance between flavor and storage potential, often providing a moderate pungency and decent shelf life.
• Examples: ‘Candy’, ‘Super Star’, ‘Red Candy Apple’.

To determine the correct day-length onion for your garden, simply identify your geographical latitude. Most seed and plant suppliers will clearly label their onion varieties as short-day, long-day, or intermediate-day, often with recommendations for suitable growing zones. Selecting the appropriate type is the single most important factor in ensuring your onions form substantial, harvestable bulbs.

Site Selection and Soil Preparation

The success of your onion crop hinges significantly on choosing the right location and meticulously preparing the soil. Onions are not particularly fussy once established, but they demand specific conditions to thrive, especially during their critical bulbing phase.

Sunlight Requirements

Onions are sun-loving plants. They require a minimum of 6-8 hours of direct sunlight daily to develop strong foliage and large, healthy bulbs. Insufficient sunlight will lead to spindly tops and small, underdeveloped bulbs. When selecting your planting site, observe how the sun moves across your garden throughout the day to ensure the chosen spot receives adequate exposure, especially during the peak growing months.

Soil pH and Structure

The ideal soil pH for onions ranges from 6.0 to 7.0, which is slightly acidic to neutral. A soil test is highly recommended to determine your current pH levels and nutrient profile. Based on the test results, you can amend your soil: add agricultural lime to raise pH or elemental sulfur to lower it. Beyond pH, soil structure is paramount. Onions prefer loose, well-drained, and fertile soil. Heavy, compacted clay soils are detrimental as they restrict root growth and can lead to waterlogging, promoting disease.

Enhancing Soil Fertility and Drainage

Regardless of your existing soil type, incorporating ample organic matter is crucial for onion success. Work in 2-4 inches of well-rotted compost, aged manure, or leaf mold into the top 10-12 inches of your soil. This not only improves drainage and aeration in heavy soils but also increases water retention in sandy soils. Organic matter also provides a slow-release source of essential nutrients and fosters a healthy microbial environment, which benefits nutrient uptake by the onion roots.

If your soil is particularly heavy clay and poor draining, consider planting onions in raised beds. Raised beds offer excellent drainage, allow for easy customization of soil composition, and warm up more quickly in spring, providing an optimal growing environment for onions.

Crop Rotation

To prevent the buildup of soil-borne diseases and pests specific to the Allium family, practice crop rotation. Avoid planting onions, or any other alliums, in the same spot year after year. A rotation cycle of at least three years is ideal. Instead, follow onions with non-allium crops like legumes (beans, peas), brassicas (cabbage, broccoli), or solanaceous plants (tomatoes, peppers) to break disease cycles and maintain soil health.

The Planting Process: Step-by-Step

With your site selected and soil prepared, the next step is planting. The timing and specific techniques vary slightly depending on whether you’re using seeds, sets, or transplants.

Timing for Your Region

The optimal planting time for onions is generally in early spring, as soon as the soil can be worked. This is typically 4-6 weeks before your average last frost date. Onions are cool-season crops that can tolerate light frosts, but prolonged periods of freezing temperatures after germination or transplanting can cause bolting. In very mild winter climates, short-day varieties can be planted in late fall for an early summer harvest.

Planting from Seeds (Indoors)

If starting seeds indoors, begin 8-12 weeks before your intended outdoor transplant date. Fill seed starting trays or shallow containers with a sterile, well-draining seed starting mix. Plant seeds about 1/4 inch deep and 1/2 inch apart. Water gently to moisten the soil, and maintain consistent moisture. Germination typically occurs in 7-10 days at temperatures between 65-75°F (18-24°C). Once seedlings emerge, provide strong light (grow lights are essential) to prevent them from becoming leggy. When seedlings are 2-3 inches tall and have developed a few true leaves, thin them to 1/2 to 1 inch apart. Before transplanting outdoors, harden off the seedlings by gradually exposing them to outdoor conditions over 7-10 days.

