How to Grow Onions

How to Grow Onions

By Matt Gibson

Onions are one of the most widely used kitchen ingredients in the world and they can be incorporated into dishes in a variety of ways. Their bold flavor makes them a favorite of chefs in various cuisines around the world. Onions are also praised for their many health benefits, such as boosting your immune system, helping to regulate blood sugar levels, and even lowering cholesterol.

Aside from their distinctive flavor and long list of medicinal benefits, onions are also relatively easy to grow. Cultivating your own onions in your garden takes a bit of patience, but when it’s time to harvest your onion bulbs, it will all be worth the wait, as homegrown onions taste and smell superior to onions purchased at the grocery store. 

Because of their hardiness, onions are fairly easy to grow. A high-yield crop, onion plants take a good amount of time to develop and mature, but gardeners can produce a large quantity of bulbs in a very small amount of total garden space. In just one to one and a half square feet, gardeners can easily produce twenty to fifty onions.

The time it takes to grow your own onions varies greatly depending on the variety you plant. Green onions can be cultivated in as little as 20 to 30 days, while dry bulb onion varieties typically take 100 to 175 days to reach maturity. 

Varieties of Onions

There are many different types of onions that gardeners can choose between when deciding what type of onion to grow. Onion varieties vary in size, shape, and color and can be grown from transplants, seed, or sets (the latter are immature bulbs that are grown the previous year and are typically the easiest to plant, the earliest to mature, and the most resistant to disease, though sets are sometimes prone to early bolting). 

White, yellow, and red bulbs vary in size from small onions, which are perfect for pickling, to large Spanish cultivars. Bulbs can be globe, top, or spindle shaped, depending on the variety. Most onion varieties can be harvested early as green onions. The multiplier, or potato onion multiplies into a bulb cluster, allowing you to replant with every harvest for a continual onion supply. One perennial bunching onion variety, called Allium fistulosum, produces amazingly flavorful scallions and is practically immune to garden pests and diseases. 

The Egyptian onion, also called the top onion, produces two harvestable clusters, one at the end of a long stem, and another which forms on top of the first cluster. The Egyptian onion also forms a bulb underground, but it is typically tossed out, as most find its flavor and odor to be too strong to enjoy. Aside from bulb onions, gardeners can grow pearl onions, sweet onions, shallots, scallions, and chives. 

Types of Onions

Onions are typically grown from either seeds, sets, or transplants. Growing onions from seed generally takes as long as five months, but growing from seed provides the greatest amount of choice between onion varieties or cultivars. Onion transplants are small seedlings that look like scallions. Transplants typically require around two months to reach full maturity. Sets are easier to successfully plant than seeds or transplants. Typically less susceptible to common diseases, onion sets mature in around two months time, but variety selection is usually very limited. Steer clear of planting onion sets with bulbs that are larger than a dime, as these are often quick to bolt. 

Gardeners can choose between bulb or bunching onions depending on how they intend to use their crop after harvesting. Bunching onions, also known as scallions or green onions, are generally grown for their tender, green top stalks and harvested before bulbs get a chance to fully develop. Bulb onions range from the smaller pearl onion varieties, to quite large Spanish cultivars, and are available in either white, yellow, or red bulbs. 

Different onion varieties are suited to different temperatures and light requirements. Onions tend to grow their tops in cool weather conditions and focus on bulb development in warm weather environments. Temperature and day length decide the timing of bulb formation. Long-day onions, which perform best in the northern states in the US, need extremely long hours of exposure to daylight to develop, requiring 14 to 16 hours of daylight every day in order to mature.

Recommended long-day onion varieties include Redwing, a Late season onion best grown from transplants which produces a nice solid red bulb, with a long-lasting red shade that doesn’t fade over time, like other red onion varieties. Another quality long-day red onion variety that can also be grown as a short-day onion is the Italian Red Torpedo. This heirloom onion forms elongated reddish-purple bulbs. Spanish varieties such as White or Yellow Sweet Spanish, Walla Walla, and Olympic onions are excellent long-season onion options.    

Better suited for mild-winter southern latitudes such as the southern and western US, short-day onions grow during the winter and fall months and begin forming bulbs once they are exposed to 12 hours of daylight per day in the early weeks of summer. If temperatures stay in the 30°s and 40°s F, onions will develop slowly.

