How to save onion seeds?

what part of a plant is an onion?

Onion in the general sense can be used for any plant in the genus Allium but used without qualifiers usually means Allium cepa, also called the garden onion. Onions (usually but not exclusively the bulbs) are edible with a distinctive strong flavour and pungent odour which is mellowed and sweetened by cooking. They generally have a papery outer skin over a fleshy, layered inner core. Used worldwide for culinary purposes, they come in a wide variety of forms and colours.

Onions may be grown from seed or very commonly from “sets”. Onion sets are produced by sowing seed very thickly one year, resulting in stunted plants which produce very small bulbs. These bulbs are very easy to set out and grow into mature bulbs the following year, but they have the reputation of producing a less durable bulb than onions grown directly from seed and thinned.

Either planting method may be used to produce spring onions or green onions, which are just onions harvested while immature, although “green onion” is also a common name for the Welsh onion, Allium fistulosum which never produces dry bulbs.

Onions are frequently used in school science laboratories because they have particularly large cells which are easily visible even through rather low-end optical microscopes.

Onion weed has slender, light green strap-like leaves that sprout in clumps and can grow to around knee height. Just like a snowdrop or a daffodil, flower stems appear from the middle of the leaves in spring and early summer producing clusters of pure white drooping flowers that open above the foliage. The flower stems themselves are characteristically three-cornered. The easiest way to confirm that you are looking at onion weed is to crush any part of the plant – it will smell of onions. The good news is that every part of this plant is edible and it can be treated a bit like a spring onion or baby leek.
Onion weed grows from a small parent bulb and spreads underground by producing additional tiny bulbils that grow into dense crowded clumps of foliage. It also spreads rapidly around the garden through seeds cast from pods that form on the three-cornered stems after flowering. Onion weed can form a dense carpet of foliage that suppresses the growth of other plants. It dies back, absorbing nutrients from foliage in autumn down into the bulbs, and then re-grows new foliage in the spring. It is a persistent weed and one that is hard to completely eradicate.
When foliage is pulled out, bulbs and bulbils in the soil below will re-grow. If onion weed is dug up to remove foliage and the parent bulb most often the smaller bulbils – that can be hard to see – get freely distributed through your soil to grow into even more plants.

Collecting Onion Seeds: How To Harvest Onion Seeds

There’s nothing like the flavor of an onion fresh from the garden. Whether it’s the narrow green ones in your salad or the fat juicy slice on your burger, onions straight from the garden are something to behold. When they find that special variety that is particularly appealing, many gardeners want to know how to collect onion seeds for future sowing. Harvesting onion seeds is a fairly simple process, but here are a few things you need to know.

Whether it’s a preference for organically grown produce, economic considerations, or just the good feeling you get from serving food you’ve grown yourself, there’s a renewed interest in home gardening. People are searching the net for the richness and flavor of old time varieties and learning about saving seed for the next garden generation. Collecting onion seeds for future production can be your contribution to the process.

Collecting Onion Seeds from the Right Plants

Before we talk about how to harvest onion seeds, we need to say a few words about what kind of onions you can harvest onion seed. Many of the seeds or sets acquired from large seed production companies are hybrids,which means the seeds are a cross between two parent varieties chosen for specific characteristics. When blended together, they give us the best of both varieties. That’s great, but if you’re planning to harvest onion seed from these hybrids, there’s a catch. The saved seeds will most likely produce onion with the traits of one parent or the other, but not both, and that’s if they germinate at all. Some companies modify a gene within the plant to produce sterile seeds. So, rule number one: Don’t harvest onion seeds from hybrids.

The next thing you need to know about collecting onion seed is that onions are biennial. Biennials only bloom and produce seed during their second year. Depending on where you live, this may add a few steps to your list of steps.

If your ground freezes during the winter, a how to collect onion seeds list will include pulling the bulbs you have chosen for seed from the ground and storing them over the winter to be replanted again in the spring. They’ll need to be kept cool at 45 to 55 F. (7-13 C.). This isn’t just for storage purposes; it’s a process called vernalization. The bulb needs cold storage for at least four weeks to trigger the growth of scapes or stalks.

Replant your bulbs in early spring when the ground has warmed to 55 F. (13 C.). After leaf growth is complete, each plant will send up one or more stalks for flowering. Like all allium species, onions produce balls covered with tiny flowers ready for pollination. Self-pollination is usual, but cross pollination can occur and in some cases should be encouraged.

How to Harvest Onion Seeds

You’ll know it’s time for harvesting onion seeds when the umbrels or flowering heads begin turning brown. Carefully clip the stalks a few inches below the head and place them in a paper bag. Set the bag in a cool, dry place for several weeks. When the heads are completely dry, shake them vigorously within the bag to release the seeds.

Keep your seeds cool and dry through the winter.