Planting from Sets

When planting onion sets, select those that are approximately 1/2 to 3/4 inch in diameter. Larger sets tend to bolt rather than form large bulbs. Create a shallow furrow in your prepared garden bed. Plant the sets with the pointed end facing upwards, gently pushing them into the soil until just the very top 1/4 to 1/3 of the set is visible above the soil surface. They should be planted about 1 inch deep. Deeper planting can restrict bulb development.

Planting from Transplants (Starts)

Transplants are typically 4-6 inches tall when purchased. If their tops are excessively long and leggy, you can trim them back to about 4-6 inches to reduce transplant shock and encourage root development. Similarly, if the roots are very long, a slight trim can be beneficial. Dig small holes in your garden bed. Place each transplant so that the white, bulbous part is just below the soil surface, and the roots are fully covered. The planting depth should be similar to how they were growing in their nursery container, usually about 1 inch deep for the root ball.

Spacing and Arrangement

Proper spacing is crucial for bulb development. For bulbing onions, plant them 4-6 inches apart within rows. If you are planting in traditional rows, allow 12-18 inches between rows to facilitate weeding and air circulation. If you’re growing in raised beds or intensively, you can plant in a grid pattern, maintaining the 4-6 inch spacing in all directions. Closer spacing will result in smaller bulbs, while wider spacing encourages larger bulbs. If you intend to harvest some as green onions (scallions), you can initially plant them 2-3 inches apart and thin out every other plant for green onions, leaving the remaining plants to bulb.

Initial Watering

Immediately after planting, water thoroughly to settle the soil around the sets or transplants and provide essential moisture. This also helps to eliminate any air pockets that could dry out delicate roots.

Essential Post-Planting Care

Once your onion plants are established, consistent care is vital for their development, ensuring robust growth and the formation of sizable, healthy bulbs. Neglecting post-planting care can significantly diminish your harvest.

Watering

Onions have shallow root systems and are sensitive to inconsistent moisture. They require consistent, deep watering, especially during dry spells and throughout the critical bulb-enlargement phase. Aim for about 1 inch of water per week, either from rainfall or irrigation. Check the soil moisture regularly by feeling the top few inches; if it’s dry, it’s time to water. As the bulbs mature and the tops begin to yellow and fall over (typically in the last 2-3 weeks before harvest), gradually reduce watering to encourage hardening of the bulbs and improve their storage potential. Avoid overhead watering in the late afternoon or evening, as this can promote fungal diseases.

Fertilizing

Onions are heavy feeders, particularly requiring nitrogen for vigorous leafy growth, which in turn fuels bulb development. Before planting, ensure your soil is enriched with organic matter and a balanced fertilizer. During the growing season, side-dress your onion plants with a nitrogen-rich fertilizer every 2-3 weeks until the bulbs begin to swell to approximately the size of a golf ball. Good organic options include blood meal, compost tea, or a balanced granular fertilizer like 10-10-10. It is crucial to stop nitrogen fertilization once the bulbs start to enlarge, as continued nitrogen can encourage lush top growth at the expense of bulb development and can also reduce storage quality.

Weed Control

Onions are poor competitors against weeds. Weeds steal vital nutrients, water, and sunlight, severely stunting onion growth and reducing yields. Regular weed control is paramount. Hand weeding is the most effective method, as deep cultivation can damage the shallow onion roots. Cultivate very shallowly and carefully if using tools. Applying a 1-2 inch layer of organic mulch, such as straw or grass clippings, can significantly suppress weeds, retain soil moisture, and moderate soil temperature. When mulching, ensure the mulch does not directly touch the developing onion bulbs, as this can trap excessive moisture and promote rot.

Pest and Disease Management

While generally resilient, onions can be susceptible to certain pests and diseases. Vigilant monitoring and preventative measures are key to keeping your crop healthy.

