Collecting Onion Seeds: How To Harvest Onion Seeds

Collecting Onion Seeds: How To Harvest Onion Seeds

By: Jackie Rhoades

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.

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How to Grow Onions: from Seed or Sets to Harvest

As strange as it may sound, onions are one of our favorite crops to grow! They’re highly underrated. It’s easy to grow onions, once you know how! Few pests bother them, and the plants are forgiving and versatile – since you can harvest and use these alliums at many different stages along the way. Not to mention, onions are the ultimate garden-to-table crop. Just about every killer soup, stew, sauce, or stir fry begins with a familiar foundation: sauteed onions and oil (or butter). Toss in some garlic? Now we’re talkin’.

Read along to learn how to grow onions from seed, seedlings, or sets. This article will teach you everything you need to know, including how to choose the varieties that will grow best in your garden, when and how to start onion seeds, plus tips for transplanting and ongoing care. Then we’ll talk about how to harvest, cure, store, and preserve your onions too. Spoiler alert: homegrown onion powder is outstanding!

When to Grow Onions

Onions are generally considered a ‘cool season crop’ because onion seedlings like to start their life when conditions are still cool. Then as the weather warms up, they switch to focus their energy on bulb formation. Therefore, spring is the best time to plant onions in most places. If you hope to grow onions from seed, you’ll need to be prepared to get a jump start and plant seeds during winter. After sprouting onions take three to five months to reach a mature size, depending on climate and variety.

Gardeners with mild winters can also grow onions in the fall for a winter harvest. Here on the temperate Central Coast of California, we can grow onions essentially year-round! In colder climates, it is also possible to plant onion sets in fall but allow them to overwinter and harvest mature onions in the spring to early summer. In that case, plant onions when the soil is still warm – at least a month before your first fall frost date. That gives them time to establish healthy roots before they go dormant for the winter.

All in all, it may take a little experimentation to figure out the best time to grow onions in your particular climate and garden. One very important key to this puzzle is to choose the right type of onions to grow for your area!

Take zone 9 for example. According to our planting calendar, zone 9 can start onions in early winter and transplant (or sow outside) in late winter. Then, zone 9 gardeners can sow more onion seeds outside in late summer to early fall. If you need a planting calendar, get one here! We’ve created free printable calendars for every USDA hardiness zone.

How Do Onions Reproduce

Onions (Allium cepa) are biennials, that produce bulbs their first year. Onion bulbs store the nutrients that the onion flowers need for the producing seeds. When onions grow stems and flowers, it is known as bolting. When this happens, the stems produce seeds for onion reproduction.

Seed to Seed Reproduction

Reproducing onions using the seed-to-seed method is inexpensive and less work than planting bulbs. It also allows you to choose from a variety of cultivars for your specific needs. Once you plant the seeds in your garden, you just leave them there to grow into bulbs, bolt and then gather the seeds. Follow these tips for producing your own onion seeds:

  • Prepare your garden soil in late summer, removing all stones, weeds and clumps of soil.
  • Sow your onion seeds about one-half-inch deep and in rows that are two-feet apart.
  • Allow your seedlings to grow beyond their juvenile stage.
  • Vernalization will happen to the plants in the fall and winter. This cooling off period gets the onion plants ready for flowering in the spring.

Bulb to Seed Reproduction

If you have a favorite type of onion, and you want to maintain the purity of the onion, planting bulbs may be your best option. With bulb-to-seed reproduction, you plant mature onion bulbs. One requirement for planting bulbs for seed production is to vernalize the bulbs to encourage bolting. This occurs naturally in your garden in the fall, when the temperature falls below 50°F (10°C) for an extended period.

Onion bulbs become established in your garden much faster than seeds do. Plant the bulbs in well-drained soil about two-inches apart. Cover them with one inch of soil.

Pollination of Onions

Like other seed producing plants, onion flowers need pollination. The male and female parts of the onion flower develop at different times, making them ideal for cross pollination by insects. The umbel, or flower cluster lasts for about four weeks. Flies are the best pollinators of onions because honey bees aren’t attracted to the nectar.

Harvesting Onion Seeds

Determining when to harvest your seeds depends on the weather and seed maturation. Harvest your seeds when they’re black. About 10 percent of the seed should be visible in the umbel. If you wait too long to harvest your seeds, the seed heads shatter and fall. After harvesting, you can separate the seeds from their capsules by rubbing them in your hands.


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.

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!

How to Grow Onions - What Can Go Wrong

When growing onions, there are a few things that can go wrong. Onion maggots can infest your plants and are almost impossible to stop. They burrow through the bulg and feed upwards. Other pests include thrips which will cause mottling and withering plants.

The best defense against these pests is to (1) Plant onions in various locations around the garden (other plants will benefit: see companion planting), (2) Keep a clean garden thrips live in weeds from previous years, and (3) rotate your crops at least once every three years.

Common Onion Growing Problems and Solutions

There is nothing new under the sun. If you are beginning this tedious project, there are many potential issues you may run into. Don’t fear them, instead, get one step ahead of them by doing your research beforehand.

  • If your plants are leafy but don’t contain any bulbs, some changes need to be made. You will need to make sure you are planting at the right time, as well as keeping them at cool temperatures. This is the only way bulbs will be able to form. You can best solve this issue by putting the onions in the fridge a few weeks before you intend to plant them, so you have chilled cloves.
  • So your plants are not growing as tall as you’d like them to. This could be a sign that worms are in the roots. In order to avoid this issue, you will want to check the dirt before you plant. If you do see worms, the soil needs to be flooded. Worm infestations can to be stopped by taking the entire infested pants out of the ground and making sure to keep the entire garden clean, especially around the infected area.
  • Are your leaves streaked or blotchy? These are called thrips, and they are common in warm weather. The best way to keep them clean is by spraying them with water and insecticidal soap, to give them a nice little bath. Humans aren’t the only ones who need good hygiene!
  • Is your root red or pink? Do you have less crop each season? A disease has likely taken over your soil. Fungus takes the form of these colors in the root, and the only way to avoid this in the future is by making sure you only plant onions in drained soil and make it a habit to rotate crops.
  • If your onions are suffering from onion necks that are thick and short plants, this is likely a lack of potassium or phosphorus. Compost dressing is the way to go here.
  • If you have a spongy or soaked bulb neck, there are some simple things you can do. Removing infected plants and keeping weeds at bay are some of the best steps if you see mold, rot, or fungus.
  • Were you aware that the flower stalks could be sucking all the nutrients up, leaving none for the actual bulb of the onion? If your bulbs are hollow, show them some love by cutting the flowers off the plant, so the nutrients go to the right place.
  • Onion bulbs that are split up into sections can only mean one thing: uneven watering. Start fixing this problem by making sure the dirt is entirely moist all over. If you are not getting better results, begin mulching the soil. This helps to allow the ground to retain moisture evenly.
  • Have you ever tasted an onion that was supposed to be sweet, but ended up being bitter? The onion is likely not bad it is actually stressed out. The stress of heat and water can cause bitterness. Knowing this, begin growing your sweet onions in cooler weather with even watering, even using mulch if necessary.

Do you have a favorite onion seed variety that you grow?