Successful onion production in North Carolina depends heavily on proper soil selection and careful soil management. Because onions have a shallow root system, they are highly sensitive to poor drainage, compaction, and uneven nutrient and water availability.

Soil Type and Texture
Onions perform best on well-drained sandy loam or loam soils. These soil types provide good aeration, allow rapid drainage after rainfall or irrigation, and promote uniform bulb expansion. Heavy clay soils are generally not recommended unless drainage is significantly improved, as they restrict root growth and increase the risk of bulb rot and other soilborne diseases.

Soil pH and Chemical Conditions
The optimal soil pH range for onions is 6.0 to 6.5. Soils below this range may limit nutrient availability and root growth, while excessively high pH can lead to micronutrient imbalances. Lime applications should be made based on soil test recommendations and incorporated well ahead of planting.

Organic Matter
Moderate levels of organic matter are beneficial for improving soil structure and moisture-holding capacity. However, large amounts of undecomposed organic residues should be avoided, as they may interfere with bulb development and increase disease risk.

Soil Preparation
Fields should be tilled to remove compaction layers and produce a fine, firm seedbed. A smooth, clod-free surface is especially important for direct-seeded onions to ensure uniform emergence and stand establishment. Excessive tillage should be avoided to prevent soil structure degradation.

Crop Rotation and Field History
Onions should be grown in fields with no recent history of Allium crops. A 3–4 year rotation away from onions and related crops is recommended to reduce soilborne diseases and insect pressure. Fields with a known history of white rot or severe onion diseases should be avoided entirely.

Onions in North Carolina may be established using direct seeding or transplants, with the choice depending on desired bulb size, available equipment, and management capacity.

Direct Seeding
Direct seeding can be used successfully for commercial bulb onion production when stands are well managed. Seed is typically planted in late fall or late winter (approximately September 10–October 15 or January 15–February 25, depending on location and weather). Seed should be placed ½ to ¾ inch deep.

For standard production, seed 8–12 seeds per foot of row, which generally requires 2–4 lb of seed per acre when two seed lines per bed are used. This spacing typically produces a high yield of medium-sized bulbs. When larger bulbs are desired, precision seeding at 3–4 inches between plants can be used, reducing seeding rates to approximately ½–1 lb per acre, depending on row configuration.

Precision seeding improves stand uniformity and bulb size, which is critical for achieving premium grades. Coated seed is recommended when using belt or cup-type planters, while uncoated seed is preferred for vacuum planters. If plants become excessively large during late fall (greater than pencil diameter), shallow undercutting approximately 2 inches below the soil surface may be used to slow growth and reduce the risk of bolting.

Transplants
Transplanting provides more uniform stands and predictable bulb sizing. Transplants are typically set from late December through February. Plants should be approximately 6 inches tall and ½ the diameter of a pencil at transplanting.

Transplants should be set at a uniform depth, with the basal plate positioned 1–1.5 inches below the soil surface. Spacing depends on the target bulb size:

• 2–4 plants per foot of row for large or jumbo bulbs

• 4–6 plants per foot of row for medium-sized bulbs

Uniform spacing is critical for consistent bulb development. The use of markers or mechanical guides during transplanting is strongly recommended to maintain even plant distribution and maximize marketable yield.

Careful cultivar selection is critical for successful onion production in North Carolina due to the crop’s sensitivity to temperature and daylength. Because onions are a cool-season crop, planting too early in the fall can increase the risk of bolting (premature seedstalk formation). Bolted onions produce small bulbs with thick necks that are difficult to cure and generally unmarketable. When onions are planted within recommended windows, bolting is uncommon unless plants are exposed to prolonged or extreme cold conditions.

Photoperiod Response and Bulbing

Onion cultivars are classified based on the daylength required to initiate bulbing, not on planting date. Regardless of when they are planted, onions will not form bulbs until daylength reaches the cultivar’s critical threshold.

• Short-day onions bulb at approximately 10–12 hours of daylength and are best suited for eastern North Carolina, typically producing bulbs for May–June harvest.

• Intermediate-day onions bulb at approximately 12–14 hours of daylength and may extend harvest into June and early July.