Grow and Save Onion Seeds

How to Grow Onions

A culinary staple, onions are an essential vegetable in American gardens. Onions, like other members of the Allium family, are biennials, producing seeds in their second year of growth. The bulb (or common) onion has brown, yellow, or red skin and is round, elongated, or flattened. Bunching onions (or scallions) are harvested small.

When and How to Start Indoors

Plant onion seeds indoors 8–10 weeks before transplanting them outside just before the average last frost date in your area. Seeds should be sown ¼ inch deep. Onions require an open and sunny site, fertile soil, and good drainage.

Time to Germination

Seeds will germinate in 4-10 days when started indoors.

When to Transplant

Transplant outdoors just before the last frost.

Spacing Requirements

When transplanting your seedlings, space them at least 6 inches apart. Plant onion seedlings in the least weedy part of your garden; onion seedlings are small and do not compete well with weeds.

Special Considerations

Before the last frost, make a large furrow in your soil, at least 4 inches deep . Water this furrow before planting your seedlings to make transplanting easier.

Common Pests and Diseases

Several bacterial, fungal, and viral diseases can affect onion growth. Insect pests can also be a problem to various degrees in different parts of the United States. Plant your onions in a well-drained space. Diseases and fungi such as Fusarium basal rot, white rot, and Botrytis neck rot can affect crops in storage. Crop rotation can help prevent these diseases.

When and How to Harvest

The tops of bulb onion plants fall over naturally once the bulbs have matured. When half of the tops in a planting have fallen over, lift all of the bulbs and place the pulled plants in a warm, dry place (away from direct sunlight) to cure.


Onions have limitless potential in the kitchen and are indispensable for flavoring savory dishes. Onions can be roasted, fried, pickled, sautéed, and combined into dishes in dozens of other ways. Bunching onions, especially, are perfect for salads, pastas, and soups. Onion jam or compote is a great way to use a flush of red onions that will not store as well as white or yellow onions.


Cure onions for two to three weeks after harvesting by storing them in a warm place away from direct sunlight. When the onions feel paper-dry on the outside, clip off the tops and roots, and lightly brush off loose soil before storing the onions in a cool, dry place. Arrange them in a single layer or hang them in mesh bags. Always handle onions very carefully; the slightest bruise will encourage rot. Properly cured onions will store for 6–8 months in a root cellar or cool basement.

How to Save Onion Seeds

Onion seeds are not typically difficult to grow or to collect, but keep in mind that they are a biennial crop, meaning that they seed once every two years.

Life Cycle


Recommended Isolation Distance

When saving seeds from onion, separate varieties by at least 800 feet up to ½ mile. To produce seed from onion, select as many perfect onions as you can spare for seed production and store them through winter in a cool, dry, dark place. Replant them in early spring at the same bulb depth and spacing as when they were harvested.

Recommended Population Sizes

To ensure viable seeds, save seeds from at least 5 plants. To maintain a variety over time, save seeds from between 20-50 plants.

Assessing Seed Maturity

Watch first for flowers and then for seed heads to form during the late summer of the second season. Wait for the seed heads to dry. Most of the flowers will be dry, and the seeds will begin to fall out on their own.


After the plants bloom and seed heads begin to dry, gather the heads in a paper bag. Most of the seeds will fall out on their own; shake the bag to free the remainder of the seeds.

Cleaning and Processing

Separate the seeds from the stems and other matter that makes up the seed head. Allow the seeds to air-dry for a few days before storing the seeds in a cool, dry place.

Storage and Viability

When stored in a cool, dark, dry place, onion seeds will remain viable for 2 years.

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If Martha Stewart, Gordon Ramsay, Hester Blumenthal, and Anthony Bourdain all announce they’re going to show up at your house at the same time here’s what you need to do. Fry some onions. Maybe first you should make sure your phone is charged and Instagram updated, but then you should definitely fry some onions.

Nothing makes a kitchen smell like the cook knows what they’re doing like the smell of fried onions. Baked cookies are fine for an open house, but if actual chef-like smells are what you’re trying to put out in the world, nothing tops a fried onion. Lay a random weird ingredient like squid ink or dry ice on your counter and Martha, Gordon, Hester and Anthony will automatically give you chef status. And not that stupid honorary status that Universities give to actors.

Grow your own onions for cooking and they’ll be begging you to cook in their kitchens. Save the seeds from your onions and grow them over and over year after year and tell them all about how you do it? They’ll be asking you for cooking lessons. ‘Cause you’ll seem JUST that smart, JUST that authoritative, JUST that committed to the world of food.

In the past I’ve shown you how to save tomato seeds. I’ve also shown you how to save lettuce seeds. Saving onion seeds is slightly different because onions are a biennial.

A quick explanation of seed saving:


There are 3 different types of plants: annuals, biennials and perennials.

Annuals: Plants that germinate, flower, produce seeds and fruit in a single season. Then they die.

