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How to Plant and Grow Onions in Pots (Step-by-Step)

Growing onions in a pot photo collage

Onions are a cool season crop, growing from fall through winter. This means that the plants should be planted in pots at least by the middle of fall. Plants grown from seeds will need to be cultivated earlier, so that they can develop before being planted. Onions growing in pots need to be watered regularly, but only so the soil becomes damp. They must not be overwatered, because this can affect the growth and health of the plant. Onions need a lot of direct sunlight, so they must be grown in a spot in your yard that is sunny for just about the whole day.

Steps to growing onions in a pot

Steps to growing onions in a pot (illustration)

Onions are not particularly fussy plants to grow. This does depend, though, on how well they are looked after. Here are the steps to growing onions in a pot successfully.

1.  Settle on the type of onion you will plant.

Onions are described as being ‘long-day’ or ‘short-day’. This refers to when the onion plant will stop forming tops and the bulb will begin to grow. Take note of the number of daylight hours you experience where you live.

One of the other things you have to look out for is the number of onions you will grow. Each leaf of an onion plant, there will be a ring on the onion.

2.  Find the best location in your home or garden for the onions.

Onions need full sun, so you must look for a spot in your yard where you can put the pot/s which is in direct sunlight for at least 10 hours a day. Remember that long-day onion may need up to 14 hours of light. This doesn’t necessarily mean direct sunlight for all this time, but the area mustn’t be dark early. However, try to get your onions as much sunlight as possible during the day.

When the onions are young, make sure the pots are in a space that is slightly sheltered. This means they should be next to a wall, or even in a smaller yard.

3.  Decide if you will plant onions from seeds, seedlings or bulbs.

Growing onions from seeds is a longer process, because you need to plant the seeds in mid-to-late summer and look after them carefully as they grow. It does mean that you will be just about guaranteed that the plants are disease free.

The advantage of growing onions from seedlings or bulbs is that the growing process will be shorter than if you begin with seeds. The plants will also establish themselves more quickly and easily, so the plants should grow successfully.

4.  Choosing the cultivar

The onion cultivar you choose to grow must suit the climate in your area and the number of onions you want to grow. It will also depend if you want to grow the onions from seed, or transplanting seedlings.

The early Yellow Glove cultivar is a long-day onion that can be grown successfully from seeds. The Georgia Sweet onion cultivar is a short-day onion that grows more successfully from seedlings that are transplanted.

5.   Planting pots

There are a whole number of planting pots you can choose from in which to grow onions at home. The choice of pot will depend on a few factors:

a.  How much space you have in which to put the pots.

If your space is limited, it is useful to use a narrow rectangular planter, or smaller individual pots. If they are square, they can be packed together.

b.  How many plants you want to grow.

If you are going to grow just a few onion plants, then choose a small planter, or one / two individual pots, in which you can put two or three plants.

If you want to grow a number of onions, then choose a long, fairly wide planter. Depending on the number of plants, you can use two or three of these.

Onions growing in a bucket

c.  The spread of plants you are looking for.

The size and layout of your yard can determine the spread of onion plants. You may have a courtyard and put a large pot with a few onions in each corner. You may also be able to put a number of pots against a wall that gets a lot of sun. This could also mean using a bigger planter and not individual pots.

d.  Whether the pots will be a feature, or just functional.

When you grow onions at home, you usually won’t need to use fancy pots. In fact, simple, functional pots will do the job well. However, if you want to make a feature of your onion planting, then choose attractive pots that will take pride of place.

Make sure that any pot you use has holes in the bottom, because this is an essential part of the drainage of the soil, which is important for growing onions successfully.

Plant pots with holes on bottom

6.  Choose the correct soil

Onions need to grow in soil that is well-drained. This means you must choose a type of soil that is loamy and has a low clay content. The soil must also be slightly acidic, with a pH between 6.0 and 7.0. If should not be rich in nitrogen. If necessary, add phosphorus and potassium to the soil.

You should be able to buy the correct potting soil from your local garden shop or nursery.