• Common Pests:
  • Onion Thrips: Tiny, slender insects that feed on leaves, causing silvery streaks and distorted growth. Use row covers, reflective mulch, or organic insecticidal soaps for control.
  • Onion Maggots: Larvae of a fly that tunnel into the bulbs, causing rot. Practice strict crop rotation, use floating row covers early in the season to prevent adult flies from laying eggs, and ensure good garden sanitation.
  • Cutworms: Larvae that cut young seedlings at the soil line. Use protective collars around young plants or introduce beneficial nematodes.
• Common Diseases:
  • Downy Mildew: Fungal disease causing pale, elongated spots on leaves, often fuzzy. Ensure good air circulation, proper spacing, and use disease-resistant varieties.
  • Botrytis Leaf Blight: Causes small, white, sunken spots with green halos on leaves. Proper spacing, good drainage, and avoiding overhead watering help prevent it.
  • White Rot: A serious soil-borne fungal disease that causes yellowing, wilting, and a white, cottony growth at the base of the bulb. Practice strict crop rotation (at least 5 years without alliums) and remove infected plants immediately.

Maintain good garden hygiene by removing plant debris and rotating crops to minimize pest and disease pressure. Healthy, well-cared-for plants are naturally more resistant to problems.

Harvesting Your Onions

The culmination of your hard work in growing onions is the harvest. Knowing when and how to harvest is essential for obtaining full-sized, flavorful bulbs that will store well.

When to Harvest

The timing for harvesting bulbing onions typically ranges from 100-175 days after planting from seed, or 60-90 days from sets or transplants. The most reliable indicator that your onions are ready for harvest is when their green tops begin to yellow, soften, and fall over naturally. This process signifies that the plant has ceased putting energy into leaf growth and has redirected it to the bulb. Aim to harvest when about 50-75% of the tops have fallen over and started to dry. Harvesting too early can result in smaller bulbs that don’t store well, while harvesting too late might lead to renewed root growth or rotting in the ground.

For green onions (scallions), you can harvest them at any stage once their green tops are sizable enough for your culinary needs. Simply pull individual plants from the ground as desired.

The Harvesting Process

To harvest bulbing onions, choose a dry, sunny day. Gently loosen the soil around the bulbs with a garden fork or trowel to avoid damaging them. Carefully pull each onion bulb from the soil. Handle them with care to prevent bruising, as bruised onions will not store well. If the weather is wet or rain is anticipated, try to harvest before the ground becomes saturated, as wet onions are more prone to rot during curing.

Once harvested, do not wash the onions. Excess moisture can encourage rot. Instead, gently brush off any loose soil clinging to the bulbs.

Preparing for Curing

After harvesting, the onions need to be prepared for the crucial curing process. Lay the harvested onions in a single layer in a warm, dry, and well-ventilated location. This area should also be out of direct sunlight, as intense sun can cause sunscald, damaging the outer layers of the bulbs. Ideal locations include a covered patio, garage, shed, or under a porch. Good air circulation is paramount during this initial stage to begin the drying process and prevent mold or rot from setting in.

Curing and Storage

Curing is a critical post-harvest step that significantly impacts the storage life of your onions. Without proper curing, onions are highly susceptible to rot and sprouting, greatly reducing their longevity.

The Curing Process

Curing involves drying the outer layers of the onion, sealing the neck, and allowing the remaining moisture in the bulb to evaporate slowly. This creates a protective, papery skin that prevents moisture loss and entry of pathogens. The curing process typically takes 2-4 weeks, depending on the humidity and temperature of your chosen location. During this time, the onion tops and roots will shrivel and dry completely, and the outer skins will become papery and crisp. The neck of the onion, where the leaves meet the bulb, should feel completely dry and tight.

Once fully cured, use clean shears or scissors to trim the roots to about 1/2 inch from the bulb. Trim the dried tops to 1-2 inches above the bulb, being careful not to cut too close, as this can break the protective seal and allow moisture or disease into the onion. Leaving a short neck provides an extra layer of protection.

Optimal Storage Conditions

After curing, store your onions in a cool (ideally 40-50°F / 4-10°C), dry, dark, and well-ventilated location. Good air circulation is essential to prevent moisture buildup and subsequent spoilage. Suitable storage places include a root cellar, an unheated basement, a cool pantry, or even a breathable mesh bag, old pantyhose, or slatted wooden crates where air can freely circulate around each bulb. Avoid storing onions in plastic bags, as these trap moisture and promote rot.

