Start seeds indoors about 6 weeks before transplanting. Onion seeds are short-lived. If planting seeds indoors, start with fresh seeds each year.
Plant onions as soon as the ground can be worked in the spring, usually late March or April. Make sure temperature doesn’t go below 20 degrees F.
If you are starting from transplants instead of seeds, skip the above steps. Plant the transplants 1" deep, with 4 to 5" between each plant and in rows 12 to 18" apart.
When to Water
Onions need about 1 inch of water per week, and that is including rain water. Onions will look healthy even if they are bone dry underneath. Add mulch to preserve moisture while killing weeds. If you want sweeter onions, water more.
Onions are of the hardiest plants to cold weather. Furthermore they can grow well in a wide range of temperatures. The best temperatures for growing onions are from 55 to 75°F. They produce higher quality bulbs if the weather is cool during their early growth stage, then they like increased temperatures for optimum maturity.
Blight, purple blotch, and thrips can be a problem for onions. Both blight and purple blotch are caused by fungus, and are more common during periods of high moisture. Blight appears as small white spots surrounded by a greenish halo. Purple blotch causes a purplish discoloration of leaves. Proper plant spacing helps increase air flow and reduces both blight and purple blotch. Thrips are insects that sometimes attack onion plants, causing the leaves to turn grey.
Soil needs to be well-drained, loose, and rich in nitrogen; compact soil affects bulb development.
Onion plants can reach a height of 1 to 2 feet.Take that into consideration when planning your garden. Make sure there is enough distance between your tall vegetables and short vegetables. That way both will get enough sun light.
When the tops become yellow and begin to fall over, it's harvest time! At that point, bend the tops down or even stomp on them to speed the final ripening proces. Loosen the soil to encourage drying, and after a few days turn them up and let them cure on dry ground. Always handle your harvested onions carefully, the slightest bruise could cause rot to set in.
Onions grow best in temperate climates without extremes of heat or cold. The time of year when onions are planted affects when they form bulbs. If you plant your onions too late in the season, they may not form bulbs properly.
Work aged manure or fertilizer into the soil a month before planting. Onions are heavy feeders and need constant nourishment to produce big bulbs. When planting, you can mix in some nitogen fertilizer, too, and side dress every few weeks until the bulbing process begins. Fertilize every few weeks with nitrogen to get big bulbs. Stop fertilizing when the onions push the soil away and the bulbing process has started. Do not put the soil back around the onions; since the bulb needs to emerge above the soil.
Onions come in two different classes: long-day and short-day onions. Long-day onions are more appropriate for northern states because they are adapted to longer days. Southern states should use short-day varieties of onions.
Onions have shallow roots and require a bit of help to get established. They are also vulnerable to competition from weeds, so weed regularly. Onions can be grown from transplants, small dry bulbs (also known as sets), or seeds. If growing onions from seed, make sure to rake the soil after heavy rains so that the onion stalks can break through, and water the soil regularly in the morning.
Onions are biennial plants (which means that they can survive winter and grow for 2 years) but they can be harvested annually.
Onions like full sun with 6 to 7 hours of direct sunlight. Select a location with full sun where your onions won't be shaded by other plants.
Beans, Peas, and Sage should not be planted next to onions.
The following vegetables will do well when planted next to onions: Beets, Broccoli, Cabbage, Carrots, Lettuce, Peppers, Potatoes, Spinach, Tomatoes. Planting certain vegetables next to each other can deter insects and encourage growth.
Next year, DO NOT plant onions, garlic, chives and leeks in the same location where you planted onions this year. Because they are all members of the Allium (Amaryllidaceae) family. Instead use that spot to plant other vegetables from a different family such as:
- Carrot (Umbelliferae): celery, carrots, parsley
- Legume (Leguminosae): peas, beans, lentils
- Mustard/Brassica (Cruciferae): broccoli, caulifower, kale, cabbage, Brussels sprouts, turnips, radishes.
- Sunflower (Compositae): lettuce, artichokes.
- Cucurbit (Cucurbitaceae): pumpkins, melons, squash, gourds, cucumbers.
- Nightshade (Solanaceae): tomatoes, potatoes, eggplant, peppers
- Goosefoot (Chenopodiaceae): spinach, Swiss chard, beets.
Growing Onion Overview
In General, onions are not hard to grow as long as they get enough fertilizers, sun light and warm temperatures between 55 and 70 degrees F. For more planting information, please visit our Farming Tips homepage.