How To Grow Onions From Bulbs (Gardener’s Guide)

If you love the taste of fresh onions in your cooking but hate paying for them at the store, then growing your own onions is a great solution. Onion bulbs are easy to grow and can be kept both indoors and outdoors. Here’s a gardener’s guide on how to grow onions from bulbs:

To grow onions from bulbs, you will need some onion bulbs that you can source from your local nursery or garden center. Plant the bulbs in well-drained soil in a sunny spot. Water deeply and infrequently so the soil stays moist but not soggy. In 2-3 weeks, green sprouts will poke out of the soil.

However, you must equip yourself with the necessary knowledge and skillset to grow healthy, vigorous bulb onions. In this comprehensive gardener’s guide, I will show you how to grow onions from bulbs, detailing every step. Let’s get started!

1. Consider the Growing Requirements of Bulb Onions

The key to growing onions from bulbs is creating the right environment for their growth. While a bountiful harvest is the ultimate goal, you must also consider the preferences of your chosen onion variety.

To ensure a healthy crop, here are the basic requirements you need to take note of:

Sunlight: Onions need at least six hours of sunlight every day. The light helps with the development of the leaves and the formation of the bulbs. Choose a spot near a sunny window if you’re growing onions indoors. Provide some afternoon shade for areas with long, hot summers to prevent the bulbs from drying out.
Soil: Onions prefer loose, fertile, well-drained loam soil. Compacted soils with poor drainage are a no-no, as they can cause the bulbs to rot. If your garden’s soil is heavy, mix in some organic matter to help improve its nutrient and water retention capacity.
Soil pH: Onions prefer slightly acidic soil with a pH of between 6-7. If unsure about your soil’s acidity level, have it tested by your local gardening center or cooperative extension office. Alternatively, you can also use a home soil test kit.
Water: Keep the soil moist but not soggy, as onions are prone to rotting when wet. Water regularly, about one to two inches (5 cm) per week, depending on the weather conditions. Mulching also helps retain moisture and suppresses weeds while regulating soil temperature.
Fertilizer: A light application of balanced fertilizer is all that onions need. Too much nitrogen will encourage leaf growth at the expense of the bulbs. Apply a fertilizer high in potassium a few weeks before harvesting to promote bulb development.

2. Source Bulb Onions

Like most plants, the quality of your bulb onions will largely depend on the quality of the seeds or bulbs you start with. To ensure a good harvest, source your onions from a reputable nursery or garden center. Look for healthy, plump bulbs that are free of blemishes or bruises. Avoid those that are soft, shriveled, or have started to sprout.

Ensure you get the right variety of onion bulbs best suited for your local climate and growing conditions. For example, if you’re growing onions in a warmer climate, choose a variety that matures quickly, such as ‘Texas Early White. For a cooler climate, go with a longer-day variety such as ‘Walla Walla.’

Another factor to consider is whether you want to grow onions for storage or fresh eating. Storage onions have a thicker skin and are best harvested when the tops have started to yellow and fall over. On the other hand, fresh-eating onions have a thinner skin and are harvested when the tops are still green.

When in doubt, ask your nursery or garden center staff for guidance in choosing suitable onion bulbs for your needs.

3. Select a Planting Site

As with most plants, the location of your onion patch is just as important as the quality of the seeds or bulbs.

To ensure a successful crop, here are a few things to consider when selecting a planting site:

Choose a spot that gets abundant sunlight. Onions need at least six hours of sunlight every day to thrive.
The soil should be loose, fertile, and well-drained. Avoid areas with compacted soils as they have poor drainage and can cause the bulbs to rot.
The soil’s pH should be slightly acidic, 6-7, with a high nutrient and water retention capacity.
It should be close to a water source for easy irrigation.

Alternatively, you can grow bulb onions indoors in a sunny spot near a window. Choose a planting container at least 12 inches (30 cm) deep with drainage holes to prevent the bulbs from rotting.

Do you know how deep soil needs to be for onions to grow well? Click on the link to read my in-depth guide on the topic. You’ll also learn what soil is best for onions and when it is best for growing onions: How Deep Does Soil Need to Be for Onions?

4. Prepare the Soil

Whether growing onions in the ground or a pot, preparing the soil is crucial to their success. As mentioned earlier, onions need loose, fertile, and well-drained soil to thrive. They are also quite sensitive to soil conditions and can be easily damaged by too much or too little water.