• Long-day onions bulb at 14–16 hours of daylength. While traditionally not recommended for the Southeast, newer long-day cultivars have shown improved adaptation, including strong bolting resistance and good bulb size when overwintered under North Carolina conditions.

In addition to proper daylength response, cultivars with a small neck diameter at maturity are preferred, as they cure more easily and have better storage and market quality.

Bolting Tolerance and Overwintering

Cold tolerance varies among cultivars. For example, cultivars such as Juno exhibit high cold tolerance but tend to be more pungent than Grano-type onions. Breeding efforts have produced intermediate- and long-day cultivars with enhanced bolting resistance, good overwinter survival, and the ability to produce large, uniform bulbs when planted in fall and held through winter. Some cultivars overwinter particularly well and offer expanded harvest flexibility for North Carolina growers.

Recommended Onion Varieties for Eastern North Carolina

Recommendations for Growers

• Match cultivar daylength requirement with the intended planting window and harvest timing.

• Avoid planting too early in fall to reduce bolting risk.

• Consider newer intermediate- and long-day cultivars where overwintering conditions and management allow, especially for extending harvest into late June and July.

• Trial unfamiliar cultivars on limited acreage before large-scale adoption.

Proper alignment of cultivar selection with planting date, climate conditions, and market goals is essential to minimize bolting, maximize bulb size, and improve profitability in North Carolina onion production systems.

Soil testing should be conducted prior to planting to determine lime and nutrient requirements. Soil pH should be adjusted to an optimal range of 6.0 to 6.5. Phosphorus (P), potassium (K), and recommended micronutrients should be applied preplant and incorporated uniformly into the bed to ensure availability in the shallow root zone.

Nitrogen Management

Nitrogen (N) is the most critical nutrient for onion production in North Carolina. Under typical commercial conditions, growers should target a total seasonal nitrogen application of 120 to 150 lb N per acre.

• Apply approximately 30–40% of the total N preplant.

• Apply the remaining nitrogen in split applications during early vegetative growth.

• Nitrogen applications should be reduced or discontinued once bulbing begins.

Excessive late-season nitrogen can delay maturity, increase thick necks, reduce curing efficiency, and negatively affect bulb storability.

Phosphorus and Potassium

Adequate phosphorus is important for early root development and stand establishment, particularly in cool soils. Potassium is essential for bulb firmness, scale development, and overall quality.

• Apply P and K according to soil test recommendations.

• On sandy soils, potassium may be applied in split applications to reduce leaching losses.

Sulfur and Micronutrients

Onions have a relatively high requirement for sulfur (S), which plays a role in plant growth and flavor development. Sulfur deficiencies are more common on sandy, low-organic-matter soils. Zinc and boron deficiencies may also occur in some North Carolina soils and should be corrected only when confirmed by soil or tissue testing.

Fertigation and Nutrient Placement

Drip irrigation systems allow for fertigation, which improves nutrient use efficiency by placing nutrients directly into the active root zone. Fertigation is especially effective for applying nitrogen and potassium in small, frequent doses that match onion nutrient uptake patterns.

Proper irrigation management is critical for onion production in North Carolina due to the crop’s shallow root system and high sensitivity to water stress. Both under- and over-irrigation can significantly reduce bulb size, yield, and market quality. Effective irrigation programs focus on maintaining consistent soil moisture in the active root zone while avoiding saturated soil conditions.

Rooting Depth and Water Uptake

Onions typically extract most of their water from the upper 6–8 inches of the soil profile. As a result, they have limited ability to access deeper soil moisture and are less able to recover from short periods of water stress compared to deeper-rooted vegetable crops.

Irrigation Systems

Drip irrigation is strongly recommended for onion production in North Carolina. Advantages include:

• Improved water-use efficiency

• More uniform soil moisture in the shallow root zone

• Reduced foliar wetting and disease pressure

• Ability to apply nutrients through fertigation

Overhead irrigation may be used but increases the risk of foliar diseases and provides less precise moisture control.

Irrigation Scheduling

Irrigation should be scheduled to maintain soil moisture near field capacity in the upper soil profile. Because onions are sensitive to moisture fluctuations, frequent, light irrigations are preferred over infrequent, heavy applications.

• Early vegetative growth: Maintain uniform moisture to promote canopy development.

