How to Grow Onions in Containers like a Pro! From Pot to Plate

grow-onions-in-containers
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Onions and other alliums are easy to grow. Each bulb requires only a small space, allowing you to grow a lot in a small area. Even if you only have a small garden or patio, there is always room to grow onions in containers.

Know Before You Grow Onions In Containers

Onions grow bulbs in response to the amount of daylight they receive. If they don’t get the right hours of light, you could end up with just leaves. The terms to categorize onions are long day, short day, or day-neutral.

Grow long day in the north and short day in the south. Day-neutral can be grown in the north and south. Draw a line from San Francisco to the tip of South Carolina to determine your area.

Short day onions

Short day onions only require 10-12 hours of sunlight to bulb. They can be grown in the north but bulbs will not be as large. These are usually sweeter varieties with a short storage life.

Day-neutral onions

Day-neutral onions require 12-14 hours of daylight and are usually sweeter onion varieties with moderate storage life.

Long day onions

Long day onions require 14-16 hours of light to grow full sized bulbs. Most onions of this variety contain more sulfur which gives them a longer storage life.

Start From Seed or Sets To Grow Onions In Containers

There are two possible ways to get started. You can buy onion sets or start your onions from seed.

Planting Onion Sets

Onion sets are the easiest way to get started if you want to grow onions in containers. If you are unfamiliar with onion sets, these are just small premature bulbs that grow into mature onions. They can be bought at garden centers; you can even find onion sets on Amazon. Sets are usually only available for bulbing onions. If you want to grow leeks, shallots or green onions, you have to start with seeds.

Mixed Red, White and Yellow Onion Sets 8 oz | Naturally Grown Non-GMO Bulbs – Easy to Grow Onion…
  • Gardeners love our red, white and yellow onion bulb set collection! Onion sets produce salad onions or full size onions if left to mature.

When planting the sets, simply push them 1 to 2 inches deep in the soil. Space them about 4 inches apart in all directions and keep them at least 3 inches from the sides of the container. This spacing leaves enough room for the bulbs to grow to around 3 inches in diameter.

Onion leaves grow upright, so don’t worry about crowding. After planting, soak them well and make sure they stay consistantly moist. In about 7-10 days the onion greens should start popping up.

Starting Onion Seeds Indoors

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It’s easy to grow onions from seed indoors, just make sure to sow at least 10-12 weeks before planting them outdoors. Fill your seed starting trays about 1/4″ from the top with a good quality sterile seed starting mix. Scatter at least 10 seeds per cell and cover with soil. The idea is to plant the onion seeds densely, the seedlings can easily be separated later.

Onion seeds germinate after about 7-10 days. If you don’t have a south-facing window for at least 8 hours of sunlight, use a grow light for seed starting. Prune the seedlings to about 2 inches high once they reach 6 inches tall to focus energy on the bulb area.

When you start onion seedlings under grow lights, they need to be hardened off before placing them in full sun all day. Use the same method as your other veggies, giving them more direct sunlight each day for a week or two. Be sure to include this time in your planting schedule.

You must separate the seedlings when it is time to plant them outdoors. Since the onion seedlings have thick fibrous roots, they are easy to separate. Just pop some seedlings with soil from the tray. Rustle the mass of roots to loosen the soil. Once all of the soil has been removed from the roots, you should be able to separate the seedlings with ease. When planting seedlings, use 4 inch spacing in all directions.

Choosing The Best Type Of Container

Onion bulbs sit only a few inches deep in the soil. However, we recommend using a container at least 10 inches deep for good root growth. Using a deeper container will also prevent the soil from drying out too quickly on hot summer days.

Plastic containers

The best results are obtained if you grow onions in containers made of plastic. They allow the soil to hold moisture longer than clay pots. Also, plastic containers are lightweight and come in many shapes and sizes. Round pots are most common, but trough type plastic pots are easy to find and work well for growing onions.

