How Deep Does Soil Need To Be for Onions?

Onions are an excellent choice for many at-home gardeners since they can be used in so many ways. They’re probably the most versatile vegetable you can grow. However, knowing what onions need is essential if you want them to be big and beautiful, and using the correct soil is a fundamental part of that process. 

The soil needs to be at least 10 inches (25 cm) deep for onions to grow successfully. Onion roots typically grow directly down in the ground in the beginning; in later stages, they tend to spread out. Giving onions enough space in the soil to sprout is essential for them to grow well. 

In the rest of this article, we’ll discuss exactly how deep those roots grow and what kind of soil onions love the most. We’ll also talk about how to work the soil in preparation for planting onions, the best time of year to do so, and some essential tips for growing onions successfully. Follow these tips, and your onions will grow big and juicy!

How Deep Do Onion Roots Grow?

Onions are relatively hardy little things, but their roots tend to be a little thin in the beginning stages of growth. The roots need soil with adequate nutrients, including nitrogen and phosphorus, to become strong. You can always help them along with some organic matter such as compost, and this way, those roots will grow broad and strong.

Onion roots need space to get at least 10 inches (25 cm) deep into the soil. They aren’t considered to have exceptionally long roots. Still, since the roots are relatively delicate at the start, they will need enough space to sprout without coming up against any obstacles. 

If you’re planting your onions indoors before moving them outdoors, you’ll need to choose a container that is deep and wide enough to accommodate their delicate nature. A wide container will help the roots sprout outwards.

Once the roots have sprouted, they will begin to thicken. This can extend, in an outdoor setting, to around 30 inches (76.2 cm), which is quite long. However, they tend to spread outwards rather than down, so you don’t necessarily need a 30-inch (76.2 cm) deep soil bed. This means they will need adequate space to stretch out lengthways in the soil and get those all-important nutrients. 

Best Type of Soil for Onions

Having plenty of soil to allow the roots to grow sufficiently is one thing, but you’ll also need to ensure that you’re using the best soil. 

One of the most critical factors for onions is the soil’s drainage capacity. The earth will become a soggy mess if it doesn’t drain well enough. This stops the onion’s roots from being about to take in the nutrients it needs, and the onion will eventually give up.

To avoid this, try to go for loamy soil with a good aeration system and drainage capacity. Mixing the topsoil with organic materials is an excellent way to ensure your soil is nicely aerated.

On that note, it’s probably best to check that you’ve got sandy soils at hand. Clay soils are far too heavy for those delicate roots to push through, so the lighter the soil, the better.

Additionally, onions love the sun. While they’re generally considered relatively hardy creatures that can withstand the cold fairly well, onions will sunbathe quite happily and grow nice and big in the process. Make sure you’re placing your onions in soil that gets a good amount of direct sunlight. 

Onions also love a slightly acidic environment. This means they’ll withstand soil with a pH of up to 7.0 quite well. If you don’t have a soil pH meter, it’s definitely worth the purchase. This way, you can check how acidic your soil is and modify it accordingly. 

Finally, a decent compost mix is almost a necessity if you’re thinking about growing onions. Compost is incredible stuff and almost entirely free to make yourself. All you have to do is fill up a sealable container with kitchen waste such as vegetable peels, egg shells and boxes, fresh soil, and a little water. 

Over time, you’ll have a really moist mixture of compost that will make your soil (and, in turn, your blooming onions) thrive. 

How To Work the Soil in Preparation for Planting Onions

Onions like a firm, soft soil that has a good aeration system. This means your soil shouldn’t be completely compressed. Avoid packing your soil too tightly; instead, you should opt to press the soil very lightly to create a firm, bouncy home for your onions. 

Tilling the soil beforehand is an excellent way to ensure those roots can dig deep into the ground. The topsoil is where the roots will begin to take hold, so it should make easy work for the roots. 

Additionally, you should moisten the soil before planting. This doesn’t mean completely saturating the soil. Rather, it’s better to keep it slightly damp so air can still get through. Don’t plant your onions when it’s raining, as well. Try to wait until the soil dries out slightly before planting them.

Onions are only one type of vegetable that needs deep soil. I’ve compiled a list of other vegetables that need deep soil to grow. Check it out to learn more about planting and growing these vegetables: 5 Vegetables That Need Deep Soil to Grow

What’s the Best Time of Year To Grow Onions?

While onions can survive most reasonable temperatures, it’s best to plant onions outdoors in the early spring. This means preparing and tilling your soil after the last frost.

This period usually begins in March and gives your onions the best chance of survival. The warm temperatures coming into spring will give the roots strength to push down into the soil. 

Planting your onions in the Springtime means you can begin the harvesting process as early as late Summer! This means all your Autumn recipes will be flush with gorgeous, tasty onions. 

However, you can plant your onions in the Autumn if you wish, as long as the temperatures aren’t freezing. Around September, if there are a few more weeks of relative sunlight and warmth, it’s safe to plant them. 

Tips for Growing Onions Successfully

It’s all well and good to decide to grow onions, but there are a few things to bear in mind to make the endeavor successful. Read on below for a few essential tips on growing big, beautiful onions:

  • While many people like to begin growing onions using an onion starter set, this isn’t always the best option. These sets will deter your onions from growing big, and you’ll end up with smaller bulbs. They’ll still taste excellent, but if size is a factor for you, don’t use sets.
  • When planting your onion seeds, plant them very close together so they can create a full, dense spread of roots. When it’s time to replant them in the ground, place the bulbs slightly further away from each other to give them the best chance of digging deep into the soil. 
  • While onions adore direct, natural sunlight, they still need to stay moist. If the weather is very warm, always take the time to check on your onion beds to see if they need a spritz of water. Keeping your onions well-watered will encourage the roots to stretch further into the soil, making for bigger, juicier onions. 
  • Since onion roots can be relatively delicate, don’t use a hoe or a rake to weed the soil beneath them. This is especially the case if your onions are still very young. Instead, use your hands to gently tug out the weed roots. 
  • If you’re watering your onions too frequently, you might notice the bulbs becoming saggy and yellow. If this happens, refrain from watering them for a week or so to give the bulbs a chance to take in the extra nutrients from the soil.
  • If you notice flowers are beginning to bloom over your onions, take the time to remove them gently. While the flowers might look pretty, they’re leeching essential nutrients from the soil that should be going directly into your onion bulbs. Use a pair of gardening scissors for a clean cut to avoid damaging the onions. 
  • Onions can withstand cold temperatures reasonably well. However, this doesn’t mean they should be grown in the winter. In fact, they’ll probably end up hibernating until the warmer weather comes around. If you’ve planted them in Autumn and the cold weather comes early, cover them with a blanket or other protective covering to keep them from freezing over and dying. 

Following these tips will ensure that you give your onions the best chance at survival. While they are generally considered quite hardy vegetables–and most people do find them easy to grow–it’s still a good idea to pay close attention to the quality of the soil and the amount of water and sunlight they’re receiving.

Final Thoughts

The soil needs to be at least 10 inches deep for onions to sprout properly. Giving them adequate space to grow is essential, especially in the beginning stages of growth, since onion roots can be very delicate. 

Ensure that your soil is of a good quality, loamy texture with lots of organic materials such as compost. Good aeration is essential and frequent watering is also recommended, as long as you’re not entirely saturating the soil. If you do it correctly, you’ll have big, blooming onions in no time.

Alexander Picot

Alexander Picot is the principal creator of, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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