Onions are one of the most useful crops you can grow at home. They take relatively little garden space, store well after harvest, and can be used fresh or cooked in countless meals. Once you understand their unusual relationship with day length, onions are also fairly straightforward to grow.
Unlike many vegetables, onion success depends heavily on choosing a variety suited to your latitude. Some onions begin forming bulbs when days reach about 10 to 12 hours, while others need 14 to 16 hours of daylight. Planting the wrong type can result in healthy green tops but disappointing bulbs.
Onions can be grown from seed, small dormant bulbs called sets, or young transplants. Each method has advantages. Sets are convenient and fast, transplants are reliable for large bulbs, and seeds provide the widest choice of varieties.
For the best harvest, plant onions in full sun, loose well-drained soil, and give them consistent moisture while their leaves are growing. Once the bulbs mature and the tops begin falling over naturally, stop heavy watering, harvest, and cure the onions thoroughly before storage.
🧅 Onion Planting at a Glance
- Botanical name: Allium cepa
- Plant type: Biennial usually grown as an annual vegetable
- Planting options: Seeds, sets, or transplants
- Sun: Full sun
- Soil: Loose, fertile, well-drained soil
- Soil pH: About 6.0 to 7.0
- Plant spacing: About 4 to 6 inches for full-size bulbs
- Row spacing: About 12 to 18 inches
- Planting depth: Sets about 1 inch deep; transplants shallow with roots covered
- Water: Consistent moisture during active growth
- Harvest: When mature tops naturally yellow and fall
- Curing: Usually 2 to 3 weeks in a dry, airy location
- Storage: Cool, dry, well-ventilated conditions
🌱 Should You Grow Onions From Seeds, Sets, or Transplants?
All three methods can produce onions, but they are not exactly interchangeable.
Onion Seeds
Seeds provide the widest range of cultivars and are inexpensive when growing many plants.
They are ideal if you want:
- Specific storage varieties
- Unusual colors
- Heirloom onions
- Precise day-length varieties
The disadvantage is that onions grow slowly from seed and often need to be started indoors well before spring planting.
Onion Sets
Sets are small dormant onion bulbs produced the previous season.
They are easy to plant and establish quickly.
Sets are excellent for beginners, but the variety selection is limited and larger sets may be more likely to bolt.
Onion Transplants
Transplants are young plants sold in bundles or pots.
They combine convenience with better cultivar selection than most sets.
For gardeners trying to produce large storage onions, good-quality transplants are often an excellent choice.
☀️ How Much Sun Do Onions Need?
Onions need full sun.
Aim for at least 6 to 8 hours of direct sunlight each day.
The more energy the leaves can produce, the more potential the plant has to develop a large bulb.
Onions grown in too much shade may produce:
- Thin leaves
- Slow growth
- Small bulbs
- Poor maturity
Do not plant onions beneath tall crops that will shade them later in the season.
🧭 Understanding Short-Day, Intermediate-Day & Long-Day Onions
This is one of the most important parts of onion growing.
Bulb formation is triggered largely by day length.
Different onion varieties have been bred to begin bulbing under different amounts of daylight.
🌞 Short-Day Onions
Short-day onions begin forming bulbs when days reach roughly 10 to 12 hours.
They are generally best suited to southern regions with mild winters.
Common short-day varieties include certain Vidalia-type, Texas, and Granex onions.
They are often planted in fall or winter in warm climates and harvested the following spring.
🌤️ Intermediate-Day Onions
Intermediate-day onions generally begin bulbing when days reach about 12 to 14 hours.
They work well across a broad middle band of the United States.
These varieties can be especially useful where summers are too warm for classic long-day types but winters are too cold for traditional short-day onion production.
🌅 Long-Day Onions
Long-day onions require roughly 14 to 16 hours of daylight to initiate strong bulb formation.
They are best suited to northern regions.
Popular storage onions are often long-day types because long northern summer days provide ideal conditions for bulb development.
⚠️ Why Day Length Matters
An onion can grow perfectly healthy foliage and still fail to produce a worthwhile bulb if the day-length type is wrong for the location.
For example, a long-day onion planted too far south may never receive enough daylight to develop properly.
