Growing Onions from Sets and Seeds

Tips on How to Grow Onions Indoors and in the Garden

The Onion, scientific name Allium cepa, is one of the most cultivated species of the genus Allium.

It is commonly referred to as the common onion, bulb onion, and the garden onion.

The Allium genus includes several other species of onions, such as the Canada onion (A. canadense), Egyptian onion (A. × proliferum), and the Japanese bunching onion (A. fistulosum).

Growing Onions
Onions Photograph by Alexandre Dulaunoy.

The onion plant is a hardy cool-season biennial that is normally grown as annual. It has a base that enlarges to form a bulb and hollow leaves.

The bulb can be red, yellow, or white.

The flower stalks of Allium cepa are taller than the leaves and are topped by clusters of lavender or white flowers.

Onion Cultivars

The majority of onion cultivars belong to the common onion group, and are normally referred to as ‘onions’, with shallots and potato onions classes put into the 'Aggregatum group' of cultivars.

Onions are one of the most popular and widely grown vegetables for home gardens as they have the advantages that they can be stored for a long period of time, and can be used in a huge range of both cooked and raw culinary dishes.

Onion Varieties

There are several varieties of onion. These are usually grouped into yellow or white; red; and sweet-eating onion varieties.

Commonly grown onion varieties (and days to to harvest)
The yellow or white variety includes: Norstar (85 days); Eskimo (85 days); Buffalo (88 days); Copper King (95 days); Lisbon White; New York Early (98 days); Bingo (100 days); Fiesta; First Edition (105 days); Prince (106 days); Legacy (108 days); Alisa Craig (110 days); Duration (110 days); Riverside Sweet Spanish; Simcoe (110 days); Frontier; Gazette; Giant Zittau; Granex (110 days); Gringo; Headliner; Joint Venture; Kelsae Sweet Giant (110 days); Sweet Sandwich; Sweet Spanish Hybrid (110 days); Early Yellow Globe (114 days); and Blanco Duro (120 days).

The red onion varieties includes California Wonder Red (85 days); Red Burgermaster; Red Mac; Red Dutch; Mars, Mercury; Red Globe; Red Baron; Lucifer (106 days); Southport Red Globe; Rio Kyda Von 120 days).

The sweet onion varieties include Vidalia (110 days); Yellow Sweet Spanish (110 days) and Walla Walla (110-300 days).

Onion Seedlings
Photograph of onion seedlings by Feeb.

Onion Soil Preparation

Choose a location that has full sun, with a well-drained and well-worked soil. Loosen the soil to six inches (15 cm) deep; any roots, stones and lumps should be removed.

Add organic fertilizer or aged compost to the bed two to three weeks before planting.

Onion Planting

Temperature is important for onions as they are quite sensitive. Onions need cool weather in order to produce their tops, and warm weather in order for bulbs to be produced.

Plant small bulblets, or planting onion sets, three to four months before your chosen harvest season.

Onion seeds are best started indoors; the seeds should be sown four to six weeks prior to the average last frost date in spring. The onion seedlings can be transplanted out into the garden as soon as the soil can be worked. Depending on the variety, plant onions in the fall or winter in mild-winter regions.

Most varieties of onions are sensitive to day length; Spanish and American onions need long day photoperiods for bulbs to be produced.

Onion Sets

Onions sets are immature onions that should be planted in the spring or towards the end of summer. Each immature onion will swell in size and should be harvested once it is mature.

It is best to purchase onion sets that have been given a heat treatment. Onion sets are treated this way in order to destroy the embryos; this helps to prevent the growing onions from bolting (creating inflorescences) and allows more energy to be focused upon bulb growth.

Onion Growing and Care

The correct spacing when planting most onion variants is two to three inches (5-8 cm) apart, and rows that are 12 to 18 inches (30-45 cm) apart.

Thin seedlings at a distance of four to six inches (10-15 cm) apart to allow for bulb development.

It normally takes two seasons before bulbs are produced from seeds. The final size of the crop is dependent on the growing space provided.

The soil used for onion growing should be kept evenly moist until the maturity of the plant takes place.

Always keep the planting beds free of weeds in order to avoid competition for water, light, and nutrients.

Onions can be planted easily between large crops such as tomatoes or cabbages, or along with lettuce, beets, summer savory, strawberries, and tomatoes.

Troubleshooting: Onion Problems

Maggots and thrips can attack growing onions. These can be simply sprayed away with a stream of water. Maggot flies can be discouraged from laying their eggs near plants by placing three to four inches (7-10 cm) of square plastic around the plants.

Some more common onion problems are that they are susceptible to smut, root and bulb rot; and downy mildew in commercial onion growing districts.

Remove and dispose diseased plants as soon as possible, and plant disease-resistant varieties in order to keep the garden clean.

Onion Harvest and Storage

Bunching onions can be harvested once the bulbs are one to two inches (3-5 cm) in diameter. After the leaves have dried, lift dry onion bulbs when they are three to five inches (7-12 cm) in diameter.

When allowing lifted onions to cure in the garden, lift the entire root from the soil or they will either become soft and watery, or may start to grow again.

Cut the tops away from stalks one and a half inch (4 cm) from the bulb, when planning not to braid the stalks.

Mature onions bulbs should be allowed to air dry for a couple of days, before storing in a cool, dry place for up to six months.

I hope that you enjoyed this guide on how to grow Onion plants. You may also enjoy my gardening guides on how to grow Orchis, Ornithogalum plant, and Lapeirousia plants.