Hydroponic Onions – The Secrets To Getting Giant Onion Bulbs

When I say hydroponics, the first thing that comes to your mind is leafy greens like lettuce and cabbage grown in a nutrient film technique. But what about root crops such as onions, there has been little to no information out there about them. It made me want to dig a little bit deeper into the possibility of growing hydroponic onions.

So, can we grow hydroponic onions? Yes, root crops like onions can be grown hydroponically with simple and easy steps just like leafy greens. However, there are some key factors that you have to consider in order to get the largest onion bulbs possible.

In fact, all of us had tried growing hydroponic onions before, we just didn’t know at the time that this was hydroponics. Remember when you were a little kid in your school and there was a project where you grow an onion inside a half-filled jar of water? Guess what? this was hydroponics.

I don’t know if you have done this back in school or not, but I am pretty sure that a great majority of people have passed through this experience.

Although growing onions in a hydroponic system aren’t that big of a deal and won’t require much work, the harvest still doesn’t satisfy most of the growers, and this is due to the fact that not everyone can grow onions with large bulbs.

There are some tips and steps that not everyone knows. These steps are crucial in producing onions with the largest bulbs. In this article, you will learn about how to grow hydroponic onions from seed to harvest. Moreover, you will get to know all of the critical tips that result in an increase in bulb size.

Best Onion Varieties For Hydroponics

In order for your to get the largest onion bulbs possible, you have to choose the right onion variety. There are three groups that determine onion varieties:

  • Short-day Onions
  • Intermediate Onions
  • Long-day Onions

So, let’s explain how the three of these work. For an onion to reach the harvest stage, it goes through 2 phases. The first phase is the vegetative stage where it forms leaves. An onion has to grow to reach the number of 13 leaves. Every leaf is responsible for the formation of a layer in the bulb.

The second phase is the bulbing phase where the bulb layers start to form and grow larger day by day. The 13 leaves that were formed in the vegetative stage will form the required 13 layers in the bulb so that the onion can reach the harvest stage.

Now that you know exactly how an onion grows, let’s connect the dots together. The previously mentioned onion varieties determine when will the onion leaves the vegetative stage and enter the bulbing stage.

Difference between Short, Intermediate, and Long day onions

A short-day onion is a variety that transfers from the vegetative stage to the bulbing stage when it reaches 10-12 hours of daylight. They favor the southern regions where the days are relatively shorter than in any other region. Cooler temperatures are also favorable in this variety.

A long-day onion is a variety that requires longer day hours to change from the vegetative stage of forming leaves to the bulbing stage of increasing bulb growth. It needs an average of 14-16 hours of daylight to enter the bulbing stage.

Intermediate onions are the varieties that need 12-14 hours of daylight to enter the bulbing stage. It can accommodate being grown in both, regions with long days, and regions with short days. However, this variety has the smallest size of all the onion varieties, so I do not suggest the idea of growing it.

The Best Onion Variety To Grow In Hydroponics

So, what is the best onion variety to grow using hydroponics? Short-day onions are the most suitable onion variety to grow using any hydroponic system.

If you are planning on growing onions using hydroponics, then you are probably going to use grow lights and heating systems as well. Appliances like this tend to consume too much electricity which might cost you a fortune at the end of the month, especially when you are trying to grow a large volume of onions.

Short-day onions require less daylight time to transfer from the vegetative stage to the bulbing stage. Thus instead of growing long-day onions and having the need to keep the growing lights running for 16 hours every day, you will only open them for 10 to 12 hours.

There is a popular misconception that the shorter the daylight, the smaller the onion is going to grow in size which is completely untrue. Short-day onions are the same as long-day onions when it comes to the size of the harvest. However, intermediate onions are significantly smaller when it is compared to others.

Another benefit of growing a short-day variety is their ability to tolerate cold. This variety is originally grown in the southern regions where temperatures are low.

Short-day onions are a huge money saver that will still grow large in size despite the shorter light time. In my opinion, anyone who uses any variety other than short-day onions for their hydroponic system is wasting his time and money.

The short-day varieties that you should choose from are:

  • Texas Super Sweet
  • Yellow Garnex
  • White Garnex
  • Texas Sweet White
  • Sweet Red
  • Georgia Sweet
  • White Bermuda

Congratulations! Now that you have known exactly which onion variety is the most suitable for your hydroponics, it is time to choose the most appropriate hydroponic system for growing onions.

The Best Hydroponic System For Onions

Choosing a hydroponic system to grow onions can be a tough part. After all, onions are root crops that are completely different from other popular leafy crops.

Three hydroponic systems can be used to grow root crops like onions: They are the Nutrient Film Technique, Deep Water Culture, and Kratky method.

Nutrient Film Technique For Hydroponic Onions

I love this hydroponic system. It gives me the ability to grow a high volume of plants at the same time using less space than any other hydroponic system.

However, there are some critical modifications that you need to add before growing your first batch of onions. Spacing is crucial for onions. You must provide the right spacing between each onion to decrease the competition of light when the onion leaves get tall in the harvest stage.

The proper spacing is around 4 feet between every two adjacent onions. You can either customize it yourself or just buy a normal NFT system and leave an empty pocket between every two onions.

