7 Super Simple Tips for Planting Onion Bulbs

Onions have been cultivated worldwide for thousands of years. They’re practically effortless to grow in your home veggie garden with the right environmental conditions. In this post, you’ll find out all about planting onion bulbs so you can enjoy delicious homegrown onions come harvest time.

The bulb onion (Allium cepa) is the most widespread member of the Allium plant family. Other alliums include garlic, chives, leeks, scallions, and shallots.

Most bulb onion varieties have a long growing season, taking between 90 and 150 days from planting seeds to harvest. In contrast, green onions and bunching onions are ready to pick in 20-30 days.

Gardeners have numerous options for planting onions. Start your onion seeds indoors in late winter or early spring. Or, purchase second-year onion sets or nursery-grown onion seedlings to transplant after the last frost date. Keep reading to learn how to plant onion bulbs in your home garden.

Best Tricks for Planting Onion Bulbs

The best approach to planting onions depends on your USDA hardiness zone, which onion varieties you grow, and your available garden space. Plant onions as early as possible since they have a long growing season, especially if you live in a colder climate.

Although bulb onions are usually annual crops, the plants actually have a biennial life cycle. They develop roots and leaves during the first growing season, then produce flowers and seeds in their second year.

Planting onions from bulbs or sets is the fastest and easiest onion growing technique. There’s minimal risk of frost damage, and growers usually have a higher success rate than they get from sowing seeds or transplanting seedlings. Most garden centers stock various types of onion sets starting in early spring.

Onion sets are miniature onion bulbs intended for planting next year’s crop. They’re grown close together to produce small, hardy bulbs. Because they’re in their second year, they mature faster and are ready for harvest in 60-75 days.

Be sure to harvest onions planted from sets before they begin developing flower stalks. When choosing onion sets, look for bulbs that are dime-size or smaller. Larger bulbs often bolt faster.

Best Time of Year for Planting Onion Bulbs

The best time to plant onion bulbs is generally spring and fall. Factors like seasonal temperature fluctuations and day length affect onion bulb formation.

Most bulbing onions grow their leafy green tops during periods of cool weather. They start forming bulbs, which are enlarged underground leaf structures, when temperatures warm up above 75℉.

Plant onion bulbs as soon as you can work the soil in early spring for a midsummer harvest. Get onion sets in the ground two to four weeks before your area’s forecasted last frost date.

If you started onion seeds indoors, wait to transplant the seedlings until after the last spring frost. Onion seeds germinate best when the soil temperature is 65-75℉, and the seeds won’t germinate below 40℉.

Planting onion bulbs in the fall gives the plants a head start in the springtime. They become well-established in the cool weather of autumn, then go dormant for the winter when temperatures drop below freezing. After coming out of dormancy, they’ll be ready to harvest by late spring or early summer.

Prepare Your Planting Site

To prepare your garden or raised beds in the springtime, lay down two inches of organic compost or well-rotted manure about two or three weeks before planting onion bulbs. Doing so helps to warm the soil and gradually releases nutrients throughout the growing season.

Onions have a shallow root system and grow best in light soil with good drainage. If your garden soil is excessively compacted or consists of heavy clay, till organic matter like compost, manure, grass clippings, and leaves into the top six to twelve inches to break it up.

Growing Conditions for Onion Plants

Select a planting site for your onions with full sun and rich, well-draining soil. Onion plants prefer neutral to slightly acidic soil with a pH level between 6.0 and 7.0.

Onions grow best in consistently moist soil and need about an inch of water per week, or more during particularly hot or dry weather. However, avoid overwatering to prevent problems with fungal disease or rot. The soil should have a consistency similar to a wrung-out sponge.

With their shallow roots, onions suffer when competing with weeds for water and nutrients. Mulching around the plants minimizes weed growth, holds in moisture, and insulates the soil.

Leave plenty of space between your onion plants, so they grow large, juicy bulbs. Spacing between each bulb should be approximately three or four inches or more for wider varieties.

After harvesting, store homegrown onions in a cool, dry place with good air circulation and temperatures consistently below 40℉. Don’t let them freeze.

Best Fertilizer for Onions

Onion plants are heavy feeders and benefit from regular fertilizing as they develop. Plant fertilizer contains three fundamental macronutrients: nitrogen (N), phosphorus (P), and potassium (K). Nitrogen promotes leaf growth, phosphorus encourages robust root systems, and potassium stimulates fruiting and flowering processes.