Examples of annuals in the vegetable garden: Tomatoes, Squash, Cucumbers, Beans, Peas, Melons, Peppers, Potatoes.

Biennials: Plants that produce fruit in a single season but don’t flower (which is where the seeds come from) until their second year.

Examples of biennials in the vegetable garden: Kale, Swiss Chard, Onions, Broccoli, Beets, Rutabaga.

Perennials: Plants that go through the entire cycle of producing and flowering every year, over and over for many seasons.

Examples of perennials in the vegetable garden: Raspberries, Strawberries, Rhubarb, Asparagus.

A simple way to identify a plant as either annual or biennial is if the vegetable contains seeds right in the actual vegetable it’s an annual. If there is no seed in the vegetable then it’s a biennial.

Also if every year you go outside and there’s that stupid plant again even though you wish you could kill it, … then it’s a perennial.

This isn’t true all the time and there are exceptions but it’s a good rule of thumb to go by.

So how do you save the seeds from a biennial? What zone you live in will dictate how you do it. If you live in a zone where the cold doesn’t kill your plants, you simply leave a few of your chosen biennials in the ground. In the spring they’ll start to grow again, sending up a shoot with a flower on top.

That was the case with these green (spring) onions I left in the ground last year.

Other plants like swiss chard or beets may get killed by the cold so saving those seeds is a bit different. I’ll get into how to save beet and carrot seeds in another post.

Onions will survive the winter in my zone of 6b. So getting them to flower is just a matter of leaving them in the ground and remembering not to pull them in up the spring when you wonder what the hell this weird onion is doing in the middle of your garden.

Anywhere from May to July (depending on the variety and your gardening zone) the onion will send up a shoot with a big pretty flower on the top. Just let it keep growing. Eventually tiny seeds will form. When the seeds have formed cut the flower stalk and allow the flower to dry. Once it does you can just shake the seeds out.

If you want to plant immediately you can also hand pick the seeds out of each tiny, individual flower.

The reason I cut the stalk off and let it dry on my porch is so I don’t lose all of the seeds. If it dries in the garden all the seeds will drop into the soil.

I harvested these seeds in June and will be bringing them up to the garden with me today so I have a new supply of green onions. And yes, I’ll be leaving a couple of them in the ground so next year I can harvest more seeds and do the same thing over and over again year after year. I’m a perennial gardener.

One final tip before I go … don’t let Martha and Gordon sit beside each other at dinner. Just trust me on that one.

Saving Onion and Shallot Seeds

Shallots are a perennial form of Allium cepa that were once propagated almost exclusively through asexual reproduction. While shallot seeds are now available in garden catalogs, most commercially available shallot seeds are hybrid varieties and are therefore not suitable for seed saving.

Multiplier onion is a name generally applied to certain other types of perennial onions that are primarily reproduced vegetatively. Within Allium cepa, the main types of multiplier onions are potato onions (bulbing types) and walking onions (top-setting types). Seed savers commonly exchange these crops by sharing bulbs and topsets.

Green onion, scallion, and bunching onion are common names that do not relate to a particular species of Allium. Some scallions are produced from cultivars of Allium cepa that are harvested before the bulbs begin to develop or while the bulbs are still small. Other scallions, including the common variety ‘Evergreen Hardy White’, belong to the species Allium fistulosum and do not form bulbs at all.


The origins of Allium cepa are uncertain. The best evidence indicates that Allium cepa was domesticated from a wild species in the Middle East at least 5,000 years ago. By 2500 BCE, onions were being cultivated in Egypt. Onions were brought to India by 600 CE, to Europe by 900, to Russia by 1300, and to the Americas by 1500.

Growing Common Onions for Seeds

Some plants flower in response to photoperiod; onion plants bulb in response to photoperiod. Technically, all common onions are long-day plants—bulbing is initiated when day length reaches or surpasses a critical photoperiod. However, within this long-day species exist vari­eties that, confusingly, are classified as short-day, intermediate-day, or long-day types. Short-day onions generally require at least 10-hour days to begin bulb development; intermediate-day onions require days between 12 and 14 hours long; and long-day types have a critical photope­riod of 14 or more hours. In the United States, short-day onions are typically grown in the South, and long-day onions are grown in the North.

Common onions are biennial, forming a bulb in their first year of growth, then flowering and producing seeds—provided their vernal­ization requirement has been met—in their second season. Onions generally require expo­sure to temperatures below 54°F (12°C) for 8 to 10 weeks before they enter their reproductive phase; however, temperatures between 48 and 54°F (9 and 12°C) are optimal for vernalization. Vernalization can occur below this range, but onions in the ground are prone to rot in moist soil when the soil temperature is below 45°F (7°C). In drier conditions, bulbs may survive temperatures of 20°F (-7°C) or lower, depending on the variety. In cold climates, onions are often dug in the fall and stored in mesh bags or crates in a dry, well-ventilated location, ideally with a relative humidity of 60 to 75 percent. Optimal temperatures for vernalizing onions in storage are between 32 and 40°F (0 and 4°C).