A short-day onion planted too far north may begin bulbing while the plant is still small, producing tiny onions.
Always check whether a cultivar is:
- Short-day
- Intermediate-day
- Long-day
- Day-neutral, if specifically marketed that way
Choose for your region before worrying about anything else.
📅 When Is the Best Time to Plant Onions?
Onions tolerate cool weather and are usually planted earlier than tomatoes, peppers, or sweet potatoes.
In many climates, onion sets and transplants can go into the garden several weeks before the average last spring frost, once the ground can be worked.
The exact timing depends on:
- Winter severity
- Variety
- Day-length type
- Whether you’re planting seed, sets, or transplants
Southern gardeners may plant short-day onions in fall or winter.
Northern gardeners commonly plant long-day onions in early spring.
❄️ Can Onions Tolerate Frost?
Young onion plants tolerate light frost surprisingly well.
Established plants can usually handle cool spring weather better than warm-season crops.
However, repeated severe freezes can damage plants, and extreme cold can injure young transplants.
If a hard freeze is expected shortly after planting, temporary row cover can provide protection.
🌡️ What Temperature Do Onions Prefer?
Onions establish well during cool weather but mature during increasingly warm conditions.
Seeds germinate in relatively cool soil, though germination becomes faster as temperatures rise.
Foliage grows strongly during mild spring weather.
Later, warming temperatures and increasing day length encourage bulb development.
This transition from cool establishment weather into warmer growing conditions is ideal for onions.
🌱 When to Start Onion Seeds Indoors
If growing onions from seed in a cold-winter climate, start them indoors approximately 8 to 12 weeks before outdoor transplanting.
Onion seedlings grow relatively slowly.
Starting early gives them time to produce enough foliage before they enter their bulb-forming stage.
Sow seeds shallowly in a sterile seed-starting mix.
Keep the mix evenly moist and provide strong light after germination.
✂️ Can You Trim Onion Seedlings?
Yes.
Indoor-grown onion seedlings often become long and floppy.
You can trim the leaves periodically to around 3 to 4 inches tall.
This makes seedlings easier to manage and does not normally harm them.
Use clean scissors.
Do not cut so severely that almost no green foliage remains.
🌱 Best Soil for Onions
Onions grow best in loose, fertile, well-drained soil.
Bulbs need room to expand without fighting dense clods or compacted clay.
Ideal soil is:
- Loose
- Rich in organic matter
- Moisture-retentive
- Well drained
- Free of stones and severe compaction
Raised beds are particularly useful for onions where native soil is heavy.
🧪 What Soil pH Do Onions Prefer?
Onions generally grow best in slightly acidic to neutral soil with a pH around 6.0 to 7.0.
If the soil is extremely acidic, growth and nutrient uptake can suffer.
A soil test is the best way to determine whether amendments are actually necessary.
Avoid adding lime simply because onions prefer near-neutral soil.
If the existing pH is already suitable, lime can push it too high.
🌿 Preparing the Onion Bed
Prepare the bed before planting.
Remove perennial weeds and loosen compacted soil.
Incorporate compost if the soil needs additional organic matter.
Onions have relatively shallow root systems, so they compete poorly with weeds.
A clean, loose planting bed makes early growth much easier.
Avoid burying large amounts of uncomposted organic material directly beneath the plants.
🧅 How to Plant Onion Sets
Plant onion sets with the pointed end facing upward.
Set them approximately 1 inch deep, leaving the neck near the soil surface.
Do not bury sets several inches deep.
Deep planting can interfere with bulb development.
Space sets according to their intended use.
For full-size onions, leave approximately 4 to 6 inches between plants.
For green onions, they can be planted more closely.
📏 How Far Apart Should You Plant Onions?
For mature bulb onions, a good general spacing is:
- 4 to 6 inches between plants
- 12 to 18 inches between rows
Closer spacing produces smaller bulbs.
Wider spacing gives individual plants more room to develop larger onions.
If you want to harvest some young plants as green onions, plant more closely initially and thin them as they grow.
Eat the thinnings rather than wasting them.
🌱 How to Plant Onion Transplants
Onion transplants are often sold in bundles with exposed roots.