To prepare the soil:

Loosen it with a spade or tiller to a depth of about eight inches (20 cm). If the soil is particularly compacted or clay-like, you may need to loosen it to a depth of 12 inches (30 cm). Remove any debris, rocks, or roots you may find and trash them.
Add a 2 – 3 inches (5 – 8 cm) layer of organic matter such as compost, peat moss, or well-rotted manure to improve the soil’s drainage and water retention capacity.
Work in the organic matter in the top 6-8 inches (15 – 20 cm) of soil and then level it out with a rake.
It’s also a good idea to take a soil test to check your soil’s nutrient levels and pH. This will help you determine if you need to add any amendments to improve the soil quality.
Water the soil deeply a day or two before planting to help the bulbs develop a robust root system.

If you’re growing onions in a pot:

Choose a high-quality potting mix to provide the bulbs with the necessary nutrients. You can purchase potting mix at your local nursery or garden center. Alternatively, you can make your own by mixing equal parts of perlite, vermiculite, and compost.
Water the potting mix well and then let it drain completely before planting the bulbs. This will help prevent the bulbs from rotting.
It’s also a good idea to add a slow-release fertilizer to the potting mix before planting. This will give the onions a steady supply of nutrients throughout their growing season.

5. Plant the Bulb Onions

With the soil prepared and the planting site chosen, it’s finally time to plant the bulbs. The best time to plant onions is in early spring, about four to six weeks before the last frost date. This timing gives the onions a head start on the growing season and ensures they mature before the summer heat sets in.

There are two ways to plant onion bulbs:

Directly in the soil.
Starting them indoors in seed trays or pots.

Planting Directly in the Soil

If you live in an area with mild winters, you can plant your onion bulbs directly into the ground without the threat of frigid temperatures damaging the roots.

However, if you’re unsure whether your area has a suitable climate, it’s essential to acquaint yourself with the USDA plant hardiness zone map. This map shows the average minimum temperature in different regions across the country and can help you determine whether it’s safe to plant your onions directly into the ground.

Follow these steps to plant your bulbs:

Use a trowel or your hands to make a hole in the soil that’s twice the width and depth of the bulb.
Space the holes six to eight inches (15 – 20 cm) apart in rows that are 12 to 18 inches (30 – 45 cm) apart.
Place the onion bulb in the hole with the pointed end facing up.
Fill in the hole with soil and lightly pack it down. Tamp the soil gently with your hands or the back of a rake to remove any air pockets.
Water the soil well to help the bulbs settle in and encourage root growth.

Starting Bulb Onions Indoors

If you want to jumpstart the growing season or live in an area with cold winters, you can start your onions indoors in seed trays or pots. Freezing temperatures inhibit enzyme activity, damaging the roots and preventing the onions from growing. By starting them indoors, you can ensure they get a head start on the season and avoid any frost damage.

To start your onions indoors, follow these steps:

Fill seed trays or pots with a high-quality potting mix. A potting mix provides the onions with the essential nutrients they need to grow.
Make a small hole in the potting mix, and then place an onion bulb in the hole with the pointed end facing up.
Fill the hole with a potting mix, and then water the soil well.
Place the seed trays or pots in a sunny location, such as a windowsill or under grow lights.
Water the onions regularly to keep the soil moist but not soggy. Fertilize the onions every two weeks with a half-strength solution of liquid fertilizer.

When the onion bulbs start sprouting, thin out the seedlings so that only the strongest ones remain. Transplant the seedlings into larger pots or plant them outdoors after the last frost date.

Transplanting the Onions

Once your bulb onions have grown enough, you’ll need to transplant them into the garden or larger pots. This process allows the onions to spread their roots and grow to their full potential.

However, you must harden off the onions before transplanting them. Hardening off is the process of slowly acclimating the plants to outdoor conditions. This prevents them from going into shock and ensures they can thrive in their new environment.

To harden off your onions:

Start by placing them outdoors in a shady spot for a few hours the first day.
Gradually increase the time they spend outdoors each day, and then move them to a sunnier location.
After a week of hardening off, your onions should be ready to transplant.

Follow these steps to transplant your onions:

Prepare the planting bed by tilling the soil and removing any rocks or debris.
Loosen the soil with a shovel or trowel, and then make a hole twice the width and depth of the onion bulb.
Space the holes 6-8 inches (15 – 20 cm) apart in rows that are 12 to 18 inches (30 – 45 cm) apart.
Carefully remove the onion bulbs from their pots or seed trays. Avoid damaging the roots or sprouts.
Place the onion bulbs in the holes with the pointed end facing up and backfill with soil.
Gently tamp down the soil to remove any air pockets and water the onions well.