Growing onions for seed begins by cultivating onions as if they were being grown for eating. At the end of the season, when the onion tops have fallen over, the bulbs are harvested and allowed to cure (if vernalizing in storage) in a warm, well-ventilated location out of direct sun until the tops, necks, and skins have fully dried. The dormant bulbs can then be stored until spring, though success with this process varies by cultivar. Storage onions and varieties that do well in long-term storage are easy to vernalize in this manner. Some onions may break dormancy and sprout during the storage period, but they can still be planted out for seed provided they are healthy. In the spring, when soil temperature is above 55°F (13°C), onions can be set out in the garden with 6 inches (15 cm) between plants in rows 36 inches (91 cm) apart, or spaced at least 15 inches (38 cm) on center.

Even where mild climates permit in-ground vernalization, it is still a good practice to lift onions. Lifting allows for a visual assessment of bulb traits and health, and also provides an opportunity to increase spacing between plants before they begin flowering. This is best done in spring, after vernalization, as onions that are lifted and replanted in fall often have a hard time re-establishing in cold soils.


Flowering, Pollination, and Seed Set

After vernalization, each onion plant sends up one or several leafless flower stalks called scapes—the inflorescence is initially enclosed within a large bract called a spathe. Onions bear hundreds of small white perfect flowers held in spherical umbels. Each perfect flower is protandrous—pollen is shed before the stigma becomes receptive—and onions rely on insects, including honey bees, syrphid flies, halictid bees, and drone flies, for pollina­tion. Flower stalks may grow to more than 3 feet (91 cm) tall, and staking will prevent lodging.

Variety Maintenance

In the garden, onion varieties should be isolated by 800 feet to half a mile (244 to 805 m). Gardeners who exchange seeds with others or who garden in settings that do not provide many landscape barriers may choose to use the upper end of this range as a starting point when determining an isola­tion distance. Commercial seed growers separate large plantings of different varieties by 1 to 2 miles (1.6 to 3.2 km). These isolation distances may seem daunting, but onions being grown for seed need to be isolated only in their second season, and only from other cross-compatible varieties that have also been vernalized and will therefore be flowering at the same time. Because of this, many gardeners can grow one variety of onion to seed each season. Multiplier onions, shallots, and common onions can cross-pollinate if they flower at the same time.

When a sufficient isolation distance cannot be provided, isolation cages can be used; polli­nators such as honey bees, blue bottle flies, and houseflies must be released into the cages.

Although onions are self-compatible, growing a large population helps to ensure a successful seed set and to conserve genetic diversity within a variety. Viable seeds can be obtained by growing 5 plants or fewer, though 20 to 50 plants is the recommended population size for seed saving. The upper end of this range helps prevent inbreeding depression and is preferable if a gardener’s intent is to save seeds for multiple generations or to share seeds with others. For those saving seeds for genetic preservation, a population size of at least 80 plants is recom­mended. Some loss can be expected from rot or disease during overwintering, so gardeners may want to grow more plants than the desired population size.

When roguing or selecting onions, seed savers should consider traits such as bulb color and shape.

Seed Maturity and Harvest

Onion seeds typically mature about 45 days after pollination. The seeds develop inside small fruits called capsules, which are pale green during development but fade as they dry. Each onion capsule has three chambers, and at seed matu­rity the capsule splits open at the top, exposing the black seeds contained inside. Although each onion flower has six ovules, and thus the potential to produce six seeds, only three or four mature seeds typically form in each capsule. The capsules located at the top of the seed head open first. Harvesting can occur any time after capsules begin to dehisce, and it may be best to harvest early in wet climates. If the plants are healthy and the weather is dry, it is ideal to harvest umbels when at least 20 percent of their capsules have split open.

Onion seeds can be harvested by cutting the scapes about 6 to 8 inches (15 to 20 cm) below the seed heads and placing the harvested material in an open container or bag. The umbels should be allowed to continue drying in a protected, well-ventilated space for 7 to 14 days; the cap­sules will continue to shatter as they dry.

Cleaning and Storage

Onion seeds can be threshed by gently rubbing the capsules between one’s hands or against a screen. Onion seeds are delicate compared to many other seeds; rubbing too aggressively can damage their thin seed coats.

After threshing, seeds should be screened and winnowed. At times, it can be hard to separate onion seeds from chaff, and floating (p. 85) is an alternative method of cleaning the seed lot. Immediately after floating, spread the seeds in a thin layer on screens to dry. Fans can be used to expedite the drying process, if desired.

When stored under cool, dry conditions, onion seeds can be expected to remain viable for two years.