7.  Prepare the pot

Put a few small pebbles in the bottom of the pot/s in which you are going to grow the onions. These will help to keep the soil from washing away through the holes in the bottom of the pot. Do not block the holes, though, because excess water needs to be able to drain out of the pot completely.

Place each pot in a drip tray, so that any extra water will drain and not make a mess.

8.  Prepare the soil

Pour about ¾ of the soil you will use for the pots in which you’ll want to grow the onions into a bucket. Add enough compost (or natural fertilizer) to the soil, so that it is enough to fill to about an inch (2.5 cm) below the rim.

Test the soil when you have mixed it up. For this, you should use a soil test kit. If the soil is too acidic, add some nitrogen. If it is too alkaline, you can add some phosphorus and potassium.

9.  Plant the seeds, seedlings or bulbs

When the pot and soil are thoroughly prepared, it is time to plant the onion plants. Onions are a winter crop, so they should effectively be planted out as established plants at the end of fall, or early in winter. This is the key to when you plant the seeds, seedlings or bulbs.

a.  Planting seeds

When you grow onions from seed, you should plant in early fall. You can sew the seeds directly onto the soil and then cover them with a thin layer of soil. Soak the soil in the pot, so that it is damp to about 1.5 inches (about 4 cm) deep. Water the seeds about every three days, making sure they are kept damp. Never over water the seeds.

b.  Transplanting the seedlings

Seedlings can be bought in early fall. Set them in a sunny area in your garden and let them grow for a couple of weeks. Transplant the seedlings into the pots at least by the end of fall. This means uprooting the whole seedling, including the bulb. Gently shake off the soil that is clinging to the roots and bulb.

Make a depression in the soil in the pot that is wide enough to fit the whole bulb. The depression must be deep enough so that the bulb will fit into it. Cover the bulb with a thin layer of soil, so that the bulb is covered by about ¼ inch (4mm) of soil.

Water the onion seedlings, making sure the soil is damp just below the bulb. It should not actually be wet, but moist.

c.  Plant the bulbs

When you plant an onion bulb, make a depression in the soil that is deep and wide enough to accommodate the whole bulb easily. This means you should be able to cover it with a layer of soil about 1.2 inch (1.25 cm) deep.

Soak the soil around the bulb, so that it is moist to just below the depth of the bulb.

10.  Fertilizer

If you want to grow onions successfully, you must make sure they are fertilized properly. This means keeping the soil at the optimum pH. Don’t use fertilizer too frequently, or your plant may be burnt by the fertilizer.

Try to use an organic fertilizer if you can.

11.  Water

Watering onion growing in pot

When it comes to watering your onion plants that are being grown in pots, remember two basic things:

  • Onions need to grow in soil that is well-drained.
  • Pots hold water more easily than soil in a bed.

The soil in which the onions grow must be kept damp. This should mean they get about 2 inches of water a week. This may change in warmer weather.

The trick is to make sure the soil in the pot remains damp. Use your finger to test the soil. If it is dry to about ¼ inch (7 mm), you will need to soak it.

12.  Avoiding pests and diseases

Onions growing in a planter

Pests

Generally, onions do not affect onion plants. The one problem is the onion maggot, a small, grey fly. They are usually active at the beginning of spring. They will invade the roots, eating away at them. If left uncontrolled, the onion maggots can destroy the roots, which will affect the growth of the plant.

The onion maggot flies lay eggs in the soil near an onion plant, so they may affect your pots, but it is relatively unlikely. To prevent the maggots getting hold of your onions, make the soil in the pot is consistently well-drained.

Diseases

To prevent the common diseases affecting your onion plants, you should begin by choosing healthy, preferably resistant cultivars. When the onions are growing, keep the pot/s clear of weeds. Make very sure that you don’t over-water the plants, because this can cause the roots to become waterlogged and root rot may develop.

Conclusion

To grow onions successfully in pots, you need to plant carefully and make sure they are looked after from the beginning. The plants need a lot of sun and not too much water. The soil in the pot must be able to drain easily and you should keep the soil as slightly acidic. All of these factors are important when you grow onions in pots.

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