Growing Onions from Seed – How to / Better than Planting Sets?

One of the most popular plants in the vegetable garden, cultivating a crop of onion bulbs is a reliable, fairly low maintenance task. Many growers chose to cultivate their onion crop from sets. However you can also grow the plants from seed.

There are a number of benefits to growing onions from seed. Not only is it pleasingly affordable and straightforward it also reduces the chances of bolt occurring. Easier than you may think, this guide to growing onions from seed will take you through everything that you need to know.

The onion is a popular part of the vegetable garden.  

Why Should I Try Growing Onions from Seed?

There are a number of benefits to growing onions from seed.

One of the main reasons is that it helps to avoid plants setting flowers or bolting. To properly understand what bolt is, it is important to understand the lifecycle of the onion plant.

The onion is a biennial plant. This means that it grows and matures over 2 years. In the first year the seed germinates to form a bulb. If planted early enough the bulb is able to grow and mature for harvest.

If left in the ground overwinter the bulb experiences a dormant period before continuing to grow during the second spring and summer. As the temperature cools at the end of the second summer a flower spike is produced. Flowering and, if fertilized, setting seed. This completes the growth cycle of the onion.

Spending two years cultivating an edible bulb is both time consuming and, in some opinions, a waste of space. Gardeners are able to speed up the cycle by either planting a purchased set or growing seeds and harvesting them in the first year.

Growing onions from seed or seedlings enables you to choose from a wider variety of cultivars. Whilst this is a longer process, seed packets are also more affordable than onion sets, enabling you to cultivate a larger crop for less expense.

What are Onion Sets?

Commonly sold in garden stores, onion sets are small onions grown from seed the previous year. Instead of being allowed to mature the plants are harvested as immature bulbs, kept dormant during the winter and replanted the following spring.

Once planted onion sets mature into full grown bulbs during their second year. The head start that sets have is the main advantage that they afford the grower over cultivation from seeds or seedlings.

Cultivating onion bulbs from sets enables you to harvest the bulbs sooner. They are also, typically, larger than onions grown from seed. However, choosing to grow from sets severely limits your choice. Only the most common white, yellow and purple types are available as sets. If you want to grow something a little different you will need to purchase seeds or seedlings.

Onion sets are immature bulbs. 

Onions grown from sets are more prone to bolt than those from seed. This is because the plants are in their second year and are closer to maturity,

If you are growing onions from seed and your plants still fall victim to bolt, try delaying planting until the warm weather arrives. Prolonged exposure to temperatures below 45 ℉ can trick the plants into thinking they have experienced winter. This, in turn, triggers a hormonal response which leads to flowering.

Suitable for growers in northern latitudes, some types such as Hytech and Fen Globe are able to survive heat treating. This is a process that slowly kills the dormant embryo, helping to prevent bolting. Heat treating onion bulbs is more common in Europe and the UK than the United States possibly because it only works on a few of the long day cultivars.

Growing Onions from Seed or Sets?

This is a regular debate amongst even the most experienced gardeners. To put it simply, you can grow onions in 3 ways:

• By sowing seeds directly into the soil,
• Starting seeds undercover and transplanting to their final position in the spring,
• Cultivating onion plants from sets or immature bulbs.

Which method is best for you and your garden depends on a number of factors. The two most important of which are your climate and how much daylight your growing onion plants can receive. You should also consider the best types of onions for your area.

When making your choice it is important to know that onion plants are grouped into 3 categories:

• Short Day,
• Day Neutral,
• Long Day.

Each of these has its own specific light and climate needs.

Short day varieties such as White Bermuda, Southern Belle, Burgundy and Granex require 10 to 12 hours of light every day. These are an ideal choice for growers in southern and western gardens or mild climates, usually USDA Zone 7 and warmer. While short day types also grow in northern climates they are unlikely to mature to their full size.

Day Neutral, or intermediate types grow in most conditions, as long as the environment isn’t too extreme. Ideally the bulbs will receive 12 to 15 hours of light every day. Reliable types of Day Neutral include Red Amposta, Superstar, Candy, Yellow Globe and Cabernet.