Allium cepa

FAMILY: Amaryllidaceae

LIFE CYCLE: Biennial or perennial

SUGGESTED SPACING: When growing onions for seed, increase spacing to 6 inches (15 cm) between plants in rows 36 inches (91 cm) apart, or to at least 15 inches (38 cm) on center. Staking is recommended.

OTHER REQUIREMENTS: Onions are long-day plants; varieties should be selected for growing based on day length. Onions also require a vernalization period of temperatures below 54°F (12°C) for 8–10 weeks in order to flower and set seeds.

FLOWER TYPE: Perfect, self-fertile flowers are protandrous and shed pollen prior to stigma receptivity. Clusters of flowers are held in spherical umbels.

POLLINATION: Insect-pollinated

MATING SYSTEM: Mixed. Protandry facilitates cross-polli­nation. However, onions self-pollinate when insects transfer pollen between flowers on the same plant.


FRUIT TYPE: Dry, dehiscent fruit (capsule)

SEED MATURITY: Seed maturity occurs in the second growing season, when capsules split open to expose mature black seeds. Shattering is common.

SCREEN SIZE: 4⁄64–9⁄64 inch (1.5–3.5 mm)


ISOLATION DISTANCE: 800 feet–1⁄2 mile (244–805 m)

Population Size

For Viable Seeds: 5 plants

For Variety Maintenance: 20–50 plants

For Genetic Preservation: 80 plants

For more on seed saving, see our Seed Saving Guide.

Reprinted with permission from The Seed Garden, by Micaela Colley & Jared Zystro, edited by Lee Buttala & Shanyn Siegel and published by Seed Savers Exchange, 2015. Buy this book from our store: The Seed Garden.

Onion Seed Production: Quick Reference

Onion Seed Production: Quick Reference – Organic Seed Alliance

Figure 2. Onion umbels drying on landscape fabric.

Onions are a biennial so seed production must take into account a 2-year reproductive cycle.


Year 1: Producing the onion crop

Onions prefer dry hot summers and develop best in sandy or silty loam soils with high water-holding capacity and a consistent water supply. They require high nitrogen and phosphorus, but excess nitrogen can be detrimental. Since onion seed populations need to be in the range of 120-200 plants, a greater number of bulbs will be needed the first year to account for any selection and bulbs that deteriorate during winter storage. A good number of onion plants for the first year might be 250-300. Plant transplants 3-4 inches apart to provide room for bulb formation. After the bulbs have formed, selections can be made based on important traits for the population. After 50% of the tops have fallen over, knock the rest down and wait a few days before digging the bulbs up and windrowing them for 1-2 weeks. Turn bulbs to avoid sunburn. The best onion bulbs should have their tops cut and be stored in burlap sacks in a humid (65-75% relative humidity, RH), cool (32-34°F) area with good air circulation.

Figure 1. Onions just before harvest. When 50% of the tops have fallen, knock the others over.

Year 2: Producing the onion seed

Since onion is highly outcrossing and insect-pollinated, it should be isolated from other onion varieties by at least 0.5-1 mile. More distance is necessary if varieties are different colors and/or different shapes. Select onion bulbs that have no disease, sprouting, or elongation after storage for planting. If selecting for single centers or other internal qualities, cut the top third of each bulb off horizontally to evaluate internal characteristics before planting. Allow selected bulbs to sit and heal over for 2-3 days before transplanting.

Figure 2. Onion umbels drying on landscape fabric.

At least 120 plants should be used to maintain population integrity. The bulbs should be planted 12 inches apart in rows that are 18-24 inches apart. Plants may need to be staked to avoid falling over or to protect them from being knocked over by wind. Clear, warm weather is beneficial for pollination, and temperatures between 65°F and 95°F are best for seed set. Overhead watering can increase the potential for disease and should be avoided during flowering and seed set. Approximately 3-4 weeks after fertilization seed enters the “dough stage.” Three more weeks after that the seed should be mature. Begin harvesting once the capsules are open and the black seeds are visible. Cut seed heads 4-6 inches below the umbel and lay on landscape cloth. Allow to dry for up to 2 weeks and turn to avoid sunburn if in direct sun. It may take 2-3 weeks to harvest all bulbs as flower maturity can be variable. Thresh the seed by rubbing by hand, through screens, or combining. A combination of winnowing and screening will be needed to get the seed fully clean. For lots with low germination or persistent bracts and stems, flotation may be a useful technique as the bracts, stems and immature seeds will float and can be easily skimmed off. If flotation is used, dry the seed immediately and do not allow seed to be immersed in water any longer than is necessary. In other words, don’t start the process unless you intend to finish it.

Figure 3. Floating onion seed. Clockwise from the top left: Add water and pour off any floating debris or light seed. Be prepared with a strainer to catch heavier seed.Figure 4. From left to right: Continue pouring until all light material is gone. Pour final water out, catching seed in a strainer with holes smaller than the seed.