However, you can grow onions in any plastic pot as long as it’s food grade plastic. Storage bins with holes in the bottom are useful for growing onions, especially large bulbing onions.

Grow bags

There are two types of grow bags, fabric and plastic. Plastic grow bags are a much better choice for growing onions which prefer a consistently moist soil. Yes, I know the fabric bags are all the rage in gardening lately but they dry out much too fast. Besides the extra work of watering every day, it’s not good for your onions to have these daily moisture swings. Nice, evenly moist soil will produce the best results.

We grow onions in plastic grow bags and they do great. The plastic bags hold moisture well and can be purchased in many sizes. We prefer the 10 gallon size for growing onions. However, you can find them all the way up to a 30 gallon size for a reasonable price.

Clay pots

I know everyone loves the look of terracotta in the garden, and we do too. However, if you want to grow onions in containers, clay pots are not a good choice. Onions do best if soil is consistently moist and the terracotta pots will dry out very quickly due to the porous clay. You’ll need to water them almost daily. If you have some large clay pots laying around and you don’t mind the frequent watering, give them a try.

Best Soil to Grow Onions in Containers

Onions will do best in loose, well-draining soil that is rich in organic matter with a neutral or slightly acidic ph. To grow onions in containers, we make our own potting soil mixture.

  • 5 gallons sphagnum peat moss
  • 5 gallons compost
  • 5 gallons perlite
  • 1/4 cup dolomite lime

The sphagnum moss has a low pH, adding lime will help balance it out.

As you probably know, bagged potting soil can get costly. Making your own soil mix is way more cost effective if you need a lot of it. However, if you’re just growing a few containers, any good store-bought potting mix will work well.

What Month Do You Plant Onions Outdoors

In most parts of the US onion sets are usually planted outdoors in late March or early April when temperatures will stay over 28F. The onion sets will start growing as the temperature warms.

Seedlings started from seed indoors should be moved into the garden once the outside temperatures ae consistently above 50F. For most of the US this is mid to late May, after Mother’s Day is the general rule. Be sure to harden off the seedlings before moving them into full sun all day.

How To Care For Onions In Containers

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Light needed to grow onions in containers

Onions grow best in full sun. When you grow onions in containers, finding the perfect sunny spot is easy. If an area is being shaded by the changing of the sun with the seasons, you can always move it to a new location. If the leaves are straight up and not leaning in any direction, you know your bulbs are getting the right amount of sun.

Water requirements

When growing onions in containers, it’s important to keep the soil consistently moist. If you provide the right moisture level, the onion bulbs will be plump and firm. Wilted greens are a sign that you’re not watering enough. Soil that is too wet can encourage fungal diseases. A good way to check the moisture is to press your finger into the soil. You should feel the moisture at least up to the first knuckle.

What is the best fertilizer to grow onions in containers

Onions need a lot of nitrogen. Therefore, a fertilizer with the first of the three highest numbers is ideal. A fertilizer that uses ammonium sulfate as a nitrogen source is best for onions and also adds sulfur to the soil. It is also helpful to supplement the potting soil with bone meal before planting.

Hi-Yield (32177) Ammonium Sulfate 21-0-0 (4 lbs.)
  • The Ammonium sulfate supplement provides Nitrogen for a dark green color and rapid growth of plants.

Common Problems When Growing Onions

Pest and Diseases

Pest/DiseaseSymptomsTreatment
AphidsCurling leaves, stunted growthSpray with insecticidal soap or neem oil
ThripsSilver streaks, distorted leavesApply insecticidal soap or neem oil
Onion MaggotsYellowing, wilting plantsRemove affected plants, use row covers
White RotYellowing, wilting leavesRemove and destroy infected plants
Downy MildewYellowing, wilting leaves, purple spotsApply fungicide, improve air circulation
Botrytis Neck RotBrown spots, rotting at the neckRemove affected plants, improve ventilation
Fusarium Basal RotWilting, yellowing leaves, brown discoloration at the baseRemove affected plants, avoid overwatering
Leaf MinerTunneling tracks on leavesRemove affected leaves, use row covers
Onion SmutDark streaks on leaves, distorted growthRemove affected plants, improve sanitation
Onion ThripsTiny insects, silver streaks on leavesApply insecticidal soap or neem oil

More information about onion diseases

Harvesting: Grow Onions In Containers

When are onions ready to harvest?