Suggested short-day onion varieties include Granex, Cipollini, and Texas Grano 1015Y. Granex onions, which are globular, sweet-tasting bulbs that mature early in the season. Granex onions are more commonly referred to as Vidalia onions. The flat, doughnut-shaped Cipollini onion variety are often grown for storing, as they are known to keep well in storage for up to five months. Texas Grano 1015Y are an improved strain of the popular yellow onion cultivar Yellow Grano, which were developed in a laboratory. The new onion strain was created to store well for extended periods and to be highly disease-resistant.  

Growing Conditions for Onions 

Onions need full sunlight exposure for optimal growth and development. Keep in mind the shade that neighboring plants might provide when selecting a location to grow onions in. Onions require a well-draining, loose, nitrogen-rich soil medium. As bulbs develop beneath the soil, providing a loose, clump, stone, and debris-free soil is essential for onion growing.

Compacted, rocky, or clay-heavy soil will affect bulb development significantly, causing slowed development, bulb deformities, and even plants that don’t develop bulbs at all. Heavy or compacted soils must be amended in order to be fit for onion cultivation. 

Before planting, in the early spring prepare and amend your garden beds prior to planting by tilling the soil thoroughly, removing all clumps, rocks, and other plant debris from the soil and digging it down at least 12 inches below the surface. Onion plants are heavy feeders that require constant nourishment to develop big bulbs.

Amend your beds by working in aged manure or compost in the early spring and mix in some nitrogen fertilizer at planting time. Practicing crop rotation is recommended for all plants to help reduce pest and disease issues, and onions are no exception to the rule. Each year, remember to switch up the garden location in which you plant your onions. 

Watering onions by soaking the plants from above doesn’t always effectively get the water to the plant’s roots, where it is needed most, and leaves the soil surface damp, which can help germinate weed seeds that are present in the soil around your onion plants.

To avoid these issues while continuing to water your onions efficiently, lengthen your soaker hoses along the rows close to your onion plants. Alternatively, dig a small trench between your onion rows and fill the trench with water instead of watering the plants directly. This will keep the roots hydrated but will leave the majority of the soil’s surface dry, keeping weeds at a minimum.

How to Plant Onions

Onions are either grown using transplants, sets, or seeds. Sets are immature onion bulbs that were grown within the past year and harvested prematurely for future planting. Growing your onions from sets comes with an increased bolting risk and a limited choice of cultivars, but onion sets provide plenty of perks too.

Sets are the easiest way to plant onions, the earliest type of onions to mature for harvesting, and the most disease-resistant onion growing method. Sets are usually only labeled white or red, rarely by cultivar. Use white sets for growing green onions and select sets with small, half-inch in diameter bulbs, as bigger bulbs are more likely to bolt than smaller ones. 

Transplants are seedlings that have been recently sprouted and are generally purchased from plant nurseries or ordered on the internet and delivered in the mail. Transplants have a reduced choice in cultivars, and are subject to contracting diseases, but they develop fully matured, high-quality bulbs in a very short span of time. Transplants produce ready-for-harvest onion bulbs in 65 days or less. 

Growing from seed gives gardeners the best choice in cultivars but requires the most patience, as crops grown from seed will take up to four months to fully mature. Gardeners in cold-winter zones will have to start their onion seeds indoors. 

The day-length of the variety you grow affects how and when your onion plants will form bulbs. Short day onions, suited for southern and southwestern areas in the US will begin forming bulbs as soon as days span 10 to 12 hours, while long-day varieties grown in the northern US zones need 13 to 16 hours of sun to form bulbs. 

Onion plants enjoy cool weather in the beginning stages of their development, so most climate areas should plant onions in the spring. In mild-winter areas, onions should instead be planted in the late summer to grow as a fall or winter crop. In cool weather, onions typically grow tops, and as the weather grows warmer, bulbs begin to develop beneath the soil. 

Onion seeds need to be planted at least four to six weeks prior to the last average frost but you can get an even earlier start if you are starting them indoors or are using a cold frame to protect them from the elements. Once the seedlings reach two to three inches in height, as long as there are no more freezes on the horizon, they can be gradually hardened and then moved into your garden beds. 

When planting onion seeds outdoors, sow thickly in rows about a half an inch deep. If root maggots are a problem in your area, mix in some radish seeds to mark the rows and help drive root maggot traffic away from your onion crops. Thin your onion seedlings to one inch apart and after four weeks of growth, thin again to six inches between each plant. 