Onion seed dried to 6.5% moisture and stored under favorable conditions (40-60°F and <40%RH) will last 1-2 years. Properly dried seed that is stored in a freezer will keep indefinitely.

Figure 5. Drying onion seed after floatation.

Selection and Variety Improvement

Because of the 2-year reproductive cycle, traits will be evaluated during both bulb production and storage to improve the population.

Figure 6. Selections for bulbs to store: any disease and small bulbs are discarded (bulbs left of the marker are discarded).

Trait Timing
Seedling vigor 2-3 weeks after seedling emergence
Early bulb formation Early summer
Uniformity of neck falldown Late summer, 1-2 weeks before harvest
Neck closure Anytime after field curing
Bulb shape Anytime after field curing
Bulb color & intensity Anytime after field curing
Bulb uniformity Anytime after field curing
Skin/wrapper quality Anytime after field curing
Days to maturity At harvest
Yield: total & marketable (whole plot & weight per bulb) Anytime after field curing
Disease resistance During growing season and after harvest
Insect resistance During growing season and after harvest
Bulb firmness Periodically throughout winter, 2-8 months after harvest
Sprouting resistance Periodically throughout winter, 2-8 months after harvest
Storagability Periodically throughout winter, 2-8 months after harvest
Flavor Soon after harvest, and again after several months in storage


Plant disease-free seed. Grow onions in well-drained soils. Thoroughly cure and dry bulbs after harvest: undercut or pull mature bulbs and leave in the field for 1-2 weeks followed by well ventilated storage and curing in a covered shed or greenhouse. Save only fully mature, well-cured bulbs. Avoid damaging or bruising the bulbs. Bury or compost any culled bulbs. Use a 3-4 year crop rotation if possible before producing onions on the same ground again. Store the bulbs at 33-40°F and 70-75%RH. Diseases are classified by severity with a class of 1 being the most severe and 3 the least severe.

Disease Type Severity class Favorable conditions
Neck rot (Botrytis aclada) Seedborne 1 Storage of immature bulbs with green, soft necks
Black mold (Aspergillus niger) Seedborne 2 Storage of immature, soft bulbs
Smudge (Colletotrichum circinans) Seedborne 3 Warm (80°F) and moist at harvest time
Botrytis blight (Botrytis cinerea) Seedborne 3 Moderately warm (65-80°F) and extended wet period
Basal rot (Fusarium oxysporum, sp. cepae) Seedborne 3 Soil temperatures 57-90°F, optimum 80°F
Stemphylium leaf blight (Stemphylium vesicarium) Seedborne 3 Moderate temperature (70-80°) and heavy dew, fog or rain
Purple blotch (Alternaria porri) Seedborne 3 Moderate temperature (70-80°) and heavy dew, fog or rain
Downy mildew (Peronospora destructor) Foliar
Pink root (Pyrenochaeta terrestris) Root
Onion smut (Urocystes cepulae) Foliar
Onion thrips (Thrips tabaci) Insect
Onion maggots (Hylemyia antiqua) Insect

This resource was made possible thanks to the Montana Department of Agriculture’s Specialty Crop Block Grant Program.

Why planting onion seeds is better than planting sets (and how to do it right)

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As a former organic market farmer, I’ve grown onions in every possible way. I’ve grown them from onion sets, from nursery-grown transplants, and from their little black seeds. Needless to say, I’ve learned quite a few tricks along the way, but I will tell you without a doubt that my best onion crops always start with planting onion seeds, not by planting onion sets or even by planting nursery-grown transplants. For me, planting onions from seed has always yielded the best results. But here’s the thing – you can’t just grow onions from seed like you do other vegetables. There’s a trick to doing it right.

Why planting onion seeds is better than planting sets

Onion sets are immature bulbs that were grown from seed that was planted in mid-summer of the previous year. The partially-grown bulbs are pulled from the soil in the fall and stored in a dormant state through the winter to be replanted the following spring. Many gardeners plant onions from sets because they’re widely available and it’s easy, but there are a few reasons why this may not be the best way to grow a good onion crop.

Planting onions from sets doesn’t always produce the biggest bulbs.

First, most gardeners make the mistake of choosing and planting the largest onion sets they can find when they should be picking the smallest sets instead. Texas A&M, Michigan State, and other university Extension Services note that bigger onion sets stop growing and go to flower sooner than smaller sets. When it comes to growing onions from sets, bigger definitely isn’t better; you’ll grow substantially larger onions by planting smaller sets.

Related post: Time-saving tips for the vegetable gardener

Onion sets are easy to find at garden centers, big box stores, and even in the produce section of the grocery store, but just because they’re easy to find, doesn’t make them the best onions to grow. Typically, only two or three varieties of onions are commonly available as sets, but there are dozens and dozens of onion varieties available from seed that are likely to do better in your garden. Just like growing tomatoes and peppers from seed, growing onions from seed means you’ll have a wider range of varietal options. But, exactly which onion varieties are best for your garden, depends on where your garden is located.