Onion bulbs have specific cultivation needs.

Long Day types are the best choice for growers in USDA Zones 6 and cooler. Requiring at least 13 hours of light every day as well as a long growing season, many growers like to start these cultivars indoors, transplanting outside in the spring.

Growers in southern climates should avoid Long Day types of onions because their gardens do not receive enough daylight to form mature bulbs. Some of the most popular types of Long Day onion include Ringmaster, Ailsa Craig, Paterson, Red Zeppelin and White Sweet Spanish.

How to Grow Onions from Seed

To make growing onions from seed easier, select a variety or varieties that are suitable for your growing conditions. If you are growing a larger crop you may also want to consider the storage capabilities of the different varieties.

Timing is one of the most important aspects of growing onions from seed. Knowing when to sow the seeds gives them the best chance of growing and maturing before the fall. Depending on your climate and the cultivar, you can sow your chosen onion seeds in either the fall or spring.

Gardeners who enjoy warm winters can start short day types in the fall for a late spring harvest.

Long day and Day neutral types should be planted in the spring. Again growers that experience mild winters can also start the seeds in the fall.

Starting seeds undercover enables you to make the most of the growing season.  

If you are growing onions from seed in the fall, remember that the plants need 4 to 6 weeks of warm weather to germinate and establish themselves before the cooler winter temperatures arrive. Aim to sow your seeds 8 to 10 weeks before your first average frost date.

Growers that are starting the bulbs in the spring, or undercover in winter for a spring transition, should remember that growing onions from seed in a single growing season produces smaller bulbs than growing over two years or from sets. If you want a full bodied onion crop, sets are probably the better choice.

How to Sow Onion Seeds

The most important part of growing onions from seed is sowing them correctly. This helps to promote germination and the development of healthy seedlings.

Growing onions from seed requires a long germination and growth period. This means that seeds are best started undercover in winter before transplanting to their final growing position in the spring.

Sow the seeds in late January or early February in small pots or modular Seed Starting Trays filled with a moistened, seed starting mix. Sow no more than 3 onion seeds per module or pot. This gives the seedlings room to develop.

Cover with a thin layer of compost before firming down the soil and watering with a Plant Mister Spray Bottle. Do not use a watering can. The flow of water from a watering can may be too harsh, washing away or drowning the seeds.

Place the trays or pots on a light shelf in a greenhouse or a cold frame. Regularly check the seedlings for signs of germination. Keep the soil evenly moist. The temperature around the seedlings should average between 50 and 60 ℉. In favorable conditions germination takes 8 to 10 days.

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Following germination, thin out the seedlings to 1 seed per module or pot. Continue to grow the seedlings on in a light location, regularly moistening the soil.

As the seedlings grow, clip the tops to keep the seedlings to a manageable size. The seedlings shouldn’t be allowed to exceed 2.5 inches in height when in their trays.

Largely cold hardy plants, around 4 weeks before your last predicted frost date start to harden off the seedlings.

How to Transplant

Growing onions from seed requires loose, well draining fertile soil. Your chosen growing position should receive around 6 hours of sunlight every day.

Before planting, dig the soil over a couple of times, breaking up large clumps of earth and removing stones that may impact bulb development. This is also a good opportunity to work in any necessary soil amendments such as compost or an all-purpose organic fertilizer.

Handle the seedlings with care when transplanting. Carefully remove them from their trays and gently tease the roots free. Make a small home in the ground and plant. Gently firm down the soil around the seedling and gently water.

How far apart you should space your onion bulbs depends on the variety that you are growing. Consult the seed packet for the exact spacing.

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When selecting your growing position, remember that onions and other allium plants should not be grown in the same soil in consecutive years.

Regularly changing the growing position helps to keep your soil healthy and crops productive. While this may sound complicated, our guide to adopting a simple crop rotation system has all the information that you need.

Growing Onions From Seed Directly in the Ground

If you enjoy a long growing period you can try growing onions from seed directly in the garden.

In late February or early March prepare your growing position by digging over the soil, working in amendments and removing any rocks and stones.