• Bulb initiation and enlargement: Critical period for water supply; water stress during this stage reduces bulb size and increases doubling.

• Late season: Gradually reduce irrigation as bulbs reach maturity to promote neck drying, skin development, and improved curing.

Water Stress and Excess Moisture

Water stress at any growth stage can reduce yield, but stress during bulb enlargement is particularly damaging. Symptoms of inadequate irrigation include reduced bulb size, increased pungency, and poor uniformity. Conversely, excessive irrigation or poor drainage can result in shallow rooting, increased disease pressure, delayed maturity, and reduced storage life.

Monitoring Soil Moisture

Soil moisture monitoring tools such as tensiometers, granular matrix sensors, or volumetric water content sensors can help guide irrigation decisions. Monitoring should focus on the upper 6–8 inches of soil, where most onion roots are active.

Effective weed control is essential for successful onion production in North Carolina. Onions are poor competitors with weeds, particularly during early growth, due to their slow emergence, upright growth habit, and shallow root system. Uncontrolled weeds can significantly reduce bulb size, yield, and harvest efficiency.

Critical Weed-Free Period

The most critical period for weed control in onions occurs during the first 6 to 8 weeks after emergence or transplanting. Weed competition during this stage can result in permanent yield loss, even if weeds are controlled later in the season. Maintaining clean fields early is therefore a top priority.

Cultural Practices

• Field selection: Choose fields with low weed pressure and avoid sites with known infestations of difficult-to-control species.

• Crop rotation: Rotating onions with crops that allow aggressive cultivation or different herbicide modes of action helps reduce weed seedbanks.

• Bed preparation: A clean, well-prepared seedbed reduces early weed emergence and improves herbicide performance.

• Mulches: Plastic or organic mulches can be effective in reducing in-row weed pressure in some production systems, though their use is more common in specialty or small-scale operations.

Mechanical Control

Shallow cultivation can be used to manage weeds between rows, but care must be taken to avoid root damage. Cultivation should be:

• Shallow and timely, especially early in the season

• Performed when weeds are small

• Avoided once bulbs begin to enlarge near the soil surface

Hand weeding is often necessary, particularly for in-row weeds, but is labor intensive and should be minimized through early-season control strategies.

Chemical Control

Herbicides are commonly used in commercial onion production and should be selected based on:

• Weed spectrum

• Crop growth stage

• Soil type

Preemergence herbicides are particularly important for early-season weed suppression. Postemergence herbicides may be used to control escaped weeds, but onion tolerance is limited, and applications must be made at labeled growth stages.

Always follow current label directions and consult the North Carolina Agricultural Chemicals Manual for up-to-date recommendations.

Integrated Weed Management

Successful weed control in onions relies on an integrated approach that combines cultural, mechanical, and chemical practices. Early-season weed suppression reduces reliance on hand labor and helps preserve yield potential.

Effective insect and disease management is essential for maintaining yield, bulb size, and market quality in onion production systems. Because onions have limited leaf area and shallow roots, damage from insects or diseases can quickly reduce photosynthetic capacity and bulb development. Successful management relies on early detection, preventative cultural practices, and timely intervention.

Key Insect Pests

Onion Thrips (Thrips tabaci)
Thrips are the most economically important insect pest of onions in North Carolina.

• Feed by rasping leaf tissue and sucking plant juices

• Cause silvery streaking, leaf distortion, and reduced bulb size

• Heavy infestations reduce photosynthesis and delay maturity

Management considerations

• Begin scouting early and monitor regularly, especially during warm, dry conditions

• Maintain adequate fertility and irrigation to reduce plant stress

• Rotate insecticide modes of action to reduce resistance development

• Thorough spray coverage is essential due to thrips feeding in leaf axils

Other occasional insect pests include onion maggot and cutworms, though these are typically less common and more localized.

