How to Grow Onions: The Complete Guide from Sets and Seeds to Harvest and Storage

Learn how to grow onions at home — from choosing sets, transplants, or seeds and understanding day-length requirements to planting, fertilizing, watering, and curing for months of storage.

Homegrown onions offer a level of flavor variety that grocery stores rarely match — sweet Vidalia-style slicing onions, pungent storage onions that keep through winter, mild scallions ready in weeks, and striking red onions that add color to any dish. They are also surprisingly easy to grow once you understand the one factor that governs their behavior above all others: day length.

Onions are photoperiod-sensitive crops — they form bulbs in response to specific day-length thresholds that differ by variety type. Plant a short-day variety in the North or a long-day variety in the South, and you’ll get disappointing results despite excellent care. Match variety to latitude, provide consistent moisture and fertilizer, and onions reward you with generous yields of one of the kitchen’s most essential vegetables.

At Outz News Garden, Maria Walker covers the complete onion growing guide — understanding the day-length rule that makes varieties succeed or fail in your region, choosing between sets, transplants, and seeds, planting, fertilizing for large bulbs, managing the weeds and water that determine final size, and harvesting and curing for months of storage. For the companion planting benefits onions provide to surrounding crops, see our companion planting guide.

The Day-Length Rule: The Most Important Onion Concept

According to University of Maryland Extension, onion cultivars have different requirements for the number of hours of daylight required to make a bulb. If the seed catalog lists the variety as long day, it sets bulbs when it receives 15 to 16 hours of daylight and is adapted to Northern summers. Short-day varieties set bulbs with about 12 hours of daylight and are used in the deep South for winter production. There are also “intermediate” cultivars.

In practical terms:

• Long-day onions (15+ hours): for gardeners in the northern US (roughly above 36° latitude — Zone 7 and north). These varieties include most storage onions: Copra, Stuttgarter, Patterson, Yellow Sweet Spanish. They form bulbs during the long summer days of northern regions and are not suitable for the South.
• Short-day onions (12 hours): for gardeners in the southern US (below 36° latitude — Zone 8 and south). Sweet Vidalias and similar mild onions are short-day varieties planted in fall and harvested in spring in warm southern climates. They will not form proper bulbs in the North.
• Intermediate-day onions (13 to 14 hours): the most flexible type — perform reasonably well in both northern and southern gardens. Candy, Red Candy Apple, and Super Star are widely grown intermediate varieties that suit a broad range of US climates. University of Maryland Extension notes that Mid-Atlantic gardeners can experiment with all groups, although long-day and intermediate types will probably perform best.

Getting the day-length category right for your latitude is the single most impactful onion decision. A beautiful, healthy long-day onion planted in Georgia will produce only small, underdeveloped bulbs because the 15-hour threshold is never reached in southern latitudes — not a care failure, but a wrong variety choice.

Onion Types: Bulb Onions, Scallions, and More

According to University of Minnesota Extension, onions (Allium cepa) have relatives that include garlic, chives, leeks, and shallots. The Allium family offers a range of garden onion options beyond the standard storage bulb:

Bulb Onions (Storage and Fresh)

• Yellow storage onions: the most productive and longest-storing type. Pungent flavor that mellows with cooking. Excellent varieties: Copra (long-day; stores 9+ months), Patterson (long-day; excellent disease resistance), Candy (intermediate; milder flavor than standard yellow types).
• Red onions: milder and sweeter than yellow storage types; beautiful color in salads and roasted dishes. Color fades when cooked. Varieties: Red Wing (long-day), Red Candy Apple (intermediate).
• White onions: sharpest flavor of the three colors; used in Mexican and Asian cuisines; generally shorter storage life than yellow types. White Sweet Spanish is a widely grown long-day white variety.
• Sweet onions: low sulfur content produces mild, almost non-pungent flavor excellent for fresh eating. Walla Walla Sweet (long-day), Candy (intermediate), and genuine Vidalia types (short-day). Sweet onions store significantly shorter than pungent storage types — typically only 1 to 4 months.

Scallions and Bunching Onions

University of Minnesota Extension confirms that scallions (Allium fistulosum), also called bunching onions and green onions, have green and white stalks that do not form bulbs. They are the quickest onion harvest available — ready in 60 to 70 days — and are the most forgiving and productive choice for beginners. Scallions can be direct-sown in early spring and harvested throughout the season; new plantings every 2 to 3 weeks provide continuous supply.

Shallots and Multiplier Onions

University of Maryland Extension notes that shallots and multiplier onions can be planted in fall (September to October) or spring (late March to April). Fall-planted shallots produce larger bulbs. These mild, gourmet onion relatives are grown from small cloves planted pointy side up at 4 to 6 inch spacing and are among the easiest alliums to grow.

Starting Onions: Three Methods Compared

Sets (Easiest, Fastest)

According to Penn State Extension, onion sets — small bulbs grown the previous year — are the most beginner-friendly starting option. PSU recommends sets or transplants rather than direct seeding for novice gardeners. Sets are widely available at garden centers in spring and can be planted as soon as the soil can be worked.