Before planting:
- Separate the plants gently.
- Remove any dead material.
- Trim extremely long roots slightly if necessary for easier planting.
- Place roots into a shallow hole.
- Cover the roots while leaving the growing point above soil.
Do not bury the entire white stem deeply.
A shallow planting encourages bulbs to expand naturally near the soil surface.
🌿 Should Onion Bulbs Be Covered With Soil?
As onions mature, the tops of the bulbs often become visible above the soil.
This is normal.
Do not repeatedly mound soil over developing onion bulbs.
Bulbs expand partly above the soil line and do not need to be completely buried.
Hilling soil over them can increase the risk of rot and interfere with normal maturation.
🌱 How to Plant Onion Seeds Directly Outdoors
Onion seed can also be sown directly outdoors where the growing season is long enough.
Sow seeds roughly ¼ to ½ inch deep.
Keep the soil consistently moist during germination.
Once seedlings are established, thin them gradually.
Direct seeding works especially well in climates with long cool growing seasons or for gardeners growing bunching onions rather than very large storage bulbs.
🌿 Growing Onions for Green Onions
If your goal is green onions or scallions rather than large bulbs, day length becomes less important.
Harvest plants while they are young and the bases remain relatively small.
You can sow seeds or plant sets fairly closely together.
Green onions can be harvested whenever the stems become large enough to use.
This is a useful way to make use of onion thinnings.
💧 How Often Should You Water Onions?
Onions perform best with consistent moisture during active growth.
Their shallow roots mean they cannot draw water from deep soil as efficiently as some crops.
Aim for roughly 1 inch of water per week, adjusting for rainfall, temperature, and soil type.
More may be needed during hot, dry weather.
Avoid allowing the bed to alternate repeatedly between extreme dryness and saturation.
🌅 What Time of Day Should You Water Onions?
Morning is best.
Water at soil level whenever possible.
Drip irrigation or a soaker hose keeps foliage drier and delivers moisture directly to the root zone.
If using overhead irrigation, water early enough that foliage dries during the day.
🚫 Can You Overwater Onions?
Yes.
Constantly saturated soil can cause:
- Root rot
- Bulb rot
- Weak roots
- Yellow leaves
- Disease
Good drainage is essential.
If your soil remains waterlogged after heavy rain, raised beds may provide a better growing environment.
🌾 Do Onions Need Fertilizer?
Onions benefit from fertile soil because they must produce substantial foliage before bulb formation begins.
Nitrogen is particularly useful during the early leafy growth stage.
However, timing matters.
Once bulbs begin enlarging, excessive nitrogen can delay maturity and encourage continued leaf growth.
A soil test provides the best fertilizer recommendation.
If additional fertilizer is needed, apply it during active early growth rather than continuing heavy nitrogen feeding late into the season.
🌱 Why Onion Leaves Matter
The green leaves are the plant’s solar panels.
Generally, the more healthy leaves an onion produces before bulbing begins, the greater its potential bulb size.
Each leaf contributes to the bulb’s layered structure.
That is one reason onions need:
- Full sun
- Adequate nitrogen early
- Consistent water
- Good weed control
Anything that severely reduces leaf growth can reduce the final harvest.
🌿 Should You Mulch Onions?
Yes, but keep the mulch relatively light.
Good options include:
- Straw
- Shredded leaves
- Fine compost
- Grass clippings from untreated lawns
Mulch helps conserve moisture and suppress weeds.
Avoid piling thick mulch directly around the necks of the onions because excessive moisture near developing bulbs can encourage rot.
🌱 Weed Control Is Especially Important
Onions do not compete well with weeds.
Their foliage is narrow and casts little shade, while their root systems remain relatively shallow.
Weeds can quickly steal:
- Water
- Nutrients
- Sunlight
- Growing space
Hand weed carefully or use shallow cultivation.
Avoid deep hoeing, which can damage onion roots.
Once bulbs begin enlarging, be especially careful not to injure them.
🌱 Can You Grow Onions in Raised Beds?
Yes.
Raised beds are excellent for onions because they provide:
- Loose soil
- Good drainage
- Easy spacing
- Convenient weed control
- Easier harvesting
Because raised beds can dry more quickly than ground-level gardens, monitor moisture regularly.