When planting onion sets or transplants, dig two-inch deep planting holes spaced four to six inches apart. Use closer spacing if you intend to harvest some of the plants early as green onions. When planting onion sets, create a furrow two inches deep and put the sets in with their stem ends pointed upwards four to six inches apart, then re-fill the furrow with soil. One pound of onion sets will provide a 50 foot row. 

Care for Onions

Onions are very hardy, and don’t require a lot of additional effort from the gardener. Onion care involves fertilizing every two weeks with a nitrogen fertilizer to encourage bulb growth until the bulb begins to push the soil away, signaling the beginning of bulb development, then cease feedings. Do not repack the soil around your onions, as the bulb should be allowed to reemerge from the soil as it develops. 

If your onion beds are mulched, manual watering will rarely be needed. Including rainwater, onions only need one inch of water per week. For sweeter onions, increase watering to one and a half to two inches per week. Be sure to provide water manually during droughts or especially dry periods. Onion plants will look deceivingly healthy even when their soil is utterly dry, so test the soil for moisture manually and provide water when necessary.  

Garden Pests and Diseases of Onions

Onions, like any other garden vegetable, has a few natural enemies. When it comes to garden pests, onions are most often troubled by root maggots and thrips. Onion root maggots flies lay their eggs around the base of onion plants. Root maggot larvae hatch and begin burrowing into the stems of the plant, feeding on it from the ground up, until the plant dies from the damage. Root maggots can be deterred by crop rotation, which will help avoid any major root maggot infestations. Laying out diatomaceous earth can drive away the flies, and covering onion seedlings prevents eggs from being laid. 

Thrips are tiny yellowish-brown flying insects that feed on the leaves of the onion plant, often causing the leaves to twist and curl. Repeated attacks from thrips can cause the plant to stop growing entirely, causing bulbs to stop developing. Thrips can be avoided by planting thrips-resistant varieties and keeping your onion plants far away from any grain crops. Thrips infestations can be treated with neem oil and insecticidal soaps for small scale infestations. 

The two most common diseases that typically plague onion plants are both caused by improper moisture levels. Onions sometimes fall prey to neck, stem, or bulb rot, which is usually a result of overly damp soils, insufficient drainage, and soil aeration issues. Improving drainage and aeration by tilling and amending your soil should keep your soil from being soggy.

Alternatively, If the onion plant is exposed to insufficient water and is kept in dry soil while the bulbs are forming, bulbs will often split or even double. Splitting can be avoided by keeping a close eye on the moisture level in your soil and taking care to provide the necessary amount of water to keep the soil slightly moist. 

How to Harvest Onions

Onions can be harvested at any stage. Pull them up early when thinning out a row, and they can be used as green onions. Allow the plant to produce their bulbous roots until they mature to harvest full size bulbs. You will notice that they are ready to be harvested when about half of the tops of your onion crop have fallen over and the bulbs’ outer skin has a dry, papery texture. If your onions are kept in the ground until half or more of the crop’s tops have collapsed will last longer in storage. 

After you notice half of the tops have fallen down, tenderly coax the rest of the leaves down without snapping them off of the bulb. Then, allow your bulbs to rest on the ground for several days before trying to lift them. Instead of pulling out mature onion bulbs, dig them out carefully. You won’t have to dig too deep into the earth to get the job done.

You only have to do enough to get the remaining roots of the plant to loosen up enough to release the bulb. Shake and brush off any loose soil without rinsing the bulb with water. Allow the bulbs to cure in a warm, dry location that has ample ventilation. Keep the leaves on the plant and use your freshly harvested homegrown onions at any time you choose. 

How to Store Onions

The best method for storing onions is to wait until the outside skin of the onions dry out and the neck area where the bulbs meet the leaves begins to shrivel, which signals that the onions have finished curing. At this point, they can be stored in a cool, dry location for optimal shelf life. Onions will stay fresh longer cool temperatures below 40 degrees F, but should never be allowed to freeze. Onions can be kept in mesh bags or strung together by braiding the tops together and hanging the strands vertically. Just take care to avoid letting them get suffocated or piled on top of each other. 

Mulching and weeding will help your onions get the nutrients that they need to produce large, healthy bulbs for you to harvest. Following this guide, you should have no trouble growing onions in your garden prolifically. There’s nothing better than growing and enjoying your own vegetables while saving money on produce throughout the year as a result.  

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