The soil should have a temperature of around 50 to 60 ℉  before you sow the seeds. Any cooler and the seeds struggle to germinate. You can warm the soil by covering it with Reusable Plant Protector Cloches or blankets for a few weeks before sowing.

Sow the seeds during the last week of March. Before sowing, mark out a shallow trench and water. Sow the seeds 8 inches apart before covering with a thin layer of soil.

Following germination, thin out the seedlings when they are around 4 inches tall. Consult the seed packet for the exact spacing for your chosen variety.

Growing bulbs require lots of room to develop.  

Warning, thinning out onion plants causes them to release a distinctive aroma. This attracts the destructive onion fly. Covering the plants with a Garden Mesh Netting or row cover helps to protect them.

How to Care for Onions Growing from Seed

A longer process, growing onions from seed involves more work than simply cultivating the crop from onion sets.

One of the most important aspects of growing onions from seed is to regularly weed the soil. Weeds can sap the soil of moisture and nutrients, stunting the growth of your crops. Aim to weed the soil around your onion bulbs regularly, keeping the soil neat and tidy.

Once the plants reach 10 inches in height you can apply an organic mulch around the bulbs. This not only deters weed growth, it also helps the soil to retain moisture and, as the mulch breaks down, adds nutrients to the soil boosting growth.

When applying your mulch, be careful not to let it contact the growing bulbs. This encourages disease or infestations to form.

Keep the soil around the bulbs clear and weed free.  

When to Water

Water your crop regularly. Do not let the soil dry out. This is particularly important when growing crops with shallow roots like onions.

Not only does overly dry soil cause stress to growing plants, it can also cause them to mature before they are fully grown, spoiling the shape and flavor.

When watering young plants and seedlings it is better to use a gentle watering can than a hose. Gently watering young plants helps to prevent issues such as drowning or accidental washing away of developing seedlings.

Do not water wet soil. Plants that are allowed to sit in wet soil are prone to developing disease or issues such as rot.

How to Fertilize

Growing onions from seed benefit from a regular, low dose of nutrients.

Do not over fertilize the plants. This can encourage the overproduction of foliage at the expense of bulb development.

One of the best ways to provide growing onions from seed with an appropriate nutrient supply is to work a long lasting blood, fish and bone mix into the soil. This should be done once a month. When working fertilizer into the soil, be careful not to damage the shallow root system.

In June or early July apply a light dose of sulfate of potash. This helps to encourage the onion bulbs to ripen.

Growing Onions From Seed Problems

If done correctly this is largely a problem free process.

Bolt, a common problem for many onion growers, is less likely to occur when growing onions from seed. Usually triggered by an exposure to low temperatures after a warm spell, loose soil or accidental root disturbance can also cause the plants to flower. If your plant does bolt, cut off the flower stalk. While still edible you are unable to store onion bulbs that have flowered.

Powdery mildew, mold, smut and rust are all common during warm, humid spells. More common in soils that are nitrogen rich. Whilst unsightly these issues are rarely serious. One of the most common signs of fungal disease is a discoloration of the leaves.

Regularly check the plants and leaves for signs of disease or infestation. 

Should any serious fungal issues develop apply an onion licenced anti-fungal spray. This alleviates the symptoms.

Regularly check your plants for signs of disease. Whilst treatable early on, most problems, once established are difficult to cure.

One of the most serious issues affecting onion plants is Onion White Rot. This is a serious disease which causes white mold to form on the base of the plant. Should Onion White Rot occur, dig up and destroy the affected plants. Do not place them on a compost heap, this can cause the spores to spread further around your garden.

Onion White Rot spores can live in the soil for up to 8 years. Healthy garden practices such as crop rotation are not effective in this situation. Instead avoid using the affected area of soil completely. You should also take care not to transfer the infected soil to other parts of the garden.

If you have an area of soil that is infected and unusable, constructing a raised bed or planter filled with fertile, raised bed soil on the affected area, enables you to continue planting in the area.

The destructive onion fly can lay eggs around the base of the plant. The eggs hatch to release destructive maggots which attack the roots of the plant, stunting growth. Small plants are unlikely to survive. One of the most obvious signs of an onion fly infection is wilting, yellow leaves. Upon closer inspection you will notice maggots in the soil.

