Onions & Shallots
allium cepa
A Basic Growing Guide
Description
Onions. A root vegetable with staying power. And I don’t mean the odor of sulfur that stays on your breath…I mean that while other fruits and vegetables have come and gone in popularity over the years (hello brussels sprouts) the onion has been gracing kitchen tables for 5,000 years. They belong to the Amaryllidaceae (Amaryllis) Family and are in an edible subgroup together with garlic, leeks, and chives. Onions are biennial plants, which means they flower in their second year, but unless you are going for onion seeds to sow later on, you’ll want to harvest them before they flower!
As you are selecting the variety of onion to grow, you will notice labels like short-day, intermediate day, and long-day. This has nothing to do with how long they grow, but WHERE on the earth they can be grown. Locations above 37° latitude, like the northern U.S., Canada, the U.K., all have longer days in the summer; it can be light for 14+ hours! Long-day onions need that much daylight to form a mature bulb. Intermediate-day onions can be grown at latitudes between 32-42°. Short-day onions can be grown at 25-35° latitudes. Tennessee spans latitudes between 34°59’N and 36°41’N, which makes it well-suited for growing intermediate-day onions. Long-day onions can be grown in Tennessee for their greens (green onions). Short-day onions could be successful in the southernmost parts of Tennessee.
Onions are fairly easy to grow, but there can be a lot of information to sift through at the beginning when selecting the variety. And even more information to sift through when it comes time to harvest, cure, and store onions. But don’t let that intimidate you! There might be a lot to read, but the things you do are not difficult. After you’ve gone through one season, you’ll get the hang of it.
Some Common Varieties
Bulb Onions:
Yellow Onions – The most versatile variety with a balance of sweetness and sharpness. We like ‘Elsye’ for its disease and pest resistance.
Red Onions – Mild and slightly sweet, great for raw dishes. Try ‘Cabernet’.
White Onions – Crisp with a sharp, clean flavor, often used in Mexican cuisine. We like ‘Super Star’ and ‘Sierra Blanca’.
Sweet Onions – Low in sulfur, making them extra mild and perfect for eating raw. ‘Zoey’ and ‘Candy’ are good varieties.
Green Onions (Bunching Onions, Scallions) – Onions that are harvested early for their tender, mild stalks. You can either cut them continually for their greens, or pull up the entire plant if you would like the white parts on the underground part of the plant. Some varieties of onions are bred for their small bulb size and are labeled as bunching onions, like ‘Parade’. But you can grow any onion as a green onion, just space them closer together and harvest early. This is also an opportunity for growers with fewer hours of summer daylight to grow long-day onion varieties as green onions.
Shallots – Shallots are a cousin of the onion. They grow in small clumps of bulbs almost like garlic. Growing, harvesting, curing, and storing shallots is the same process as onions, so I’ve included them here. Shallots are milder in flavor, but can be used instead of onions in almost any recipe, and they store well. We like ‘Matador’.
Onions Through The Season
Planting Instructions
Light Requirements
Onions should be planted in a full sun location. But you also need to check on your latitude in order to know what type of onion to plant. In Tennessee, we are usually growing “Intermediate-day” onions.
When
In Middle Tennessee, plant onions in late winter to early spring (February-March) for summer harvests. They can also be planted in fall (September-October) for overwintering and early spring harvest.
Seed or Transplant?
Onions can be grown from seeds, sets, or transplants.
Seeds take a longer time to mature. They are a good option if you plan to harvest your onions early for the greens.
Sets are the most common method for home gardens. Onion sets are onions that were started from seed the previous summer, harvested as small bulbs in the fall, and then stored until spring. You can buy a bag of sets and they will have a better chance than seeds of producing a large bulb by the time the growing season is over. Plant them soon after receiving or store in a cool, dry place until ready.
Transplants are young seedlings ready for planting. Because they have a long head start, they will be larger by the end of the season. You can buy them as transplants or sow them as seeds inside 6-8 weeks before last frost date.
Depth
Seeds – Sow 1/4 inch deep in loose, well-draining soil.
Sets & Transplants – Plant 1.5 -2.5 inch deep, with the top just peeking above the soil.