• Plant sets with the pointed (root) end down, tip barely below the soil surface (½ to 1 inch deep)
• Space 2 inches apart for scallions; 4 inches for moderate bulbs; 8 inches for large bulbs (per UMD Extension specifications)
• Sets are less expensive and more foolproof than transplants or seeds
• Limitation: variety selection with sets is limited — usually just basic yellow, red, and white types

Transplants (Best Overall Balance)

University of Minnesota Extension notes that some seed companies sell onion transplants — small seedlings that can tolerate light frosts and can be planted when temperatures reach 50°F. Transplants offer the best combination of variety access and ease of establishment. Start your own indoors 10 to 12 weeks before outdoor planting, or purchase transplants from garden centers for immediate planting.

Starting transplants indoors (per UMN guidance):

• Sow seeds ¾ inch deep in a larger flat (not individual cells) — onion and leek seedlings can be started together in one flat and transplanted while still small without harm
• Keep evenly moist; provide grow lights for 14 to 16 hours daily
• At transplanting time: loosen plants to separate, trim roots to ½ inch, trim tops to 4 inches
• Plant 2 inches deep, 3 to 4 inches apart, in rows 12 to 16 inches apart

Direct Seeding (Most Variety Choice, Most Time)

Direct seeding provides the widest variety selection but requires the earliest timing. University of Minnesota Extension specifies direct seeding as soon as the soil is workable in spring — onion seeds germinate at cool temperatures and benefit from the early start. After seedlings emerge, thin to 3 to 4 inches apart.

Site Selection and Soil Preparation

Site Requirements

• Full sun: University of Minnesota Extension specifies that all onions require full sun for the best growth. Overcast skies and cool temperatures during the growing season will delay bulb formation. Onions in partial shade produce smaller bulbs and are more susceptible to disease.
• Well-draining soil: onions are susceptible to fungal diseases in waterlogged conditions. University of Maryland Extension notes that onion bulbs emerge above the soil as they enlarge — a sign that their roots are in the top soil layer and drainage matters greatly.
• Weed-free area: UMD Extension is specific: onions compete poorly with weeds because of their shallow root systems. A weedy onion bed consistently produces smaller bulbs than a clean one.

Soil Preparation

University of Maryland Extension categorizes onions as having a high requirement for nutrients. University of Minnesota Extension specifies that soil for onions should be well-drained and high in organic matter. Practical preparation:

• Work 2 to 3 inches of compost into the top 8 inches before planting
• Apply balanced fertilizer before planting
• Soil pH 6.0 to 7.0 — test and adjust if needed
• Raised beds are excellent for onions, providing the drainage and loose soil that encourage straight, full-size bulb development
• Do not hill soil over developing bulbs: University of Maryland Extension is explicit — do not hill up the soil on onions, as this encourages stem rot. Bulbs naturally emerge above the soil surface as they develop.

Watering and Fertilizing for Maximum Bulb Size

Watering

University of Maryland Extension makes a statement that captures onion watering requirements perfectly: onion bulb size is directly proportional to the amount of water applied to the onions during the growing season. University of Minnesota Extension reinforces this: onions are shallow-rooted and require constant moisture for proper growth, specifying that if the planting does not receive 1 inch of rain each week, soak the soil thoroughly at least once a week.

• Maintain consistent 1 inch of water per week throughout the growing season — more in hot weather or sandy soil
• Water at the soil level — wet foliage increases disease pressure
• Mulch with weed-free straw or herbicide-free grass clippings to a depth of 3 to 4 inches — University of Minnesota Extension specifically recommends this mulch depth to prevent weed growth and reduce the need for frequent cultivation
• Stop watering when bulbs approach maturity: University of Maryland Extension specifies stopping when tops begin to fall. University of Minnesota Extension adds: stop watering when bulbs have reached full size and the tops begin to fall. Continued watering at this stage causes disease and reduces storage quality.

Fertilizing

University of Maryland Extension specifies a three-stage fertilizing approach: apply fertilizer before planting, use starter fertilizer for transplants, and side-dress 1 to 2 weeks after bulb enlargement begins. University of Minnesota Extension specifies: spread fertilizer alongside the row of onions, about six inches away from the plants, and scratch it into the soil.

• Before planting: incorporate balanced granular fertilizer with compost
• Starter fertilizer at transplanting: high-phosphorus solution to support root establishment
• Side-dress with nitrogen when plants are 6 to 8 inches tall and again when bulbing begins — this is the critical fertilizing window for maximum bulb size
• Stop fertilizing when tops begin falling — late fertilizing delays maturity and reduces storage quality

Weeding: More Critical Than Many Realize

University of Minnesota Extension is direct about onion weeding: using a hoe or hand tool, make a shallow cut to kill weeds just below the soil level before they become a problem — do not hoe or chop too deeply, as injured onion bulbs are more prone to disease.