Difficult to cure, it is far easier to protect your plants by covering them with a mesh. This also protects the plants from allium leaf miner.

Finally birds, in particular pigeons, are notorious for pulling up rows of seedlings, destroying crops. Protect small plants with a net or fleece until they are well established.

Companion Planting

Companion planting is an easy way to promote growth and keep crops healthy. In basic terms companion planting is the practice of cultivating mutually beneficial plants together.

Members of the cabbage family, such as kale or broccoli, thrive when planted close to onion plants. This is because the aroma of the onion repels cabbage loving pests. Other good companion plants include:

• Peppers,
• Tomatoes,
• Strawberries.

When growing onions from seed, it is important to keep the soil around the plants weed free. One way to do this is to plant smother crops that discourage weed growth. Amongst the most reliable smother crops are leafy greens such as rocket and lettuce.

Leafy greens are a good smother crop to prevent weed growth. 

Do not practice growing onions from seed near peas, sage, beans or asparagus.

Finally space your allium plants out. Cultivating onion plants or members of the allium family together in a large group can encourage pests and infestation.

How to Prepare Growing Onions for Harvest

In late summer, remove the mulch and earth to expose around two-thirds of the bulb to the sun. As the bulbs swell cease watering and fertilizing.

Bulbs are ready to harvest when the leaves yellow and the stems start to bend. During wet summers the stalks of developing onion plants can stay green for too long, delaying the ripening process. To encourage ripening, pull the bulbs up slightly, breaking a few roots.

Harvesting and Storing Your Crop

Continue to care for your onion plants until the bulbs reach a suitable size. This varies depending on the variety you are growing.

Once the bulbs are at an appropriate size and the leaves brown or droop wait around a week before harvesting the entire crop.

To harvest your onion bulbs, lift them from the ground by carefully digging around the bulbs with a garden fork. Do not dig too closely to the bulb. Damaged or blemished bulbs will not store well. If you do attempt to store damaged bubs they may spoil your entire crop.

Dry or cure the bulbs before storing.  

If you are not using the onions in the immediate future you will need to cure the crop for long term storage.

To cure your onion crop, spread out the bulbs on newspaper or on the shelves of a well-ventilated greenhouse. Space the bulbs out so that air is able to circulate evenly around the bulbs.

It takes 2 to 3 weeks to fully cure onion bulbs. Once cured the skin of the bulbs will feel papery to the touch. The leaves will also shrivel.

Cut away the roots and remove any loose skin.

Store your cured onion bulbs in a cool, dry, well-ventilated space away from direct sunlight. A garage or cellar is ideal. You can also hang the crops in net bags. Hanging your cured onion bulbs enables air to circulate around the bulbs, reducing the chances of mold forming.

Hanging cured bulbs is a good way to store them.  

How to Harvest Onion Seeds

Onion seeds are readily available and affordable. However, if you have the space and time you can also harvest your own. Remember these are biennial plants meaning that they won’t set flowers or develop viable seeds until the second year.

Growers in warmer climates can allow some excess onion plants to remain in the ground overwinter enabling the plants to flower the following spring. If you are cultivating in a cooler climate where the onion plant is unlikely to survive the winter months you will need to dig up and store the bulbs overwinter before replanting the following spring.

After the plants have flowered, allow the blooms to go to seed. As the seeds ripen, cut the flowerheads and a little stem from the plant and place in a paper bag. Cutting a little stem as well as the flowerhead makes it easier to handle the plants.

Dry the flowerheads in the paper bag for around a month. Once dry you can release the seeds by shaking the bag. This releases the seeds from the flower head.

Store the seeds in an airtight jar in a cool, dry place until you are ready to use them.

Allowing some crops to remain in the ground, set flower and seed enables you to harvest fresh seeds for use the following year.

Warning, seeds harvested from F1 varieties do not grow true to type. The plants these seeds produce are likely to be a disappointment.

A reliable part of the vegetable garden, growing onions from seed can be a time consuming process. However it also has a number of benefits.