Nursery-grown onion transplants are another way to grow onions, but growing your own plants from seed often yields better results.

Which type of onion is best for your garden?

There are three different types of onions and picking the right type is key to growing a great crop.

  1. Short-day onions are varieties that form bulbs as soon the days reach 10 to 12 hours in length. They’re perfect for southern gardeners below the 35th parallel whose days are slightly shorter throughout the growing season. If you grow short-day onions in the north, you’ll end up with tiny bulbs that go to flower early in the season because the bulbs stop growing as the days lengthen. Common short day onions are ‘Southern Belle’, ‘White Bermuda’, and ‘Granex’, to name a few.
  2. Long-day onions are varieties that form bulbs when the days reach about 14 hours in length. They’re best for gardeners in the northern tier of the U.S. and Canada. Long-day onions won’t form bulbs south of the 35th parallel because the days aren’t long enough to trigger bulb formation. Common long-day onion varieties include ‘Walla Walla’, ‘Ring Master’, ‘Red Zeppelin’, ‘Yellow Sweet Spanish’.
  3. If you live somewhere across the mid-section of the U.S., grow day-neutral onion varieties (also called intermediate day). Varieties like ‘Red Amposta’, ‘Early Yellow Globe’, ‘Cabernet’, and ‘Superstar’ are a good fit. These varieties begin to set bulbs when days range from 12 to 14 hours in length.

Aside from the ability to grow a wide variety of the right onions for your climate, growing onions from seed also means you’ll grow larger bulbs. But, this is only true if you grow onion seeds the right way.

Two ways to plant onion seeds

When growing onion from seed, there are two ways to grow a successful crop.

Planting onion seeds under lights

Related post: The best way to start seeds: Grow lights or sunny windowsill?

Onions are cool-season crops that require 90 days or more to reach maturity. Because of this long growing season requirement and their preference for cooler weather, planting onion seeds directly into the garden in the spring makes it difficult for the bulbs to reach a good size before warm temperatures arrive. This means the seeds have to be started many weeks in advance of moving the plants outside into the garden. To make matters worse, onion seedlings are also slow growing. So, if you want to grow onion seeds indoors under grow lights, you should start them 10 to 12 weeks before it’s time to plant them into the garden in early spring.

But, planting onion seeds indoors under grow lights is a bit more nuanced than growing other vegetables from seed. When growing the seeds of tomatoes, eggplants, and other veggies indoors under grow lights, the lights should be on for 16 to 18 hours per day. But, if you grow onion seeds indoors under grow lights and leave the lights on for that long, it will initiate an early bulb set and result in puny onions. That means that if you want to start onion seeds indoors under grow lights, start very early and only leave the lights on for 10 to 12 hours per day.

To me, all of that seems like an awful lot of work, so I’m now planting onion seeds using a different method that’s far easier and a lot more fun. It’s called winter sowing.

My favorite method: Planting onion seeds via winter sowing

If you want to skip the hassle of grow lights, heating mats, and other seed-starting equipment, growing onion seeds via winter sowing is the way to go. It works like a charm and is super easy. All you need is a packet of onion seeds, a plastic lidded container, and some potting soil formulated for seed starting. I start planting onion seeds via winter sowing anytime between early December and mid-February.

Planting onion seed via winter sowing is a great way to grow big onions.

Here are the steps I use to winter sow onion seeds:

  • Poke three or four 1/2″ wide drainage holes in the bottom of the plastic container (I use clamshell-type take-out containers or empty plastic lettuce packages). Also make two 1/2″ wide ventilation holes in the top of the lid.
  • Open the container and fill it with three inches of potting soil.
  • Sprinkle the onion seeds on top of the soil, casually spacing them about 1/4″ to 1/2″ apart.
  • Cover the seeds with a sprinkling of potting soil and water them in well.
  • Put the lid on the container and label it with a piece of tape and a permanent marker.

Once the seeds are planted, put the container in a protected, shady spot outdoors. I keep mine on a picnic table against the back of our house. It doesn’t matter if it’s freezing cold and snowy outside when you plant the seeds; they’ll just sit dormant until it’s the perfect time for them to sprout (just like Mother Nature intended!). Don’t bother clearing off any snow or protecting the containers from freezing weather. The seeds will be fine.

Related post: Winter sowing containers

Containers planted with onion seeds should be left outdoors in a sheltered, shady site.

When the temperatures and day length are just right, your onion seeds will start to sprout inside the container. At that time, you need to start monitoring the moisture level inside the container, watering your seedlings when necessary. Open the lid on warm days and close it at night. If you get a hard freeze in the spring, after the seedlings have germinated, toss a blanket or towel over the container at night for added insulation.

Watch this video to see how to grow seeds by winter sowing.

