Growing Perfect Onions in Small Gardens: Square Foot Gardening Tips

In this guide to square-foot gardening for onions, we will share with you all the information you need to grow onions within a confined space.

Onions in Small Gardens: Gardener's hands trim the greens of onions in a square foot garden, tending to the onion crop.

Whether you have a small urban garden or a backyard, the innovative approach of square-foot gardening can help you grow various types of onions efficiently. This guide will cover the basics of square-foot gardening and provide specific insights on planting techniques, the optimal number of onions per square foot, and expert care tips.

Also covered is every step of the process, from soil preparation to harvest, to ensure even those new to gardening can achieve a successful yield.

So, if you’re looking to add some flavor to your garden with home-grown onions, read on for further advice.

What is Square Foot Gardening?

Square foot gardening is a revolutionary approach to home gardening that maximizes yield in minimal space.

Developed by Mel Bartholomew, it involves dividing the garden space into small square sections, typically one foot on each side, and planting a specific number of plants in each square. This method promotes efficient use of space, reduced water usage, and easier maintenance.

Benefits of Growing Onions in a Square Foot Garden

It is a tried and tested method of growing and offers versatility in garden planning, crop rotation and companion growing.  It also makes introducing growing vegetables to children or complete garden novices easier.

As an efficient use of space, it allows you to grow many onions in a small area, making it perfect for urban gardeners or gardeners with smaller yards. 

In addition, the structured layout of this gardening method allows for easier crop maintenance. Tasks like weeding, watering, and monitoring for pests are far more manageable within a compact space.

Preparing Your Square Foot Garden for Onions 

The success of your onion crop begins with soil. Preparation is essential as shortcuts will likely result in a low yield and poor-quality crop.

Soil Preparation

Onions do not have a substantial root system, so it is essential to prepare the ground with well-draining, nutrient-rich soil to ensure a good start. Begin by loosening the soil and enriching it with compost that is rich in organic matter or even some well-rotted manure. A nutrient-rich soil should provide all the nutrients required for the growing season. However, a potassium-rich mulch or fertilizer applied in summer will help with healthy bulb formation. 

The aim is to achieve a neutral to slightly acid soil with a pH ranging from 6.2 to 6.8 for optimal growth. It’s advisable to test your soil to ensure it falls and remains within this range (a soil testing kit can be helpful).

Thorough weeding is essential to avoid overcrowding and competition for nutrients and light.

Planning Your Layout

Start by planning how many squares you will dedicate to onions, considering the size and number of other plants in your garden. This is critical for ensuring each plant gets enough nutrients and sunlight. Also, consider factors such as access between blocks for ease of maintenance and proximity to a water supply.

A finger points to young onion sprouts growing in rich soil, showcasing early plant development in small gardens.

Planting Onions in a Square Foot Garden – Quick Guide

Soil TypeWell-drained, fertile soil rich in organic matter
Soil pHSlightly acidic to neutral (6.0-7.0)
Spacing4 – 9 onions per square foot, depending on the variety
WateringConsistent moisture during bulb formation
SunlightFull sun (6+ hours per day)
FertilizerBalanced fertilizer at planting; nitrogen-rich fertilizer until bulbing starts
Harvest TimeWhen tops begin to fall over and yellow
Additional CareMulching to retain moisture; regular weeding

Planting Technique

Sow onion seeds or plant sets about 1 inch deep in the soil. Depending on the variety, you can plant up to nine onions per square foot in a square-foot garden. Ensure each onion has enough space to develop fully.

Selecting Onion Varieties

There are a wide variety of onions to choose from; however, there are three main types of onions to consider before choosing which variety to grow. The ideal choice depends on the daylight received in duration to your geographical location.

  • Short-Day Onions: These onions start forming bulbs with 10-12 hours of daylight and are well-suited to the southern regions, for example, Arizona’s low desert. Varieties to consider include Yellow Granex, Texas Sweet, and Red Creole.
  • Intermediate-Day Onions: Best suited for the central parts of the U.S., these onions start bulbing after 12-14 hours of daylight. Yellow Candy and Super Star are our recommended choices.
  • Long-Day Onions: These thrive in the northern states and require 14-16 hours of daylight to bulb. Varieties like Walla Walla, Red River, and Highlander are good options.

How Many Onions Per Square Foot?