Major Onion Diseases

Downy Mildew (Peronospora destructor)

• Favored by cool, humid conditions and extended leaf wetness

• Causes pale green to yellow lesions and gray-purple growth on leaves

• Severe infections can result in premature leaf death and yield loss

Purple Blotch (Alternaria porri)

• Common during warm, humid weather

• Characterized by purple lesions with yellow margins on older leaves

• Can reduce bulb size and increase susceptibility to secondary infections

Botrytis Leaf Blight (Botrytis squamosa)

• Produces small white lesions that expand under favorable conditions

• Often develops during cool, moist periods

Soilborne Diseases

• White rot and other soilborne pathogens can persist for many years

• Fields with a known history of severe onion diseases should be avoided

Cultural and Preventative Practices

• Use crop rotations of 3–4 years away from onions and other Allium crops

• Select well-drained fields and use raised beds to reduce moisture-related diseases

• Avoid excessive nitrogen, particularly late in the season

• Promote good airflow by maintaining proper plant spacing

• Manage irrigation to minimize prolonged leaf wetness

Chemical Management

Fungicides and insecticides are often necessary in commercial onion production and should be applied based on:

• Scouting results

• Weather conditions

• Crop growth stage

Preventative fungicide programs are commonly used when environmental conditions favor disease development. Always rotate chemical classes to reduce the risk of resistance.

All pesticide applications must follow current label directions and recommendations outlined in the North Carolina Agricultural Chemicals Manual. Recomendations on the 2025 Southeastern US Vegetable Crop Handbook are also extremelly usefull.

Proper harvest timing and curing are critical for achieving high yields of marketable bulbs and maintaining postharvest quality. Harvesting onions either too early or too late can reduce bulb size, increase disease incidence, and shorten storage life.

Harvest Timing

Onions are ready for harvest when 50–80% of the plants have undergone neck fall, indicating that bulb enlargement is complete. At this stage, tops weaken naturally and fall over, signaling physiological maturity. Harvest should not be delayed excessively once this stage is reached, as overmature bulbs are more prone to disease and sunscald.

Harvest timing may vary depending on cultivar, planting date, and seasonal weather conditions. Fields should be harvested promptly and uniformly once the majority of plants reach maturity.

Harvest Operations

Bulbs are typically undercut to loosen them from the soil and lifted mechanically or by hand. After lifting, onions may be placed in windrows in the field to allow initial drying, provided weather conditions are dry and favorable.

Care should be taken during harvest to minimize mechanical injury, as bruised or cut bulbs are more susceptible to decay during curing and storage.

Proper postharvest handling and curing are essential for maintaining bulb quality, minimizing decay, and extending shelf life. Onions are particularly sensitive to mechanical injury and excess moisture after harvest, and careful handling during this period has a major impact on marketability.

Curing

Curing is the process of drying the neck tissue and outer scales to create a protective barrier against moisture loss and disease. Onions may be cured in the field or in a well-ventilated structure, depending on weather conditions.

• Field curing is suitable under warm, dry conditions and typically requires 7–14 days. Bulbs should be arranged in windrows with tops covering adjacent bulbs to reduce sunscald.

• Shed curing is recommended when rainfall or high humidity prevents effective field drying. Adequate airflow is critical, and forced air systems may be used to accelerate drying. Recomended for NC.

Onions are considered fully cured when the necks are dry and tight and the outer scales are papery and firmly attached.

Postharvest Handling

After curing, onion tops may be trimmed and roots clipped. Bulbs should be handled gently at all stages to avoid bruising or cutting, as injuries serve as entry points for pathogens and reduce storage life. Grading should remove damaged, diseased, or misshapen bulbs prior to storage or marketing.

Storage Conditions

For short-term storage prior to marketing, onions should be held in a cool, dry, and well-ventilated environment. Excess humidity should be avoided, as it encourages sprouting and decay. Storage temperature and relative humidity should be managed to maintain bulb firmness and prevent breakdown.

Disease Prevention

Proper curing is one of the most effective strategies for reducing postharvest diseases. Maintaining dry conditions, avoiding mechanical injury, and removing cull bulbs from storage areas help minimize the spread of decay organisms.

Onions are graded according to size and quality. A high quality pack is obtained by eliminating immature, decayed, sunburned, and mechanically injured bulbs, double bulbs, and bulbs that have started a second growth.

Buyers usually specify minimum size of the onions they will buy. This minimum size is usually 2 inches in diameter although some will buy onions 1.5 inches in diameter. Usually onions 3 to 3.5 inches bring a premium price. Onions are usually sold in 50-pound mesh bags, although fiberboard boxes provide better protection and are becoming more popular.