8 Reasons Onions Are Small (With Solutions)

Have you recently harvested your patch of onions? So have I! But just like me, you may be disappointed by how underdeveloped and small your onions are. So let’s have a look at what may have caused small onion growth, and possible solutions to avoid this happening again.


The most common growth problems causing small onions include growing the wrong onion types, warm temperatures, lack of soil nutrients, excessive amounts of weeds, incorrect watering, incorrect spacing, insufficient sunlight, and harvesting onions at the wrong time.

Read on to find out more details about each problem and how to fix it for a harvest of larger onions.

1.  Wrong Onion Types

Different varieties of onions.
Different varieties of onions

It may come as a surprise to learn that one of the most likely causes for small-sized onions (Allium cepa) is planting the wrong type of onion for your area. The area in which you reside will be suitable for specific variants of onions.

Onions are divided into three (3) categories: short-day, intermediate-day, and long-day types of onions. All three categories of onions rely on the length of day to start forming bulbs.

Short-day onions will begin to bulb when there is between 10-12 hours of daylight. These onions are best grown in the Southern States (Hardiness zones 8-10).

Intermediate-day onions will begin to bulb, predominantly in the middle regions of the U.S (Hardiness zones 6-8), when they are exposed to 12-14 hours of light.

Long-day onions, as the name suggests, will begin to bulb when daylight reaches 14-16 hours per day. Extended hours of daylight are common in the Northern States (Hardiness zones 3-5) at certain times of the year.

Planting the incorrect onion types for your area will result in stunted bulb growth and bolting (flowering) of your onion plants.

I recommend purchasing onion seeds or sets that are conducive to your growing area.

2. Warm Temperatures

As a cool season crop, onions do not grow well if temperatures start heading above 80°F (27°C). So depending on your climate, onions are best grown in temperatures of 55-75°F (13-24°C). This usually means an early spring sowing of onion seeds or sets, but could also be a fall planting.

Onions exposed to warmer temperatures can cause them to flower instead of producing larger bulbs.

My climate means I can grow onions during fall, as I have mild winters. Some varieties of onion are cold hardy to 20°F (-6°C).

So by planting your onions at the correct time of the year for your climate you will help resolve your small-onion problem.

3. Lack of Soil Nutrients

Growing Bulb Onions
Growing Bulb Onions

Onions are considered hungry plants that require nutrient-rich soil, so a deficiency in nutrients will starve the bulbs, resulting in stunted growth.

Larger amounts of phosphorus (in the soil) are needed at the beginning of an onions’ growth cycle. Onion seedlings further require high levels of nitrogen for bulb development. Then once your onion plants begin to bulb, there is no need to continue adding nitrogen.

I suggest using an organic blood meal fertilizer like this or aged manure, which can be added to the top layers of the soil, and making sure to water the fertilizer into your soil.

Initial soil preparation is key to successful bulb growth. Onions prefer soil that is free-draining and fertile. My favorite way to increase soil fertility is by using homemade compost and aged manure. By giving onions the right growing start, you will increase the likelihood of producing large healthy bulbs come harvest time.

4. Excessive Weeds

Weeds are the never-ending complaint of all gardeners and for good reason. They compete with your onions for nutrients and moisture in the soil. This competition will prevent your onions from reaching their full potential.

Weeds can often be found growing amongst onions (or growing anywhere, for that matter!), so removing them can be challenging. However, your onion garden should be kept free from weeds by removing them (gently) by hand, making sure to pull the roots out as well.

Another way to reduce pesky weeds is by surrounding your onion plants with mulch. A thick layer of mulch will also regulate the temperature of the soil and prevent your onions from bolting.

A technique that I find most effective against weeds is surrounding my onion plants with a garden weed barrier fabric. Measure the length of your garden and cut holes in the fabric where your onion sprouts will be planted. This fabric is durable and easy to use.

5. Incorrect Watering

Not only are onions “hungry”, they are also considered “thirsty” vegetables and require sufficent water to grow. Inadequate watering can lead to your onion plants bolting, resulting in tiny onions.

While these bulbs have an appetite for water and nutrients, they also won’t grow well in waterlogged soil.

There is a fine line between sufficiently watering your onions and overwatering them. If you overwater your onions, they will become susceptible to disease and begin to rot.

Soil can be very porous, or, on the other hand, it can retain a lot of water. Should your soil contain a high percentage of clay, I recommend you plant your onions in a raised garden bed as this will provide both sufficient drainage and water retention.

Alternatively, if you are struggling to maintain the moisture levels in your soil, add several cups of pebbly-looking vermiculite to your soil. Though this is really only going to work on a small planting area.

Regular compost added to the soil will help increase the soils water retaining properties.

6. Incrorrect Spacing

Planting onions isn’t as simple as sticking a seed/seedling in the ground. Onions that are planted too close together will compete for nutrients, sunlight, and water. This will cause the onion plants to produce less leaves, and in turn, produce smaller onion bulbs.

Whether you decide to use seeds, onion sets, or transplant seedlings, it’s important that there is adequate space between each onion plant so that they have sufficient space to expand and grow large, healthy onion bulbs.

The recommended spacing distance between each onion plant is 3-4 inches (7-10cm).

When planting onions at home, I suggest that you measure out the area where you are going to plant your onions. Make holes in the soil, ensuring that there is about 4-6 inches (10-15 cm) separating each onion plant.

Onions that have sufficient space to grow will not intrude on each other’s growing space, allowing you to harvest the biggest onion bulbs possible from your garden.

7. Insufficient Sunlight

One of the most common misconceptions about growing onions is that they don’t require large amounts of sunlight as they aren’t plants that produce fruits. This is a simply not true. Onion plant’s basic needs include lots of sunlight and fertile soil.

Onions that are starved of sunlight will grow thin, scrawny stems and the bulbs of these plants will not grow very large. A lack of sunlight will also cause your onions to grow at a slower rate as they don’t receive sufficient energy from the sun to photosynthesize.

Prior to growing onion plants, I suggest you find an area in your garden that will provide 6-8 hours of sunlight a day for your onions to grow. Remove or cut back any objects or foilage from the surrounding area that may obstruct the light from reaching your onion plants.

8. Harvesting at the Wrong Time

Local farmer harvesting onions.
Harvesting Onions

You may, like me, be eager to harvest your onions from the ground, however, it’s never a good idea to pull up any onions from the ground until they are totally ready to be harvested. If you harvest your onions before they are ready, you will find yourself with a handful of small, pickling-size onions as opposed to the big, juicy bulbs that you aim for.

One of the difficulties associated with growing onions is the inability to monitor the growth of your onion plants below ground. However, experience and knowledge will definitely improve your ability to know when the correct time is to harvest your onions.

Onions generally take around 90-100 days (4 months) to fully mature into large bulbs. A clear sign indicating when your onions are ready to be harvested is the large flower stalk that is produced. This flower stalk means that the onion bulb has stopped developing and the plant is getting ready to produce seeds.

Planting onions in loose soil will allow you to gently stick your finger along the side of a rooted onion bulb and feel the depth that the bulb reaches, or the size of the bulb.

Although I have removed onions from my garden prematurely and have managed to successfully replant them back in the soil, I do suggest that you first wait until you are 100% certain before completely removing them from the soil.

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