Use organic fertilizer with high nitrogen levels every four to six weeks until the bulbs’ shoulders start pushing the surrounding soil away. More leaves mean bigger bulbs at harvest time.

Types of Onions

Before planting onion bulbs, find out which varieties grow best in your area. Bulb onions fall into three basic categories: long-day, day-neutral or intermediate-day, and short-day.

Long-day onions are best for northern regions with long mid-summer daylight hours. They need 14-16 hours of daylight during later stages of growth for the bulbs to mature. Grow long-day onions in USDA zones 6 and lower.

Short-day onions grow well in southern climates where winters are mild. They typically get planted in the autumn and either go dormant through the winter or keep growing until spring and are ready to harvest in early summer. Short-day onions start forming bulbs when the day length exceeds 12 hours, and temperatures rise above 70℉. Plant short-day onions in USDA zones 7 and higher.

Day-neutral or intermediate-day onions thrive in most climates. Their bulb formation doesn’t depend on day length. Plant day-neutral onions in early autumn for a late-spring harvest if you have mild winters. Start them in late winter or early spring in colder regions.

Best Bulb Onion Varieties for Home Gardeners

• Candy (hybrid, day-neutral yellow onions, mild flavor, extra-large bulbs, 85-95 days to harvest)
• Red Wethersfield (heirloom, long-day red onions, mild flavor, large flattened bulbs, 100-110 days to harvest)
• Southern Belle (hybrid, short-day red onions, sweet flavor, globe-shaped bulbs, 110-120 days to harvest)
• Super Star (hybrid, day-neutral white onions, mild flavor, large globe-shaped bulbs, 95-105 days to harvest)
• Walla Walla (heirloom, long-day yellow onions, sweet flavor, large bulbs, 90-100 days to harvest)
• White Bermuda (heirloom, short-day white onions, mild flavor, flattened bulbs, 95-105 days to harvest)

Troubleshooting Common Onion Growing Problems

Although onions are usually low-maintenance, they’re sometimes prone to a few pest and disease problems. Healthy plants are less vulnerable to attacks from insects and pathogens. Set yourself up for success by giving your plants appropriate amounts of light, nutrients, and water.

The most frequent insects that feed on onion plants include onion maggots, leafminers, and thrips. Leafminers and thrips damage leaf tissue and leave behind white-ish scars. Spray any affected plants with an organic insecticide like neem oil or spinosad.

Onion maggots are fly larvae that burrow into allium bulbs. The damaged tissue is exposed to infection and may kill the plant. Protect your onion crop with floating row covers in early spring to keep pests from laying eggs on the plants or nearby soil. Onion flies are resistant to insecticides. Remove any infested plants and spread diatomaceous earth or ground cayenne pepper to deter adults from laying eggs.

Bulb rot is a prevalent onion plant disease that results from prolonged overwatering or poor soil drainage. Conversely, onion bulbs often split if they remain dried out for too long.

Companion Planting Onion Bulbs

It’s advantageous to design your garden by pairing compatible plants together to take advantage of their mutually beneficial attributes. Many plants attract beneficial pollinators and predatory insects, repel bothersome pests, and improve their neighbors’ flavor and overall growth. However, certain plants don’t get along well and must grow separately.

Onions are wonderfully helpful companion plants around a veggie garden. Their powerful odor repels numerous insects, including aphids, ants, cabbage loopers, carrot flies, flea beetles, potato beetles, spider mites, and mosquitoes. They also deter larger garden pests like deer and rodents.

Plant dill near your onions to ward off onion flies. It also attracts beneficial predatory insects that feed on pests. Basil and marigold effectively repel flies and thrips.

Be sure to plant your onions away from asparagus, beans, peas, and sage. These plant groups reportedly stunt one another’s growth. Additionally, avoid planting your onions next to other alliums since they’re susceptible to the same types of pests and diseases.

Most gardeners agree that growing an onion from a bulb is one of the easiest, quickest ways to get a terrific springtime onion crop. The best time for planting onions from bulbs is generally early spring and late fall. Long-day onions are well-suited for northern climates, and short-day onions grow best in warmer southern regions. Day-neutral or intermediate-day onions are more versatile and flourish in most areas.

Do you have any questions or recommendations about how to plant onion bulbs? Please share them in the comments! If you found these tips on planting onion bulbs helpful, please share this post with your fellow green thumbs.