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Can You Plant Onion Sets Too Deep

Growing onions from sets are one of my favorite things to do in the garden. Sets are those tiny onions that mature over the course of a few months. Because they’ve already got some size to them, the tendency is to plant these deeply. But, is it possible to plant onion sets too deep?

It’s possible to plant onion sets too deep; resulting in stunted growth and poor bulb development. Onion sets should be planted between 1 – 1.5 inches deep for best results.

The bulb is what we’re all after when planting onions. Unfortunately, if you plant too deeply, that bulb may never form the way you imagine it would. Planting onion sets too far down in the soil or without enough spacing can cause a plethora of problems.

Let’s get into what those are next.

Determining How Deep To Plant Onion Sets

Before you plant your onion sets, it’s important to determine how deeply they should be planted. There are basically two types of ways to plant onions. Ones that are small bulbs (sets) or from seed. Here’s a table showing the ideal planting depth of onions at various stages:

Onion TypeHow Deep To Plant In Soil
Onion Sets (transplants)1 – 1.5 inches
Onion Seeds0.25 inches deep
Large Onion Bulb2 – 3 inches deep
Planting depth of onion sets and onion seeds

The above numbers could change with onion sets; especially if you’re taking a mature onion from a store that’s beginning to sprout. In that case, you’ll need to plant deeper so the entire bulb can be covered in the soil and hidden from birds and other pests.

Because these onions sets are not overly mature, and you’re not starting from seed, you’ll need to observe the following:

  • How big the set is
  • What the variety of onion is

That last point is particularly important. Different varieties of onions grow at different rates and in different sizes. Because we’re looking for a bulb, it’s important that your onion will eventually reach a size where it can break through the soil.

However, if you were to plant a miniature variety such as shallots (and these split), then it would be a huge mistake to plant them anywhere too deep in the soil; they may never come out. Larger sets of different varieties could benefit from a little more soil so that the roots can properly catch.

Issues With Planting Your Onions Too Deep

Gardening has its share of issues from watering to selecting the right type of fertilizer. Even if you’ve got all those things right, if your plants are too deep in the soil, all the other good things you do just won’t matter.

This is especially true for onion plants. Here are a few issues that can arise from planting your sets too deep.

Stunted Growth

The number one issue with planting onion sets too deep is stunted growth. You may see the plant start to do well but eventually fall off in its development.

Some signs include seeing the leaves droop, or even the onion leaves browning. The plant won’t seem to gain any size really at this time.

Your first instinct may be to just add tons of nitrogen fertilizer. That’s going to do little to help though. It’ll be too much fertilizer for the amount of rooting you have, and the plant will simply become overwhelmed and start to develop more problems.

Even if your onion does develop after a while, it just won’t have the shape and size you’re looking for. In some cases, it won’t get much bigger than the initial set you started with; and this can be after months of grooming the plant.

Poor Bulb Development (or no bulb)

In addition to the stunted growth, you’re likely to see poor bulb development. In some cases, you won’t see a bulb forming at all!

Although you can leave the onion sets planted deep, all of the green growth on top can be misleading. This is because what’s happening deep beneath the soil is a bulb that’s struggling to expand and take shape.

The bulb needs to have a chance to round out and swell so that it becomes an edible plant.

However, when you plant too deep, this just doesn’t happen. Most gardeners will end up seeing a bulb that is flat in shape, and soft to the touch.

The softness doesn’t necessarily mean your bulb is rotting, but it doesn’t have the firmness it needs because it’s not growing properly.

Adding more water won’t fix the problem either. Overwatering can quickly become an issue and that could also eventually lead to some root rot.

It’s important to think of your onion bulb as a storage device for the plant. It’s from that bulb where the top growth occurs and the plant thrives. Take care of the bulb, and the rest will follow.

Things You Can Do If Your Onion Sets Are Planted Too Deep

So perhaps at this point, you’ve discovered that indeed, those onion sets were planted too deep. You’re probably seeing all the symptoms of stunted growth and issues with the bulb. No worries though!

There are a few things you can do to help your onion plant along and in a way, reverse these issues so you can enjoy a healthy plant just in time for harvest.

Remove Some Of The Soil

Planting crops too deep is more common than you think. Unless you’re using a ruler and eyeballing everything, it’s easy to mess this up; especially with onion sets.

Thankfully if you plant them too deep you can fix it by removing some of the soil. I wouldn’t suggest doing this all at once though, but rather over time.

The onion sets will continue to grow, and as it grows the bulb will expand and make an attempt to take form. Of course, because the soil is too packed over it, that doesn’t always happen properly. So as you see it start to take shape, gently brush back the soil as much as you can without forcing it.

This helps you not damage the plant accidentally. Over the course of a few weeks, the bulb will firm up a bit and start to swell as you would expect.

If you’re using thick in-ground soil, this works too, but it’s easier if your soil is a loose potting mix.

Plant In A Loose, High Drainage Soil

Carrying on with the previous point, the looser the soil, the better your plant will grow.

Loose soil aids in root development; allowing them to easily breathe and travel as they grow. The soil needs to drain easily, so I wouldn’t recommend filling it with too much peat moss.

In the case of planting onions too deeply, that peat moss could in a way hold in too much moisture and that’ll lead to your onion rotting if you combine that with the facts it’s having a hard time developing as it is.

You can also add more drainage holes to the bottom of your gardening container to help it dry out faster. This in combination with removing soil over time will help you produce a healthy plant.

Some Closing Thoughts

A lot of times what happens with onion sets planted too deep is that the entire growing cycle is extended. Rather than harvesting in a few months, some gardeners might end up still waiting for their onions to be ready a year later.

Sure, the leaves will get taller, and the roots may develop, but the bulb never really comes. At around 18 inches in height on the leaves, the plant will start to max out and eventually start rotting.

So as a general rule, start by planting your onion set between 1 to 1.5 inches deep. If that is not deep enough, then that’s okay. It’s much easier to add soil after the fact rather than the tedious process of removing soil little by little each week.