Spacing
Bulb Onions - Space 4-6 inches apart, with rows 12-16 inches apart.
Green Onions - They can be placed closer together, 1/4” to 1” since you don’t need room for the bulb.
Shallots - If you want smaller, single bulb shallots, give them 2” spacing. 3-4” spacing will yield larger shallots with possibly multiple bulbs.
Growth Habit
Onions form a bulb underground, as long, slender green leaves shoot up above ground. Each new leaf that forms is a new ring on the onion!
Tending
Watering
Provide 1 inch of water per week, keeping the soil evenly moist but not soggy. Reduce watering when bulbs start to mature. Stop watering when the tops begin to fall over, this signals that harvest is close at hand!
Common Pests and Treatments
Onion Maggots – Tiny white larvae of the Onion fly that tunnel into bulbs. Prevent by using floating row covers. Reduce the fly population by using yellow sticky traps to trap adult flies. Here is an in-depth article about how to manage onion maggots if they ruin your onion or garlic crop. “How to Successfully Recover from an Onion Fly Infestation”
Thrips – The nymphs are tiny and yellow and often congregate near the base of the leaves. They cause silvery streaks on leaves. The leaves can handle some thrip damage and you only need to be concerned if populations are so high that your leaves start to look lacy. Spray with neem oil to help keep populations low.
Common Diseases and How to Treat
Bulb Rot –A fungus in the soil that shows up as a white mold growing around the bulb. This stunts growth and makes leaves turn yellow. Once the fungus is in your soil, it will stay there, so you will need to move your onions to a different location next growing season.
Pink Root – Causes roots under the bulb to turn pink/purple and eventually brown, it stunts the growth of your onions. This disease will stay in your soil, so move your crop to a different location next growing season.
Feeding/Fertilizing
Plant with a balanced fertilizer, like Gardentone. Sidedress with a nitrogen rich fertilizer about a month after planting.
Special Crop Maintenance
Weeds - Onion and shallot roots are very shallow and easily disturbed by weeds and other plants. They aren’t strong enough to compete, so make sure the ground around is regularly cleared of weeds. Mulching around the onions with organic straw is a good option for suppressing weed growth.
Blanching - for green onions only - To further develop the white part of the green onions, mound soil up against the stems 2-3 times during the growing period, making the dirt a little higher each time.
Pruning
No need to prune, but remove flower stalks (if they appear) to keep energy focused on bulb development.
Harvesting
When
Green Onions- Can be harvested at any stage, typically once they reach 6-8 inches tall.
Bulb Onions and Shallots- When 50% or more of the leaves have fallen over.
How
Green Onions- Cut the green leaves from the plant, leaving the bulb in the soil to continue growing. Harvest individual leaves as needed. Or, pull the entire plant out if you want to use the white parts.
Bulb Onions and Shallots-
It is better to loosen the soil and lift the onions from below with a garden fork or dig them up with a trowel. Pulling them from the top risks tearing the root collar (which can cause it to spoil during storage).
What to do with the stalks? There is some debate about the perfect time to remove the stalks. Traditionally, the stalks are left intact at the harvest and not removed until after the curing process. Read on for how to cure and store onions.
Then What?
Onions and Shallots
Curing - This allows the outer layers of the onions to dry and the neck of the onion (where the stalks come out), to seal for storage. To cure your onions, lay them out in a single layer in a shady, warm, dry place- your shed, garage, porch. If it is humid, I recommend using a fan to help circulate the air. Leave them 2-7 days for sweet onion varieties, 1-2 weeks for shallots and onions. Over this time, the flavor will gradually change from sweet to pungent, like the onions we know and love. Once the stalks are completely dry and the necks are small and hardened, your onions have cured! You can now trim the roots. The stalks can be braided together for storage or trimmed off, leaving about an inch.
Storing - Cured onions can be stored in a cool, dry place for 2-6 months depending on the onion variety. For instance, sweet onions will only store for a few days, so refrigerate after curing. The refrigerator is a fine place for your other onions as well, but if you have a lot of them to store, try the basement or garage where they will stay dry and below 50 degrees, but above freezing. Hang them in mesh bags or in braided onion bunches, or place them in baskets only 2-3 onions deep.