Generally, you can plant four to nine onions per square foot, depending on the variety. This density is usually enough to allow each plant room to grow while maximizing your garden’s productivity. However, onions do not like growing in cramped conditions, so check the guidelines for spacing on the growing instructions and reduce the quantity if needed.

Spacing for Different Varieties

The spacing may vary slightly for different types of onions. For example, larger varieties like Walla Walla might need more space with five-to-seven-per-square-foot spacing, while smaller types like red or yellow onions can fit comfortably at the nine-per-square-foot spacing.

Child's hands harvest red onions from the soil in a home garden, demonstrating the yield of onions in small gardens.

Caring for Your Onions

Regular inspection to check for adequate watering, signs of pests, diseases, or overcrowding is essential. 

Watering and Nutrient Management

Onions require consistent moisture, especially during bulb formation. Water your onions regularly, keeping the soil moist but not waterlogged. Assuming the soil has been well prepared before planting, then a nitrogen-rich fertilizer applied during bulb formation is all that will be required.

  • Once planted, ensure plants are thoroughly watered.
  • Use an organic, Nitrogen-rich fertilizer only during bulb formation.
  • Post-fertilization requires deep watering.
  • Regularly water when the top inch of the soil becomes dry.
  • If you notice the leaves turning yellow, this could indicate overwatering or pest or disease infestation.
  • Onion tops starting to bend over is a sign that the bulbs are ready to harvest. Reduce watering to prevent bulbs from rotting.

Pest and Disease Control

As a rule, onions grown from sets have fewer problems than those grown from seeds, though the risk with seeds lessens as they develop into sets.

Check the plants regularly and look out for the following:

Onion Fly: The onion fly lays eggs at the base of the onion. When hatched, the larvae eat away the base and roots of the onion, causing the plant to die. Initial signs are yellowing and wilting leaves, with smaller onions dying off quite quickly. If lifted, you’ll see small white maggots. Fortunately, it is rarer in raised beds as the soil is generally warmer, and the maggot eggs prefer cooler temperatures to hatch. As treatment is only by prevention, it will be easier to manage in the square foot garden by simply covering young plants with a floating cover of light agriculture fleece. If the infection has occurred, it is essential to remove all plant material to prevent further infestations. The practice of crop rotation will also help.

Onion Botrytis Allii: Commonly referred to as onion neck rot, is quite serious and causes the onions to become mushy and unedible. Signs are not usually noticeable until after the bulb has been harvested, but when lifted, you might notice the outer layers are usually water-logged with a grey mold spreading down into the layers of the onion itself. Again, the only treatment is prevention. Take care not to overwater when bulbs have formed; only harvest when the necks have ripened and leaves fall over of their own accord. Avoid using high-nitrogen fertilizer after bulbs have been set. It is vitally important to remove all signs of infected plants and not to plant onions in the same area for at least two years.

When to Harvest

Harvest onions when the tops begin to yellow and fall over. Gently lift them from the soil and let them cure in a dry, well-ventilated area.

Storing Your Onions

Once cured, store onions in a cool, dry place. Proper storage can extend their shelf life, allowing you to enjoy your onions for months.

Common Challenges and Solutions

Onion bolting: This is more common in onion sets than seed and is caused by early planting and a combined temperature drop or root disturbance when weeding. It is when the plant is stressed and puts less energy into the bub and more energy into growing a stalk and flowerhead.  Bolting can be avoided by planting after frosts or during extremes of temperatures and by careful weeding, ideally done by hand.  There is no solution once bolting has happened, and the plant is best discarded.

Addressing Spacing and Nutrient Issues

Ensure each onion plant is not competing with weeds for space or nutrients. Also, ensure not to over-plant each square, as this could cause the bulbs to not grow to their full potential.

Managing Water and Disease

Correct watering is crucial in preventing diseases. Avoid overwatering and provide good drainage to prevent root rot, especially when bulbs have fully formed.


Square foot gardening offers an innovative and efficient way to grow onions. This method maximizes space, reduces maintenance, and can yield a substantial harvest.

Whether you’re a seasoned gardener or just starting, growing onions in a square-foot garden is a rewarding experience that brings the joy of fresh produce right to your doorstep.

So why not give it a try? While waiting to enjoy the produce, don’t forget to explore our guide for growing broccoli and corn in a sqaure foot garden.

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