How to Grow Onions: Complete Guide from Seed to Harvest

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This post may contain one or more affiliate links, which means I will get a small commission when you buy a product via the provided link at no cost to you! As an Amazon Associate, I earn from qualifying purchases. See my full disclaimer here.

Today I’m going to tell you everything a beginner gardener needs to know about growing onions in the vegetable garden! Are you eager to embark on a rewarding journey of growing your food from scratch? Look no further than the humble onion plant. 

Onions are an excellent choice for beginners, they are versatile, flavorful, and easy to grow. In this beginner’s guide, I will take you through the step-by-step process of growing onion bulbs in your garden, from selecting the right variety to the proper care for your onions and enjoying a bountiful harvest.

If you do it right you can even store enough onions to last through the whole winter plus you can harvest seeds so you can plant your onions again next season!

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What is the Onion’s Life Cycle?

When you want to grow onions in your kitchen garden you should know the growth cycle of an onion. Every vegetable plant has its own unique life cycle. In general, onions are classified as biennial plants, meaning they complete their life cycle over two years. However, if you have a vegetable garden and want to eat the onions you grow, you should grow them as annuals. This means, harvesting them in their first year. Here below I will explain the stages of onion growth.

The First Season (Annual Cultivation)

When onions are grown as annuals, the entire growth cycle occurs within one year. Here are the key growing stages:

Planting

Onion seeds or sets (small bulbs) are planted in early spring or late fall. Check the pack of your onion seeds or sets and see in which climate you want to grow onions for the perfect time to plant them. Onions prefer well-drained soil and full sun.

Vegetative growth

After planting, the onion will focus on vegetative growth. This means the roots will develop, and green shoots emerge from the ground.

During this stage, the onion plant forms leaves and develops the bulb. Keep in mind, that onions have shallow roots, so proper watering and weed control are essential.

onion Bulb formation stage

As the onion plant continues to grow, it will put its energy towards bulb development. Onions store nutrients in the bulb, which swells and becomes the onion we use in the kitchen.

Maturation stage and harvest

Onions are usually ready for harvest in late summer to early fall, depending on the variety and growing conditions. Once the onion tops (foliage) start to turn yellow and fall over, it’s a sign that the bulbs are mature and ready for harvest.

The second season (biennial cultivation)

If you leave the mature plant in the ground after its first year in the garden, it will continue its growth cycle:

Bolting and flowering

In the second year, the onion plant transitions to its reproductive phase. It sends up a tall flowering stalk known as a scape. This scape will produce flowers, and if pollinated, it will produce seeds.

Seed production

The seeds mature and can be collected for planting in subsequent years.

Choosing the Right Onion Variety

Before you start sowing seeds or transplanting onion sets, it’s very important to select the correct type of onion for your region and desired use.

For instance, there are several varieties which are called long-day onions or short-day onions. This separates the variety of onions that need a lot of daylight versus the onion variety that needs less daylight. Furthermore, you have red onions, green onions, and yellow onions, large bulb or small bulb onions. So there is enough to choose from!

Common varieties like ‘Yellow Sweet Spanish,’ ‘Red Burgundy,’ and ‘White Lisbon’ are excellent choices for beginners due to their adaptability and flavor.

Yellow Sweet Spanish

• Mild flavored
• Creamy-white flesh
• Thin skin, so not ideal for long storage
• Start seeds outdoors in August, harvest half of March until the beginning of May
• Start seeds outdoors at the end of February, and harvest in mid-July
• Biennial plant

Red Burgundy

• Mild sweet taste
• Deep red onions with concentric white with red rings inside
• Not suited for long-term storage
• Start indoors in early spring 8 – 10 weeks before the last frost date, and harvest after around 100 days when the onion is the size of a tennis ball

White Lisbon

• Mild and tasty
• White bulb with a dark green stem
• Best eaten fresh
• Sow in late March – July, harvest late fall from May – October

Preparing the Soil

Onions thrive in loose, well-draining soil with a pH level between 6.0 and 7.0. Begin by clearing the planting area of any weeds or debris. Loosen the soil using a garden fork or tiller, and add compost or well-rotted manure to improve its fertility and drainage. 

Avoid using fresh manure, as it can burn the roots of young onion plants and can contain harmful bacteria. Compost your manure first.

Sowing Seeds or Transplanting Sets

Onions can be grown from onion seeds, perhaps even seeds you’ve harvested last season, or sets (small bulbs or onion seedlings). Onion sets will provide you with a head start in the growing process.

If you want to start seeds indoors, start them 8 to 10 weeks before the last frost date. You can also plant onion sets directly in the ground during early spring. Plant onion sets or onion seedlings about 4-6 inches apart, with rows spaced 12-18 inches apart.

When starting your seeds indoors you can make use of a sunny spot in your home or greenhouse. If you don’t have the space to keep your seedlings in a sunny spot, you can use grow lights.

Proper Watering and Mulching

Onions require consistent moisture to grow healthy bulbs. Water them deeply, however, be cautious not to overwater, as this can lead to rotting. Applying a layer of organic mulch, such as straw, wood chips, or dried leaves, around the plants helps retain soil moisture and suppresses weed growth. As a bonus, the organic matter will enrich the soil for future growing seasons.