6. Care for Your Onions

Onions are relatively easy to care for, but there are a few things you need to do to ensure they stay healthy and productive. Proper care and maintenance will result in a bountiful harvest of delicious onions you can enjoy all season long.

Here are a few tips for caring for your onion plants:

Water deeply and infrequently: While onions are drought tolerant, they do need to be watered regularly to prevent the bulbs from drying out. Water the onions deeply and less frequently to encourage the onions to develop deep roots. Aim 1-2 inches (25 – 50mm) of water per week. Soggy soil can lead to root rot, so ensure the soil is dry before the next watering.
Weed regularly: Weeds compete with your onion plants for water and nutrients and can harbor pests and diseases. Keeping your onion beds weed-free will help your plants stay healthy and productive. Hand-pull or use a hoe to remove any weeds that pop up.
Mulch: Mulch helps to retain moisture, suppress weeds and keep the soil cool. Apply a 2-3 inches (5 – 8 cm) layer of organic mulch around your onion plants. Leave a small space around the base of the plants to prevent rot.
Fertilize: Regular fertilizer feedings will help your onions grow to their full potential. Use a balanced fertilizer or compost tea every two weeks during the growing season.
Provide shade: If you live in a hot climate, your onion plants may benefit from some afternoon shade. This will help to prevent the bulbs from overheating and drying out. Use row covers or shade cloth to protect from the sun.

7. Try To Control Pests and Diseases

Onions are relatively resistant to pests and diseases, but there are a few of them that can cause serious problems. The best way to control pests and diseases is to practice preventive measures such as keeping your plants healthy and weed-free. For example, you can sprinkle a layer of Repels-All around your onion garden to keep pests away: Is Repels-All Safe for Vegetable Gardens?

Here are a few common onion pests and diseases:

Aphids: Aphids are small, sap-sucking insects that can cause stunted growth and distorted leaves. Control aphids by blasting them off with a powerful stream of water or treating them with insecticidal soap. You can also introduce beneficial predators such as ladybugs into your garden.
Thrips: Thrips are tiny, winged insects that feed on the leaves of onion plants. They have specialized mouths that suck the plant juices, causing the leaves to become discolored and distorted. They also secret honeydew, which can lead to sooty mold. Control thrips by treating them with insecticidal soap or neem oil. You can also release beneficial insects to feed on the thrips.
Onion Maggots: Onion maggots are tiny, white larvae that feed on the roots and bulbs of onion plants. Left unchecked, they can quickly destroy an entire crop. Control onion maggots by using row covers to prevent the adults from laying eggs near the plants. You can also treat it with an insecticide such as spinosad.
Leaf Blight: Leaf blight is a fungal disease that affects the leaves of onion plants. The leaves turn yellow or brown, and the tips may die back. The disease can spread quickly in warm, humid weather. Control leaf blight by practicing crop rotation and using a fungicide such as copper sulfate.

8. Harvest and Store the Onions

Bulb onions are ready to harvest approximately 100-125 days (3-4 months) after planting. The best time to harvest is in the late summer or early fall when the tops have started to yellow and fall over. Harvest in the morning after the dew has evaporated.

To harvest your bulb onions:

Loosen the soil around the plants with a spading fork to make it easier to pull them up.
Carefully lift the plants out of the ground, being careful not to damage the bulbs.
Brush off dirt and allow the onions to dry in a sunny spot for a few hours.
Cure the onions in a warm, dry place for two weeks. This helps to toughen the skin and improve storage life.
Cut off the tops, leaving about an inch (2.54 cm) of stem attached to the bulb.
Avoid washing the onions, as this can lead to rot.
Store the onions in a cool, dry place. Ideal storage temperatures are between 32-40°F (0-45°C).

Onions can be stored for several months if under the right conditions. Check them periodically to ensure they are not drying out or sprouting. Use any onions that show signs of spoilage first.

If you’d like a more in-depth guide about harvesting your fruits and vegetables, you can read this article: How to Harvest and Store Your Fruits and Vegetables

Final Thoughts

Growing onions from bulbs is an excellent way to enjoy fresh onions all throughout the growing season, saving you money at the grocery store. By following these simple tips, you can grow a healthy crop of onions at home. Give it a try and see how easy and rewarding it can be. Bon appétit!