A 4-foot-wide bed can accommodate several rows of onions while still allowing easy access from the sides.
🪴 Can You Grow Onions in Containers?
Yes.
Onions grow well in containers as long as the pot is sufficiently deep and wide.
Choose a container at least about 10 to 12 inches deep with drainage holes.
The width determines how many onions you can grow.
Space bulb onions approximately 4 inches or more apart.
Green onions can be planted much more closely.
Use quality potting mix rather than dense garden soil.
🧅 Can You Regrow Grocery Store Onions?
You can often regrow green shoots from an onion base, but this is different from reliably producing a new large storage bulb.
An onion placed in water may produce fresh green leaves.
You can use those greens in cooking.
For a dependable crop of mature onions, start with:
- Seed
- Sets
- Transplants
These provide much more predictable results.
🌱 Can You Grow an Onion From Another Onion?
Sometimes an onion that begins sprouting in storage can be planted.
However, it is entering the second year of its natural biennial life cycle.
Its main biological goal may be flowering and producing seed rather than developing another large storage bulb.
Plant sprouted onions if you want:
- Green tops
- Flowers
- Seed
Do not expect them to reliably produce one new large bulb.
🌸 Why Are My Onions Flowering?
An onion that sends up a flower stalk is said to have bolted.
Bolting can be triggered when a plant experiences certain temperature fluctuations that make it behave as though it has gone through winter and entered its second season.
Large onion sets may be particularly prone to bolting.
Once an onion bolts, the bulb generally does not store well.
Harvest and use bolted onions relatively soon.
⚠️ Should You Remove Onion Flower Stalks?
If an onion begins flowering, you can cut off the flower stalk, but this will not fully reverse the process.
The bulb has already shifted toward reproduction.
Removing the stalk may prevent energy from going into seed production, but the onion still generally has reduced storage potential.
Use bolted bulbs first.
🌡️ How Long Do Onions Take to Grow?
The growing period varies substantially.
Onions may require roughly 90 to 150 days from seed to mature bulb, depending on variety and growing method.
Transplants and sets shorten the apparent garden season because some growth has already occurred before planting.
Day length is just as important as calendar age.
Bulbs begin forming when the cultivar receives its genetically required amount of daylight.
🧅 When Do Onions Start Forming Bulbs?
Bulbing begins once the appropriate day length is reached.
Short-day onions begin earlier under shorter daylight.
Long-day onions wait until summer days become much longer.
The plant’s size at that moment strongly influences final bulb size.
A plant with many healthy leaves can produce a larger bulb than a weak plant with only a few leaves.
This is why getting onions planted early enough is important.
❓ Why Aren’t My Onions Making Bulbs?
Several factors can cause poor bulb development.
Wrong Day-Length Type
This is one of the most common causes.
Planted Too Late
The plant did not have enough time to develop leaves before bulbing began.
Too Much Shade
Weak foliage means less energy for bulb development.
Overcrowding
Plants compete for resources.
Poor Fertility
Weak early growth can limit final bulb size.
Excessively Deep Planting
Bulbs develop best relatively close to the surface.
❓ Why Are My Onions So Small?
Small onions may result from:
- Late planting
- Crowding
- Wrong day-length variety
- Drought
- Heavy weed competition
- Poor fertility
- Disease
- Early leaf damage
- Bolting
Think of bulb size as the final result of everything the plant accomplished before bulbing began.
Strong foliage early in the season is critical.
❓ Why Are Onion Leaves Turning Yellow?
Yellow foliage can indicate:
- Natural maturity
- Overwatering
- Drought
- Nutrient deficiency
- Root disease
- Pest damage
Timing matters.
If the plants are near harvest and the outer leaves begin yellowing naturally, this is expected.
If young onions turn yellow while the soil remains saturated, poor drainage may be involved.
❓ Why Are Onion Tips Turning Brown?
Brown tips can occur because of:
- Dry weather
- Irregular watering
- Excess fertilizer salts
- Heat stress
- Disease
- Natural leaf aging
A small amount of tip browning is common and may not seriously reduce the crop.
Extensive damage deserves further investigation.