A thick layer of wood chips is an excellent form of mulching

Nutrient Management of Your Onion Plants

If you want to add extra nutrients to provide growth for your onion plants you can add organic fertilizer or compost. Provide the onion plants with a nitrogen-rich organic fertilizer when the green tops reach around 6 inches in height, and repeat this process every 3-4 weeks. Avoid excessive nitrogen, as it may promote foliage growth at the expense of bulb development.

How to Control Weeds Around Your Onion Bulbs?

Weeds that grow around your onion plants will compete for nutrients, water, and sunlight. Regularly inspect your onion patch and remove any weeds by hand, taking care not to disturb the shallow onion root system. I just check in every day in the garden to weed by hand. It’s one of those tasks that you don’t want to skip too many days in a row to prevent weeds from overtaking the garden. Pull the weeds when they are still small and have shallow roots, it will only make things easier to handle!

Also, applying a layer of mulch on the soil surface, as said before, will significantly reduce weed growth, making your gardening tasks easier.

I’m not proud of our onions from a few seasons ago. Because of the pregnancy and the accompanying ailments, we have grossly neglected our onions and garlic. Because of all the weeds, the onions didn’t grow any bigger than this. It was a total lost crop, we only harvested smaller bulbs because the weeds around them were feeding off the nutrients in the ground.

Well, let’s just say it was a learning year, the crops were lost but lessons were learned!

Our overgrown onion patch from last year’s winter garden

What is the Right Time to Harvest Onions?

Onions are typically ready for harvest when the green tops have fallen over and started to dry. Gently loosen the soil around the bulbs using a garden fork, avoiding damage to the onion itself. Let the onions sit for a few days before harvesting them.

Allow the harvested onions to cure in a dry, well-ventilated area for a couple of weeks until the necks have dried completely. Trim the foliage and store the onions in a cool, dry place for long-term storage. 

Note: Not every onion crop is suitable for long-term storage.

What to Do About Bolting Onions?

Bolting refers to the premature development of a flowering stem, which can adversely affect the quality and storage life of the onion bulb. See my blog post about bolting onions for more information.

In essence, it comes down to this, you don’t want your onions to bolt in their first year of growing. If they do, it means they have had some form of stress (warm temperatures, too much or not enough water, daylight and shot straight into their reproduction phase. They will form a beautiful flower stalk (a white, blue, or purple flower) and eventually set seed.

You can harvest the seed and use it for next year’s gardening season. This way you provide your garden with your own seeds.

Note: The onions that bolted will not send any energy anymore to the onion bulb, it will produce small onions, they are not suitable for long-term storage, and it’s best if you eat them rather quickly.

Great Companions for Onion Plants

Choosing the right companion plants for your onions can provide numerous benefits, including pest control, enhanced growth, and improved flavor. Here below I drafted a list of some plants that make great companions for onions!

Carrots

Onions and carrots form an excellent companionship in the garden. Carrots help deter onion flies, which are a common pest for onions, while onions repel carrot flies. Their root systems also complement each other, as onions’ shallow roots won’t compete with carrots’ deeper roots.

By planting carrots next to your onions you’ll make sure the onion flies will be deterred

Lettuce and Spinach

Lettuce and spinach are shallow-rooted leafy greens that grow well alongside onions. They provide shade and help conserve moisture for onions while maximizing garden space. Additionally, the tall onion foliage provides shade for lettuce and spinach during hot summer months.

Plant lettuce as a space filler in your garden. The onion stalk will provide shade

Brassicas (Cabbage, Broccoli, Cauliflower)

Onions can help deter pests such as cabbage worms, which are a common problem for brassicas. Onions also release sulfur compounds into the soil, which can help deter clubroot, a fungal disease that affects brassicas.

Beets

Beets and onions are compatible companions in the garden. They are both root vegetables.

Beets are known to repel onion flies, while onions help deter pests that can harm beets, such as leaf miners.

Parsley and Thyme

Planting herbs near onions can help repel pests and attract beneficial insects. Parsley and thyme, in particular, can provide a lovely aromatic blend and add flavor to your culinary endeavors.

Chamomile

Chamomile has beneficial effects on the growth of nearby plants, including onions. Its flowers attract pollinators and beneficial insects while also adding a pleasant scent to the garden.

Marigolds

Marigolds are often used as companion plants due to their strong scent, which helps repel many pests, including nematodes. Planting marigolds around your onion patch can provide a natural pest control method.

Remember to consider the plant growth habits and spacing requirements of companion plants to ensure they don’t overshadow or compete with your onions. By incorporating these companion plants into your onion bed in your home garden, you can create a thriving and harmonious garden ecosystem while reaping the benefits of healthier, more flavorful onions.

Well, that was it, everything a beginner gardener should know when you want to grow onions in your vegetable garden. Did I miss something? Or have you learned something new? Let me know in the comments below if you’ve grown onions or are planning to!

This post may contain one or more affiliate links, which means I will get a small commission when you buy a product via the provided link at no cost to you! As an Amazon Associate, I earn from qualifying purchases. See my full disclaimer here.

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