How to plant, grow and harvest onions
If you're looking for a low-maintenance vegetable to grow in your garden this year, put onions on your radar. The bulbs basically tend to themselves once you place them in the soil and can withstand full sun and low-water conditions. So long as you give your rows of onion bulbs plenty of space (onions will compete for water and sun if you don't!), you will be able to enjoy a plentiful, edible harvest come summer.
Related: How to Plan Your Vegetable Garden
Onion Sets vs. Seeds
There are two key ways to grow onions: from sets or seeds. Planting with onion sets is the most popular method, says Niki Jabbour of SavvyGardening and the author of Growing Under Cover. Onion sets are immature bulbs that are 3/4 inches in diameter and smaller. You can usually find these in bags of 100 in the spring—plus, they are easy to grow. "You just open the bag and plant each set an inch deep and you're done," says Jabbour. Most onion sets take between 90 to 100 days to mature to full-size bulbs.
To grow onions by seed, start by sowing the seeds indoors 10 to 12 weeks before the last spring frost date. Fill pots or trays with pre-moistened potting mix and sow the seeds 1/4 inches deep and 1/2 inch apart. As the grassy seedlings grow, clip the tops to encourage strong, stocky plants. (Jabbour keeps the tops about 5 inches tall.) Most seedlings will need at least 16 hours of light each day, lightly moist soil, and fertilization every 2 to 3 weeks with a liquid organic fertilizer diluted to half strength, adds Jabbour.
How to Choose the Right Onion Type for Your Garden
All onion varieties fall into one of three categories: short-day (which require 10 hours of light per day), intermediate-day (which require 12 to 14 hours of light per day) and long-day (which require 14 to 15 hours of light per day). The one you choose will depend on your USDA hardiness zone. "Short-day onion varieties will find the greatest success in zones 7 and south," says Rebecca Sears, Ferry-Morse's CMO and resident green thumb. Sweet or mild varieties usually fall in this category, says the team at University of Minnesota Extension.
Zones 5 and 6 are best for intermediate-day onions; zones 6 and north are best for long-day varieties, says Sears. If you're unsure of what your zone is, you can find out by using our handy guide to USDA gardening zones.
How to Start Onions Indoors
If you plan on starting onions indoors from seed, follow Jabbour's instructions, above; to time it right, keep your specific zone's last frost date in mind, so seeds have time to mature before being transplanted outdoors, says Sears. You can start sets inside, as well: "If you don't have the outdoor space available for gardening, you can actually grow onions indoors year round in containers," she says. "Just make sure they are being grown in an open and sunny area in your home with fertile soil and good drainage."
When to Move Onion Seedlings Outdoors
Wait until the seedlings are about 6 to 8 inches in height and have a few leaves before moving the onions outdoors in spring.
How to Plant Onions Outdoors
When planting your onion sets or seedlings outdoors, make sure they are planted with the pointy size up and placed 1 inch deep and 4 inches apart in the soil. "If they're planted too deep, it can negatively impact bulb development," says Sears. "Your onions will still grow shoots, but they likely won't grow bulbs."
It's also important to practice crop rotation, which is planting vegetable families in different areas of the garden from year to year, says Jabbour. "Shifting crops around reduces the occurrence of pests and diseases, as well as nutrient depletion," she says. "I practice a four year crop rotation, where a crop family isn't planted in the same bed for four years."
She recommends grouping your vegetables by family, so the "onion family," would include onions, shallots, scallions, leeks, and garlic. This is helpful, since these types of produce have similar nutrient needs.
To make sure that you grow healthy onions, space them properly. "If you don't properly space your onions when planting, the onions can compete for water and sun," says Sears. "However, planting them too far apart can result in wasted garden space and a smaller yield, so you want to ensure you're following spacing recommendations as closely as possible."
Overcrowding onions can also cause poor air flow, says Jabbour. This will encourage diseases, like botrytis and downy mildew. The best layout for the most common onion varieties, such as red burgundy onion and sweet Spanish yellow Utah jumbo onion, includes planting them about 3 inches apart in rows that are spaced nearly 18 inches apart, says Sears.
How to Grow and Care for Onions
Follow these best practices to grow onion bulbs and care for the plant in your garden.
Onions need full sun and between 10 and 16 hours of light each day (depending on the variety) during bulb formation. "If you plan on growing your onions indoors year round and don't live in an area where that much sunlight is always available, a grow light can help provide your plants with the light that they need despite the current outdoor conditions," says Sears.
Soil and Fertilizer
Fertile, moist soil enriched with compost is the best for onion production. It should also be high in nitrogen, which is necessary for healthy development. Use a macro-nutrient-rich fertilizer, like SUPERthrive Organic All-Purpose Plant Food, to feed the plant as it grows. This type of fertilizer can help with nutrient and water intake. Apply just prior to planting and then every four to six weeks during the growing season, says Sears.
Onion seedlings should stay moist, says Sears. However, once they've matured, they just need about an inch of water each week. "If leaves begin to yellow, it may be a sign that you are overwatering, which can cause your onions to become diseased and rot in the ground," says Sears. "You also want to ensure you're not under-watering your plants and that your base soil isn't getting dried out because of onions' shallow roots."
If you notice the plant is starting to yellow or wilt, however, it could be a pest infestation from onion maggots, which infest the plant stems, says the team at University of Minnesota Extension.
Common Problems When Growing Onions
While onions are easy to grow, there are still some issues that can arise. Learn how to remedy each problem with these tips.
Onion maggots can infest this crop. However, you can prevent this from happening by practicing a three- to four-year crop rotation plan. "Crop rotation is key, as onion maggots overwinter in the soil, with the adults emerging in early to mid-spring," says Jabbour. "If you plant onions in the same space each year, the emerging pest will find themselves surrounded by their favorite food." Move the onion family crops to a different spot every few years and and place lightweight row covers over the just-planted bed in early spring to limit an infestation, she says.
Downy mildew is a disease that can cause onion plants' leaves to turn yellow or develop brown patches, says Jabbour. The leaves will eventually collapse. "The disease doesn't necessarily kill the plants, but it does affect bulb size and yield," she says. "To reduce the risk of downy mildew, I practice crop rotation and space onions properly to allow good air flow."
How to Harvest Onions
In general, late summer is the best time to harvest onions. Mature, ready-to-harvest onions have yellow tops, and falling, drooping tops, says Sabine H. Schoenberg, host of Sabine's New House on Smart Healthy Green Living. To properly harvest them, undercut and lift bulbs with a spading fork, says the team at University of Minnesota Extension. Pro tip: Never harvest onions in wet conditions, as they are likely to rot in transport and storage, says Schoenberg.
Once you've harvested your onions, lay them out to dry for a bit in a cool, dry place in your home. Jabbour recommends storing onions in mesh onion bags or in a single layer in a cardboard box. Make sure to avoid storing them near apples or pears. "The ethylene gas produced by the fruit spoils the flavor and storage of many vegetables, including onions," says Schoenberg.