🐛 Common Onion Pests
Potential pests include:
- Onion thrips
- Onion maggots
- Cutworms
- Leafminers
Thrips are especially common.
They are tiny insects that feed on onion foliage and can cause leaves to appear silvery, streaked, or distorted.
Severe infestations reduce the plant’s ability to photosynthesize and can ultimately reduce bulb size.
🪰 Onion Maggots
Onion maggots are larvae of small flies that feed on roots and bulbs.
Infested plants may:
- Wilt
- Yellow
- Collapse
- Develop rotting bulbs
Crop rotation and sanitation are important management tools.
Avoid leaving discarded onions or cull bulbs in the garden.
Local Extension recommendations are useful where onion maggots are a recurring problem.
🍄 Common Onion Diseases
Possible diseases include:
- Downy mildew
- Botrytis leaf blight
- Purple blotch
- Fusarium basal rot
- White rot
- Bacterial soft rot
Disease risk increases under certain combinations of moisture, temperature, and poor airflow.
Helpful practices include:
- Crop rotation
- Good drainage
- Proper spacing
- Clean planting material
- Avoiding unnecessary overhead irrigation
- Removing diseased plants
🔄 Should You Rotate Onions?
Yes.
Onions, garlic, leeks, shallots, and other Allium crops can share some pests and diseases.
Avoid growing onions in the same bed year after year when possible.
A rotation of several years can help reduce soilborne disease pressure.
This is especially important if you have experienced white rot, Fusarium, or onion maggot problems.
🌿 Companion Plants for Onions
Onions can fit easily among many garden crops because their upright leaves occupy relatively little horizontal space.
Potential ornamental companions include:
Do not let companion plants shade the onions or compete heavily with their shallow root systems.
🥕 Can You Plant Onions With Carrots?
Yes.
Carrots and onions are often grown near one another because they occupy space differently.
Onion foliage grows upright while carrots develop long roots below ground.
However, companion planting should not be treated as guaranteed pest control.
Both crops still need proper spacing, sunlight, nutrients, and pest monitoring.
🍓 Can You Plant Onions Near Strawberries?
They can share a garden area provided both crops receive enough sunlight and airflow.
Onions have relatively upright growth and do not create much shade.
However, avoid planting them so densely that they interfere with strawberry runners or harvesting.
For strawberry planting guidance, see How to Plant Strawberries.
✂️ Should You Cut Onion Tops?
Do not routinely cut the leaves of outdoor bulb onions.
Healthy leaves are essential for building large bulbs.
Trimming indoor onion seedlings before transplanting is different because the plants are small and actively producing new foliage.
Once bulb onions are growing in the garden, allow the leaves to develop normally.
🧅 When Are Onions Ready to Harvest?
Mature bulb onions are usually ready when:
- Bulbs have reached useful size
- Neck tissue begins softening
- Leaves yellow
- Tops naturally bend or fall over
When about half to three-quarters of the tops have fallen naturally, harvest is approaching.
Do not force healthy green tops over simply because you want the onions to mature faster.
Let the plant complete the process naturally.
🚫 Should You Bend Onion Tops Over?
No.
The old practice of deliberately bending onion tops is unnecessary and can injure the plant.
The tops naturally fall as the bulb reaches maturity.
Breaking healthy leaves prematurely reduces photosynthesis and may interfere with the final stages of bulb development.
Wait for the plants to signal maturity themselves.
🌾 Should You Stop Watering Before Harvest?
Yes.
When the tops begin yellowing and naturally falling, reduce or stop irrigation.
Dry conditions help the bulbs finish maturing and prepare for curing.
If heavy rain is expected once onions are mature, harvesting them before prolonged wet weather may reduce the chance of rot.
⛏️ How to Harvest Onions
Loosen the soil gently around the bulbs with a garden fork if necessary.
Pull onions by the base rather than yanking hard on the foliage.
Handle the bulbs carefully.
Bruised or cut onions are much more likely to rot during storage.
Brush away loose soil but avoid aggressive cleaning.
☀️ What Does Curing Onions Mean?
Curing dries the outer skin and neck so the onion can be stored.
Freshly harvested onions contain substantial moisture.