You can harvest and eat onions at any stage of growth all season long. However, if you want to wait for the bulbs to reach their full size, wait until you notice the greens falling over and turning brown from the neck just above the bulb. This will happen later in the season. Falling greens are a sign that the onion bulb has stopped growing. When about 90% of the plants fall over, you can harvest the entire crop.

grow-onions-in-containers-ready-to-harvest
Onions ready for harvest

How to harvest onions

Harvesting is easy when you grow onions in containers. Grab the onion plant directly above the bulb and lift it up. In containers, the soil should be nice and loose so that the bulbs can be easily pulled out of the soil. Rock stubborn onions back and forth to loosen them. The best time of day to harvest onions is in the morning.

How to cure onions that you grow in containers

You can eat onions immediately after picking them. The curing process is only required for preservation purposes. Curing the onions makes them last longer.

If the weather is dry and there is no danger of frost, simply leave the bulbs in a single layer on the ground for a day or two. In humid conditions, the bulbs should be moved to a safe place to cure. Don’t remove the leaves while the onion is curing and try not to bang the onions against each other to avoid bruising and soft spots.

Onions that you want to keep for a long time need to be cured for two to three weeks. After fully cured, the skin will tighten and dry around the bulb. The neck should also wither. At this point you can use scissors to cut the roots off the bottom of the onion. The leaves should be cut off an inch above the bulb. Onions that still have a green neck do not keep well and should be used as soon as possible. For storage, do not wash the bulbs, simply brush the soil off the bulbs.

Proper way to store onions

Ideally, onions should be stored at around 40 F in a dry place. Store the onions in a single layer if possible. Do not store them in plastic or sealed containers. Mesh bags work well. The onions should be exposed to the air in a low humidity environment. Properly dried long-day onions can be kept in storage for up to 12 months. The sweeter onions of the short-day variety only keep for a few weeks. You can keep them in the fridge to extend their shelf life. Optionally, you can chop the sweet onion into small pieces and freeze them for later use.

Conclusion

By choosing the right container, soil, and onion variety, providing adequate sunlight, watering appropriately, and using fertilizer as needed, you can successfully grow onions in containers.

With proper care and attention, you can enjoy a bountiful harvest of fresh onions that are perfect for cooking, salads, and other culinary uses.

If you enjoyed this post you might also like our article about how to grow chives in pots.

FAQ: Grow Onions In Containers

How deep do pots need to be for onions?

When you grow onions in containers, be sure the depth is at least 10″ for proper root growth. While the bulb sits in just the top couple inches, onions do send roots down pretty deep.

Do onions grow back each year?

Onions are biennial. The first year they bulb, the second year they flower. If you leave an onion in the ground over the winter, the bulb will not continue to grow the second year. Instead, the onion will go into flowering mode. Leave an onion or two in the ground over winter if you want to collect seeds at the end of the second season. Be sure to leave only one variety to prevent cross pollination.

How do you know if you’re overwatering onions?

Overwatering is usually apparent by the yellowing of leaves and the pale unhealthy appearance of the plant. Once you cut back on watering the plants should recover and return to the proper green color. Yellowing can also be a symptom of low nitrogen. If you suspect you are overwatering, try watering less before attempting to fertilize.

Can you eat onions straight from the garden?

Absolutely. Onions can be used at any stage, even the green leaves. Curing is only needed for long term storage.

How long do you dry onions after picking?

Onions should be allowed to cure for about 2 to 3 weeks after picking. They should be spread out in a well-ventilated area with low humidity, away from direct sunlight, and with good air circulation to allow them to dry properly. Once the outer skin becomes dry and papery, they are ready for storage.

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