The hydroponic onion pots should be wider than normal pots. As I have said before, onions are different from other crops, and it tends to get large when they reach the harvest stage.

If the hydroponic pot is not wide enough, the onion will either grow from the inside or break the hydroponic pot and get stuck into the NFT pipe. You have got to make sure of the pot sizing before even starting to grow the seedlings.

Deep Water Culture For Onions

DWC can be also a very suitable choice to grow hydroponic onions in. Although it cannot be used to grow very high quantities of onions or practically any crop, it remains a perfect choice for beginners to start with.

This system is easy to manage with no particular complications. However, you still need to keep the diameter of the hydroponic pot wide enough so that when the onion grows large in size, it doesn’t get stuck inside.

The Kratky Method

This method has been used for ages in elementary schools where kids used to put half-grown onions into water jars and leave them beside the glass window. The water inside the jar gets used by the onion as it grows larger and its leaves get taller.

The Kratky method can be used to grow onions as decoration inside your kitchen. Moreover, it can be used as a cool project for your kids to learn about hydroponics from a very young age.

Growing Hydroponic Onions From Seed Stage

So, when it comes to growing onions, there are 2 routes. Either you grow onions from the set stage or the seed stage.

If you would ask me which one is better to grow in a hydroponic system, I would probably tell you to grow it from the seed stage. Although it will take a longer time to grow and reach the maturity stage, growing onions from a seed stage have its perks.

Onion sets tend to have problems. When you grow a hydroponic onion from the set stage, you cannot determine its variety. These onion sets will be most probably sold at local stores with no particular classification.

So, you can grow intermediate or long-day onions and you only know it when you are halfway through the growing cycle. Actually, I can’t imagine myself going through all of this hassle, so I would rather just grow the onions from the seed stage.

How to Grow Onions From Seed Stage?

Step 1: You have to soak the seeds of your selected onion variety in the water.

Step 2: Place them inside Rockwool cubes in a tray.

Step 3: Place the tray in the dark to stimulate germination.

Step 4: Leave the tray for 6-10 days at a temperature of 65-70 ºF

You have to make sure not to place any kind of nutrient solution in the germination tray. I know that nutrient solutions were meant to promote growth for any plant, but in this case, it can result in reverse effects that might stunt the seed’s germination.

The Growing Conditions Of Hydroponic Onions

Now comes the most important part which is maintaining the hydroponic onion conditions to keep the growth rate maximized as much as possible. We have done our part while choosing the onion species to be the most suitable for being grown in a hydroponic system.


The perfect temperature for hydroponic onions is from 55ºF to 75ºF. Remember when we have chosen to grow short-day onions rather than intermediate or long-day ones? This variety is very tolerant to low temperatures. Thus, it will save you a lot of money on heating and electricity bills every month.

Nutrient Solution

This part depends on which system are you going to use. The Kratky system is the simplest one. You are going to place the nutrient solution as it is inside the jar. No further steps are required, you have to just leave the onion as it keeps using the nutrient solution till it fully grows.

When using a DWC system, make sure to regulate the nutrient solution level so that it is not too high or too low. To know exactly how to set the level of your nutrient solution, check this post that I had written.

Avoid adding nutrient solutions to the newly transplanted seedlings during the first period. This promotes more root growth that will later support your onion in further development.

Growing Lights

As I have said before, short-day onions are the most appropriate variety for any hydroponic system. It tends to save electricity by requiring shorter growing light periods.

The short-day variety needs an average of 10-12 hours of light per day, which is the shortest among all other onion varieties. I suggest using LED grow lights for their power efficiency.

The Secret Tips For Growing Hydroponic Giant Onions

Not everyone will grow hydroponic onions that are large in size, in fact, only a few know the secrets of how to grow giant onions. Pay close attention to the following tips, because I can guarantee that your onions will grow like never before from now then.


Pruning your onions can be a crucial part of preserving constant growth. Some people might think that by pruning the onion leaves, the growth will be stunted as the leaves tend to play a huge part in the formation of bulb layers. This is completely incorrect.

When onions grow, their leaves tend to grow higher in height, which later bends and causes them to break the neck connecting the bulb to the leaves. When this neck is broken, the vascular supply of carbohydrates coming from the leaves to the bulb will be stopped, leading to stunted growth.

So, to solve this problem, you have to cut the top of the leaves whenever you feel that they are bending.

High Nitrogen And Water

Water and nutrients are important for every plant, but for onions, they are life-changing. Onions tend to swallow a huge amount of water and nutrients for growth, so make sure that you pay close attention to the nutrient solution every day.

Another thing that you have to keep in mind is that onions love nitrogen. Nitrogen promotes leaf growth which is crucial in the formation of the bulb layers. But make sure that you don’t add too much of it to avoid any nutrient burn

Choosing The Right Species

If you chose to grow intermediate onions, then don’t blame me for the size of your harvest. I have advised you on the best hydroponic onion variety which is the short-day onions, so please stick to it.


2 thoughts on “Hydroponic Onions – The Secrets To Getting Giant Onion Bulbs

  1. Hi, love your article. My question is what happens if you continue to expose short term onions to longer days of light. Aside from cost is there a harm to 24 hour lighting. Is there a benefit. My 1st batch of hydroponics did well, but I seek to improve such for spring. Thankyou for this riveting article.

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