Before long-term storage, they need time in a dry, airy location.
Spread onions in a single layer or place them on screens where air can circulate.
A sheltered porch, shed, garage, or similar location can work if it remains dry and well ventilated.
⏳ How Long Should Onions Cure?
Most onions need approximately 2 to 3 weeks to cure thoroughly.
They are ready when:
- Outer skins are papery
- Necks are dry
- Tops are completely dry
- Roots are dry
After curing, trim the roots.
Tops can be trimmed to about 1 inch above the bulb for loose storage, or left longer if you plan to braid suitable varieties.
🧅 Which Onions Store Best?
Not all onions have the same storage potential.
Generally:
- Sweet onions contain more moisture and store for shorter periods.
- Pungent storage onions usually keep much longer.
Cultivars specifically described as storage onions are best if you want a supply lasting into winter.
Red, yellow, and white onions can all include storage varieties.
Cultivar matters more than color alone.
🏠 How to Store Onions
Store cured onions somewhere:
- Cool
- Dry
- Dark
- Well ventilated
Good airflow is essential.
Suitable containers include:
- Mesh bags
- Slatted crates
- Wire baskets
- Open boxes with ventilation
Avoid sealed plastic bags.
Trapped humidity encourages sprouting and rot.
❄️ Should Onions Be Refrigerated?
Whole cured storage onions generally keep best in a cool, dry, ventilated pantry or storage space rather than a humid refrigerator.
Once an onion has been cut, refrigerate the remaining portion.
Sweet onions with high moisture content may also benefit from refrigeration if they cannot be used quickly.
🧅 Why Are My Stored Onions Rotting?
Storage rot can result from:
- Incomplete curing
- Bruising during harvest
- High humidity
- Poor airflow
- Diseased bulbs
- Storage temperatures that are too warm
- Thick or wet necks
Inspect stored onions periodically.
Remove any soft or rotting bulbs immediately before decay spreads to neighboring onions.
🌱 Can You Save Onion Seeds?
Yes.
Onions are biennials, meaning they generally flower during their second growing season.
To save seed, mature onions must survive storage or overwintering and then be replanted.
They send up tall flower stalks topped by round clusters of small flowers.
Allow seed heads to mature and dry before collecting the seeds.
Keep in mind that different onion varieties can cross-pollinate.
🌸 Are Onion Flowers Good for Pollinators?
Yes.
Onion flower heads attract numerous insects.
Visitors can include:
- Bees
- Flies
- Small beneficial wasps
Most onions grown for bulbs are harvested before flowering, but onions deliberately kept for seed can become attractive pollinator plants during their second year.
🧅 Are Onions Easy to Grow?
Yes, once you understand day length.
The essential steps are:
- Choose the correct short-, intermediate-, or long-day variety for your region.
- Plant in full sun.
- Use loose, fertile, well-drained soil.
- Plant early enough for strong leaf growth before bulbing.
- Space full-size onions about 4 to 6 inches apart.
- Keep the planting shallow.
- Water consistently during active growth.
- Control weeds carefully.
- Fertilize early rather than heavily late in the season.
- Stop heavy watering as the tops naturally fall.
- Harvest carefully.
- Cure bulbs thoroughly before storage.
For many gardeners, choosing the correct day-length variety is the difference between harvesting golf-ball-size onions and large mature bulbs.
🧅 Final Thoughts
Onions are simple plants with one unusually important requirement: they must be matched to the amount of daylight they receive.
Choose short-day onions for southern regions, long-day onions for northern gardens, and intermediate-day varieties where appropriate between the two. Then plant early enough for the onions to develop a strong collection of healthy leaves before increasing day length triggers bulb formation.
Whether you start with seeds, sets, or transplants, grow onions in full sun and loose, fertile soil. Keep the planting shallow, space bulb onions about 4 to 6 inches apart, and provide consistent moisture while they are actively growing.
As summer progresses and mature tops begin yellowing and falling naturally, reduce watering and prepare for harvest. Cure the bulbs thoroughly until their necks and outer skins are dry before placing them into storage.
With the right variety and a little attention to timing, onions can become one of the most reliable and useful crops in the home vegetable garden.
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