As soon as your garden soil can be worked in the early spring, transplant your onion seedlings out into the garden (that’s usually mid-March in my Pennsylvania garden). Unlike onion seedlings grown indoors under grow lights, there’s no need to harden-off winter sown onion seeds because they’ve been outdoors from the start.

Planting onion seeds by winter sowing means the plants are subjected to the natural day-night cycle right from the time of their germination. This means that bulb set is triggered at the correct time and the plants can form large bulbs before hot temperatures arrive.

Try planting onion seeds instead of sets this year, and enjoy a prolific harvest of these beautiful bulbs.


You know me. I’m a tomato girl. So I was super-bummed when my tomato patch didn’t exactly thrive this year, with the cool, wet summer we had. Thankfully, my onions on the other hand, went bonkers for the cool and damp weather. This was by far my best onion crop yet.

Yellow, red, white – they all did well. And what especially made me happy was that some of my best specimens were from seeds I’d saved myself.

Since onions are a biennial crop, they take two years to grow to seed-bearing maturity – thus monopolizing garden bed space for two seasons rather than one. So I only plan to over-winter onions for seed-saving every 2-3 years, and when I do, I save enough for a couple years at a time.

Saving my own onion seeds is a cost-effective way of having high-quality, heirloom seeds for my garden every year, and it’s really easy to do!

Seriously. If you’re daunted by the prospect of saving seeds from a biennial crop, don’t be! It’s so easy. There are just some simple principles to keep in mind. Here’s how I go about it:


In the first year, start any onions that will be for seed-saving a little later than you usually would (April or May works well for me.) The reason for getting them a later start is that smaller, immature bulbs are more cold-hardy, and fare better through the winter than larger, mature bulbs.

Cultivate and fertilize them just like you would a usual onion crop.

As fall comes, and the weather gets cold, cover them with a good thick layer of mulch. About 4-5 inches of straw or shredded leaves works well – exactly like you would with garlic. This will help ensure they survive the winter!


In the spring, pull away most of the mulch, so the onions will be able to send up their new shoots.

This is the year when you need to worry about cross-pollination, because it’s when your onions will be flowering. There are four main species within the onion family, and varieties within a species can cross-pollinate with each other. While it’s uncommon, cross-pollination between species has been observed, (particularly between the “fistulosum” and “cepa” families, from what I have read.)

These are the four major species:

Allium schoenoprasum: Common chives

Allium tuberosum: Garlic chives

Allium fistulosum: Japanese bunching onions

Allium cepa (Comprised of three groups):

  • Aggregatum: shallots, potato onions, and multiplier onions belong to this group
  • Cepa: “regular” biennial bulb onions, for slicing and storage
  • Proliferum: this group includes Egyptian or “walking” onions

What all this means is that’s important to take precautions to prevent cross-pollination if you’re growing other kinds of onions that might be flowering in the same year as those you’ll be saving your seeds from.

Perfectly dry and ready to harvest. The umbrel in behind needs to dry longer.

There are a few ways to accomplish this!

The most straight-forward is to make sure that your seed-saving onions are planted at least 1,000 feet away from any other alliums within the same species. If you’re saving these to sell, with a guarantee of perfect purity, the isolation distance is drastically more – a full mile!

But – if you’re addicted to alliums, and spacing same-species crops at least 1,000 feet apart isn’t an option, you can always use caging to prevent cross-pollination. This article has solid tips to get you started, if you’re new to using caging methods for seed purity.

Once your onions flower, all that’s left is to wait for harvest time! When nearly all of the flowers have dried and fallen off on their own, you can grab a pair of pruning shears, and snip the seed heads into a bowl or bucket – something without holes, so you don’t lose any precious seeds! (Can you tell I made the mistake of harvesting into a loosely-woven basket once?)

Let them sit in a cool, dry location for a couple of weeks. Each little tiny seed head will start to open up, and the clean black seeds will separate easily from the umbels. You can rub them between your hands to make sure you get all the seeds out.

Separate seeds from the chaff, and store them in a cool place, just like the rest of your seeds. Onion seeds only maintain a high germination rate for 3 years at most, so it’s important to save fresh seed every 2-3 years.

The one last thing to keep in mind is that onions are prone to what’s called “inbreeding depression” after 2-3 generations. What this means is that if you were to save the seeds from only a single plant, for several years in a row, the resulting onions would tend to be smaller and less robust. It’s important to save and mix seeds from several plants, to maintain some genetic diversity and prevent this from happening.

That’s really all there is to it! I always think the most complicated part of seed-saving is taking measures to prevent cross-pollination. Aside from that, saving seeds from biennials really is easy peasy. And the joy and pride when you’re harvesting beautiful vegetables from seed you bred and saved yourself is really one of prime thrills of gardening, I think!

Happy seed-saving, my friends!

Read Next: Printable Packet for Saving Seeds

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