The Onion Growing Guide: How to Plant Onions

By Caroline Nagrodsky Posted on 4/28/23

Onions are one of the foundation vegetables. In other words they are used in cuisines all over the world as a staple. Central Asia saw the origination of onion farming over 5,000 years ago, and it has been cultivated ever since. Onions were even found mummified with ancient Egyptian mummies. They are very versatile, sliced and served raw as part of sandwiches or salads, fried, pickled, roasted, and prepared in many types of dishes, they are really one of the most popular vegetables in the world! They are related to several others, including garlic, leeks, shallots, chives and scallions.

Onion Garden Facts

Onions are high in antioxidants (especially yellow and red onions).  They are high in vitamin C, potassium, as well as vitamins B6 and B9. They are low-carb and low-fat, and are a great healthy addition to most diets. Allergies to onions are very uncommon, but the rare occurrence of an allergic reaction is a good thing to be aware of, as is the potential for onions to sometimes cause bad breath!


  • Onion (Allium cepa)
  • Biennial
  • Full sun
  • Soil should be slightly acidic 6.0-7.0 pH)
  • Well-drained rich warm soil
  • Garden planting:

Start indoors, 10-12 weeks before the last frost of spring.

Transplant to the garden 2 weeks before the last frost date

Plant the seedlings 5 inches apart, and the bulb no more than 1 inch deep. Roots should be deeper.

Rows 15 inches apart

  • Mulch surface well (6” of straw)
  • Climate Zones:

Short-day 7-12

Day-neutral 5-6 is best, but will grow anywhere

Long-day 2-6

  • Rotate crops
  • Container planting:

Large pots are needed to grow a good amount for harvest.

3 foot long pots at least, 12 inches deep, or use a large plastic tub

Start indoors, and transplant to the final container at about 10 to weeks

Plant seedlings 5 inches apart

Newly pulled onions
Source: OakleyOriginals

An important distinction for gardeners is that there are three main basic categories of onions for growing purposes. These are long-day onions, short-day onions, and day-neutral onions. Long-day onions need longer periods of direct sunlight to produce bulbs, 14-15 hours is best. These are grown in northern areas, usually zones 2-6. Short-day onions only need 10-12 hours a day, and are best in the South, zone 7 or warmer. Day-neutrals will bulb anywhere, regardless. These “middle of the road” types are commonly grown in the central region of the United States, zones 5-6, but are popular all over, and will bulb anywhere in the US. The 35th parallel is a good dividing line, for determining whether you are “north” or “south.”  As an easy reference in the US, this is right along the southern border of Tennessee, and goes along the southern edge of the Albuquerque area in the west.


Types of Onions

Within each of the three growing categories are all three of the familiar color types of onions. Specific varieties of yellow onions, red onions, and white onions are part of each of these groups. Yellow onions are the most common, with the familiar strong flavor, and these are usually cooked in various recipes. Red onions are often red or purplish, and they have the strong taste of yellow onions but have more sugar, and so are both strong and sweet. White onions are the sweetest, and are very often sliced or grated raw. Green onions or spring onions are often used as an alternative to scallions in recipes, though they are much less sweet when served raw.


As it shows in the chart, it is best to start onion seeds indoors, well before spring. They should get only 10 hours of light per day during and after germination, to prevent bulbs from starting. As the seedlings grow, usually about 4 weeks after germination, trim the seedlings down to about two inches tall. Transplant them to the garden bed two weeks before the last frost. They should be spaced about 5 inches apart within each row. Rows should be 15 inches apart. Don’t plant them too deep, about 1 inch is fine. The bulbs will form just beneath the surface of the soil. Plant too deep and bulbs won’t form! Keep them well watered! The soil should be moist but not damp. The equivalent of 1 inch of rain per week is ideal.

Red onions
Source: hconw1

Onions can tolerate a wide range of temperatures, even withstanding frosts. Hard freezes are different, this can damage the onions. If temperatures drop below 25˚F, it’s best to cover them for protection with a layer of mulch, or if needed even a tarp.


Planting Onions in Containers

Onions can also be planted in containers. These should be deep, at least 15 inches deep or more, and long enough (3 to 4 feet long) to provide for a good sized harvest of bulbs. A large plastic tub can be used instead, with two rows of plants, one on each side. Onions should be planted 5 inches apart in the row, just like in the garden. Ensure that the containers are well drained. Container grown onions do need to get enough light, so short-day onions may be best if the containers will be inside, but 12 hours a day of light is a good rule of thumb for these. Sunlight is best, but grow lights will work as well. It is best to start them off with either some fertilizer or good compost when planting.


Onion Pest and Disease Mitigation

Onions do sometimes have trouble with pests and diseases. Blast, the common name for leaf blight in onions, is a fungal disease.  Another disease is sour skin (or sour scales). This is a bacterial infection that causes a sliminess and affects the quality and taste of the onion. Regular crop rotation, ground level watering (as opposed to overhead watering), weeding carefully, and using well-drained soil is the best prevention for both pests and diseases.


Root knot nematodes are a common onion pest. The best prevention is regular crop rotation, and cover crops over the late fall and winter months. Onion flies are another pest to be wary of, especially in coastal areas.. These lay eggs on the soil surrounding the onion, and the maggots will feed on the seedlings. This can be prevented in most cases by transplanting seedlings that were started indoors into the garden, rather than directly planting the seeds in the ground. Onion eelworms are a bigger problem, if these are detected, destroy the plant completely, and don’t dispose of the plant in compost.

Onions growing
Source: Stephen Rees

Onion Companion Planting

There are several crops that work very well as companion plants for onions. Chamomile and winter savory attract good predatory insects that will help protect onions from problem pests. Onions also help to repel pests for a large number of other plants. These include cole crops like broccoli, turnips, brussels sprouts, and kale. The same is true with strawberries, beets, carrots, lettuce, tomatoes, and peppers.


Harvesting onions can be done early, for a yield of green onions. Near full maturity, the leaves and stems of the onion will start to lean over. This happens about a week or two before you should harvest. Harvest the whole bulbs with the stems, and lay them out carefully to dry, or “cure.” Avoid direct sunlight. White onions should be allowed to cure for a few days, so that the necks and stems can dry out, but then should be used. Yellow and red onions should be cured for a few weeks, but then can be stored for use over the winter if needed.


Harvesting Your Onions

There are a few important things to know if harvesting seeds from your onions. First, onions are biennial, so they last for two years, producing bulbs the first year, and seeds the second year. For seeds, leave a few heirloom or open pollinated onions in the ground, unharvested, over the winter. In the coldest climates, where the ground freezes over, pull the bulbs from the ground in late fall, and store them in the refrigerator over the winter, until the next spring, when they should be replanted once the temperature warms up into the 50s ˚F. They will re-sprout, with leaves and flowers. Once the flowers turn brown, clip off the stalks, and put them in a bag and store them in the fridge for a few weeks, until they are completely dried out. The seeds can be shaken off the flowers and collected, and stored for future use!

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