Bulb onions require a long growing season—3 to 4 months. Onions are best planted in late winter or early spring for summer through autumn harvest. Green onions or scallions can be harvested about 2 months after sowing.
Onions and Day-length: An onion’s ability to form a bulb is influenced by the hours of daylight. Onions for northern latitudes (in the Northern Hemisphere) are called long-day onions; they form bulbs when days grow long and nights are short. Onions for growing closer to the equator—where summer nights are longer—are called short-day varieties. If you live where days and nights are of equal length in summer, grow an intermediate variety.
Onions and Temperature: Onion bulb formation is strongly influenced by not only day length but also temperature and environmental conditions. Warm temperatures are critical for good bulb formation; if the weather is too cold, onions won’t form bulbs. As well, soil too rich in nitrogen will delay bulbing.
Grow onions and other roots crops in light-textured soil free of pebbles and stones. This will ensure roots do not split or become malformed.
Onions mature in 95 to 115 days for bulbing onions depending on the variety, about 60 to 70 days for green onions.
Onion Seed Starting and Planting Tips
- Grow onions from seed, seedlings, or sets; onions grown from small bulbs are called “sets”.
- Onion seed is very small and can be difficult to handle; it may be easier to start seed indoors and transplant out seedlings. However, onions started from seed in the garden grow larger and store longer.
- Seed is viable for just 1 to 2 years.
- If you plant onions from sets, choose bulbs less than 1 inch (2.5 cm) in diameter; larger bulbs tend to flower sooner.
- Start seed indoors 4 to 6 weeks before the last expected frost; transplant to the garden as soon as the soil can be worked.
- Direct sow seeds in the garden in spring as soon as the soil can be worked.
- Sow seed ¼ to ½ (6-12 mm) inch deep; plant sets about 1 inch (2.5 cm) deep for large onions, 2 inches (5 cm) deep for green onions.
- Transplant seedlings started indoors to the garden 4 to 8 weeks after sowing.
- Sow seed 1 inch (2.5 cm) apart or closer and later thin successful plant to 4 inches (7.5-10 cm) apart for bulb onions, closer for green onions.
- Space rows 12 inches (30 cm) apart.
- Sow seed in loose, fertile soil. Adding aged compost to planting beds in advance of sowing will feed the soil and aide moisture retention.
- Seeds should germinate in 4 to 10 days at an optimal temperature of 70°F (21°C) or thereabouts; germination will take longer in colder soil but germination can be slow in chilly soil.
- Optimum soil temperature to grow onions is 55°F to 75°F (13-24°C).
- Onions prefer a pH range of 6.0 to 6.8.
- Grow onions in full sun for best yield.
- High soil nitrogen will delay bulbing—as will temperatures too cool.
- Onions grown in potassium-deficient soil will not store well; onions grown in phosphorus-deficient soil will have thick necks and mature more slowly.
- Keep the planting beds free of weeds to avoid competition for moisture and nutrients.
- Avoid planting onions where garlic or leeks have grown recently.
- Fertilize with an organic fertilizer such as fish emulsion at half strength.
- Common pest enemies include Japanese beetles, onion eelworms, onion maggots, slugs, thrips, and wireworms. Protect seedlings from pests and cold for two to three weeks after planting in the garden by covering seedling with spun poly row covers.
Interplanting: Plant with beets, carrots, kohlrabi, lettuce, parsnips, spinach, and members of the cabbage family.
Container Growing Onions: Choose a container 10 to 12 inches (25-30 cm) deep.
Onion Planting Calendar
- 12-10 weeks before the last frost in spring: direct-sow scallions and onions in a plastic tunnel or cold frame, or sow indoors for transplanting into garden 5 to 4 weeks before the last frost in spring.
- 5-4 weeks before the last frost in spring: direct-sow scallions in the garden; transplant onion seedlings to the garden.
For Fall and Winter Harvest of Green Onions and Scallions:
- 10-8 weeks before the first frost in fall: direct-sow green onions and scallions in the garden and under a plastic tunnel or cold frame.
Onion Types and Classifications
There are many types of onions to choose from and grow. Here are some basics:
Onion Color Classification:
- Yellow or golden varieties: best for storing, early to harvest in mid-summer.
- Red: sweet-tasting, early to harvest in mid-summer.
- White: mild flavored.
- Long storing: choose America type, yellow, white or red.
- Tiny and round: choose pearl or pickling onions.
- Large, sweet for slicing: choose Bermuda or Spanish onions.
- Scallions: harvest at scallions size or grow bunching onions.
- Multiplier onions: also called potato onions; produce a cluster of small to medium size bulbs joined at the base. Plant from sets in late summer.
Onion Day-length Classifications: different types form bulbs in response to varying day lengths:
- Short-day: requires equal amount of darkness and light (about 10-12 hours) to set bulbs; best grown in southern regions of the Northern Hemisphere; varieties include ‘Bermuda’, ‘California Red’, ‘Granex’, ‘Grano Super Sweet’.
- Long-day: require 14-15 hours of light and 9-10 hours of darkness; varieties include: ‘Early Yellow Globe’, ‘Ebeneezer’, ‘Ruby’, ‘Sweet Spanish’.
- Intermediate-day-length: requires about 13 to 14 hours of daylight to bulb; varieties include ‘Red Torpedo’, ‘Ringmaker’.
Botanical Name: Allium cepa
Onions are a member of the Alliaceae also called Lilliaceae family, other members of this family include garlic and leeks.
More tips: How to Grow Onions.