It is important to avoid storing onions in high humidity, which can lead to sprouting or rotting. Similarly, protect them from freezing temperatures, which will damage the bulbs. Direct sunlight can also cause onions to green and sprout prematurely. Periodically inspect your stored onions and remove any that show signs of softening, sprouting, or spoilage to prevent them from affecting other bulbs.

Storage Duration

The length of time onions can be stored varies significantly by variety. Mild, sweet, short-day onions typically have a shorter shelf life, lasting anywhere from 1-3 months. Their higher water content and lower sulfur compounds contribute to their sweet flavor but reduce their keeping quality. Pungent, long-day varieties, on the other hand, are bred for long-term storage and can often last for 6-12 months under optimal conditions. Their robust flavor is due to higher sulfur content, which also acts as a natural preservative. Always check the storage characteristics of the specific onion varieties you choose to grow.

Troubleshooting Common Onion Growing Issues

Even with careful planning and consistent care, gardeners may encounter challenges when growing onions. Understanding common problems and their solutions can help you salvage your crop and improve future harvests.

Onions Not Bulbing

If your onion plants produce lush green tops but fail to form substantial bulbs, the most common culprit is planting the wrong day-length variety for your region. Short-day onions planted in a long-day region, or vice versa, will not receive the correct photoperiod to trigger bulb initiation. Other causes include insufficient sunlight (onions need 6-8 hours of direct sun), overcrowding, or a lack of essential nutrients, particularly phosphorus and potassium during the bulb formation phase.

• Solution: Always select the correct day-length variety for your geographical latitude. Ensure your planting site receives full sun, space plants appropriately (4-6 inches apart), and provide balanced fertilization, reducing nitrogen once bulbs begin to swell.

Onions Bolting (Flowering Prematurely)

Bolting occurs when an onion plant sends up a flower stalk and attempts to set seed before the bulb has fully developed. While natural for biennials, it’s undesirable for bulb production. Bolting is often triggered by stress, such as sudden temperature fluctuations (a cold snap after a period of warmth) or using onion sets that are too large (over 3/4 inch in diameter). Over-fertilization with nitrogen late in the season can also contribute.

• Solution: Plant onion sets that are appropriately sized. Protect young plants from extreme temperature swings, if possible. Once an onion bolts, the bulb will stop enlarging and its quality will deteriorate; harvest and consume it immediately, as it won’t store well.

Small Bulbs

Producing small bulbs despite healthy top growth can be frustrating. This issue often stems from a lack of consistent moisture, insufficient nutrients, or overcrowding. Onions need a steady supply of water and nutrients, especially nitrogen early on and then potassium/phosphorus for bulb development. Competition from too many neighboring plants or weeds will also limit bulb size.

• Solution: Ensure consistent, deep watering, aiming for 1 inch per week. Follow a regular fertilization schedule as outlined in the “Post-Planting Care” section. Maintain proper spacing between plants and diligently remove all weeds to eliminate competition.

Yellowing Leaves

Yellowing leaves can indicate several problems. If older, outer leaves are turning yellow first, it often points to a nitrogen deficiency, as the plant pulls nitrogen from older leaves to support new growth. However, overall yellowing combined with wilting can signal overwatering or poor drainage, leading to root rot. Pests like thrips or certain diseases can also cause discoloration.

• Solution: If nitrogen deficiency is suspected, apply a nitrogen-rich fertilizer. For overwatering, improve drainage and adjust your watering schedule. Inspect plants for signs of pests or diseases and address them promptly with appropriate organic controls.

Pungent Flavor or Poor Storage

The flavor and storage duration of your onions are largely determined by the variety you select, but post-harvest practices also play a significant role. If your sweet onions are unexpectedly pungent, or if your onions are rotting quickly despite proper variety choice, curing and storage conditions are likely at fault.

• Solution: Always choose varieties known for your desired flavor and storage characteristics. Ensure a thorough curing process (2-4 weeks in a warm, dry, well-ventilated area). Store cured onions in a cool, dry, dark place with good air circulation to maximize their shelf life.

By understanding these common issues and implementing the suggested solutions, you can effectively manage challenges and cultivate a successful, abundant crop of homegrown onions.