The approach that produces the best results:

• Weed when weeds are small seedlings — at this stage, a quick pass with a hoe eliminates them before they compete
• Cultivate shallowly (1 inch or less) to avoid injuring onion roots and bulbs
• Keep mulch consistent — mulch is the most effective time-saving weed prevention for onion beds
• Early weed control matters most — once onions are well-established with 8 to 10 inch tops, their canopy shades out many subsequent weed seedlings

Harvesting and Curing Onions

Harvest Timing

University of Minnesota Extension’s harvesting timing is clear: harvest onions when about half the tops are falling over and dry. University of Maryland Extension provides additional specificity: bulb onions should be harvested when about two-thirds of the dried tops have fallen over.

The natural falling of tops is the plant’s signal that bulb development is complete — it has redirected its energy from leaf growth to bulb maturation. Harvesting before tops fall produces onions with fresh necks that do not cure well and rot in storage; waiting until all tops fall allows disease to enter through the weakened neck tissue.

Curing for Long Storage

Curing is what converts freshly harvested onions into the firm, papery, long-storing ingredient of the winter kitchen. University of Maryland Extension specifies: onions may be pulled and left in the field for several days to dry, then cured with tops intact in a well-ventilated attic or porch out of the direct sun for 1 to 2 weeks. University of Minnesota Extension is emphatic: curing is essential if you want to store onions. You must dry them in a warm, dry, well-ventilated area.

• Pull onions and lay in the garden for 3 to 5 days if weather is dry — sun-curing rapidly dries the outer layers
• Move to a warm (75 to 80°F), dry, shaded location with excellent airflow for final curing — 2 to 4 weeks total
• Curing is complete when the neck (top of the bulb) is completely dry, tight, and shrunken — no moisture remains in the neck tissue
• Trim tops to 1 inch above the bulb; trim roots close
• Cure harvest on dry, cool days when possible — UMN Extension notes that harvesting on dry, cool days limits post-harvest diseases

Storage

University of Maryland Extension specifies: onions can be kept under very cool (32°F), dry (65 to 70% relative humidity) conditions for up to 6 to 7 months. Practical home storage options:

• Mesh bags or open crates in a cool (32 to 50°F), dry basement or garage — 2 to 6 months depending on variety
• Braided onion tops for traditional hanging storage in a cool location
• Sweet onions store significantly shorter than pungent varieties — use sweet onions first; save storage-type yellow onions for long-term keeping
• Warm temperatures induce sprouting; sub-freezing temperatures injure bulbs — keep within the recommended range

Common Onion Problems

According to Penn State Extension’s onion production guide, many onion diseases can be prevented using good crop rotation, high-quality soil with good air drainage, and disease-resistant onion varieties. The most common home garden issues:

• Small bulbs: most often caused by insufficient light, insufficient water during bulb development, wrong variety for your day-length zone, planting too late, or overcrowding. Address each factor before replanting.
• Bolting (flowering): onions exposed to temperatures below 40°F for extended periods before bulbing can bolt — producing a flower stalk and rendering the bulb unusable for storage. Use appropriate varieties for your climate and planting dates.
• Neck rot in storage: caused by Botrytis fungi entering through poorly cured necks. Ensure necks are completely dry before storing; avoid storing damaged bulbs with healthy ones.
• Onion maggots: larvae that bore into stems, causing yellowing and wilting. Row cover from planting prevents adult flies from laying eggs at plant bases — the most effective preventive measure.
• Thrips: tiny insects causing silvery streaking on leaves. Maintain adequate moisture (stressed plants are more susceptible); use reflective mulch to deter thrips; apply insecticidal soap for heavy infestations.

Quick-Reference Onion Growing Guide

• Choose the right day-length type for your latitude — the single most critical decision
• Full sun all day — no exceptions for good bulb development
• Sets for beginners; transplants for variety access; direct seed for maximum options
• Plant early — as soon as soil can be worked in spring
• Water is directly proportional to bulb size — 1 inch per week consistently
• Weed early and shallowly — onions cannot compete with weeds
• Never hill soil over bulbs — causes neck rot
• Harvest when half to two-thirds of tops have fallen
• Cure 2 to 4 weeks in warm, dry, ventilated conditions before storing

Learning how to grow onions at home adds depth and variety to the kitchen that store-bought onions rarely offer — sweet slicing varieties, pungent long-storing yellows, mild red onions for salads, and continuous scallion harvests through the season. Once you understand the day-length principle that governs variety selection and provide the consistent moisture that builds large bulbs, onions become one of the most rewarding and satisfying vegetable garden crops available.

Start with sets or transplants in your first season — they give the most reliable results with the least knowledge required. Add seed-starting the following year to access the full range of varieties. And discover that homegrown cured onions, braided and hanging in a cool pantry through the winter, are one of the garden’s most satisfying long-term harvests.

Share your onion growing successes and harvest photos in the comments! And for the complete allium family in your garden, see our garlic growing guide.

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Maria

Maria Walker is a certified horticulturist and gardening specialist with over 15 years of hands-on experience in plant care, garden design, and sustainable growing practices.
She holds a Bachelor’s degree in Horticulture Science and a Master’s degree in Sustainable Agriculture — and has spent her career helping people of all skill levels create beautiful, thriving gardens.
Maria launched Outz News Garden with one simple mission: to make gardening accessible and inspiring for everyone, from first-time planters to seasoned green thumbs.