Onions may not be the first crop that comes to your mind when you think of the word “hydroponics”, but in fact they’re a great root vegetable to grow! The best hydroponic systems to grow onions in are DWC, NFT, and kratky. Your hydroponic onions will need a lot of nitrogen and water to thrive.

Let’s talk about hydroponic onions in depth so you know what you’re doing when you start your first crop!

Can You Grow Hydroponic Onions?

You can grow hydroponic onions! There are plenty of varieties and hydroponic setups to choose from when growing hydroponic onions. This blog will discuss everything you need to know in detail so you can grow your own!

Best Onion Varieties For Hydroponics

There are three different varieties of onions: short-day onions, intermediate onions, and long-day onions. They each have their pros and cons.

Difference between Short, Intermediate, and Long day onions

There are three categories of onions:

  1. Short-day variety
  2. Intermediate variety
  3. Long-day variety

To elaborate: onions go through two stages before they’re ready to harvest. During the first stage, the onion forms leaves. It needs to grow 13 leaves, as each leaf forms one layer in the bulb.

The second stage is called the bulbing phase. This is when the bulb layers begin to form and grow larger each day. The leaves build their respective layers until the onion is ready to be harvested.

So the categories of onions determine when the onion leaves the vegetative phase and enters the bulbing phase.

A short-day onion needs 10-12 hours of sunlight each day in order to change from the vegetative stage into the bulbing stage. They like cooler temperatures as well.

Long-day onions need longer days to enter the bulbing stage: 14-16 hours of sun to be exact.

Intermediate onions fall somewhere in between: 12-14 hours of light each day. Intermediate onions are versatile because they thrive in regions that have short or long days. That being said, intermediate onions have the smallest bulbs. Since you’re able to control how many hours of light your hydroponic onions get, intermediate onions might not make much sense to grow hydroponically.

How Long Does It Take To Grow Onions Hydroponically?

onion varieties

How long it takes to grow onions hydroponically depends on a few factors. It depends on what kind of onions you’re growing, as well as if you’re harvesting the bulbs or just the tops. 

If you’re just harvesting the tops, you can have a crop ready in as little as three days! Remember, hydroponic plants grow very quickly.

For a full-fledged bulb, you’re looking at something closer to 90 days.

Hydroponic green onions grow the fastest out of all onions, and they can mature in about a month.

Onion seeds usually take around a week to germinate (if you’re starting from seed).

The Best Hydroponic System For Onions

Hydroponic systems are often a matter of preference. That being said, NFT, DWC, and the kratky method are all great hydroponic systems for onions.

Nutrient Film Technique For Hydroponic Onions

Nutrient film technique is a great hydroponic system for onions because you can grow a lot of plants using less space than other systems.

However, you need to make a few adjustments before growing onions in an NFT system.

For one, spacing is very important. You need to make sure there’s enough space between each onion so they’re not competing for light.

Although it may seem like a lot, you need at least four feet between each adjacent onion. You can DIY this spacing or just leave an empty hole between each onion.

You also need wider pots than usual because of how large hydroponic onions get.

Deep Water Culture For Onions

DWC also makes a lot of sense to grow hydroponic onions because of how large the water reservoir tends to be. While it doesn’t have the biggest yields, it’s great for beginners and perfect for this particular crop.

Deep water culture systems are easy to take care of and you likely won’t run into too many problems, if any!

The Kratky Method

The kratky method is likely quite familiar to you, even if you don’t know it by name. It’s when you put a half-grown onion in a jar of water and the onion grows.

The kratky method is the perfect choice for beginners. It’s also a cool project for classrooms and children (or adults!).

How to grow Hydroponic Giant Onions?

There are a couple vital steps if you want to grow hydroponic giant onions.


Pruning onions is actually super important. It’s a common misconception that pruning the leaves will result in smaller onions since the leaves cause the bulbs to form, but this couldn’t be further from the truth.

As onions grow, the leaves grow taller and taller. Eventually the leaves can break and disconnect from the bulb. Obviously this will stunt the growth of the onion.

So whenever the leaves start to bend, cut off the tops to prevent breakage.

High Nitrogen And Water

While vegetable fertilizers and water are important for all crops, they’re a dealbreaker for onions. Onions are incredibly hungry for water and nutrients, so it’s vital to monitor the nutrient solution closely at all times.

Onions also love nitrogen because it promotes leaf growth, which, in turn, promotes larger bulb layers. Just remember that it’s a fine line because you don’t want to cause nutrient burn!

Choosing The Right Species

Short-day onions are your best bet if you want the largest onion bulbs. Intermediate or long-day onions also have their perks, so consider all your pros and cons when choosing the right onion species.

[H2] What Do You Need To Grow Hydroponic Onions?

Let’s go over what you need to grow hydroponic onions!


It can be tricky to balance onion nutrients. 

Nitrogen is arguably the most important nutrient when growing hydroponic onions. However, adding too much nitrogen can lead to nitrogen burn or make the plant put too much energy into its foliage.

On the other hand, too little nitrogen will decrease plant growth and strength.

You also need to monitor pH and electrical conductivity (EC) when considering hydroponic onion nutrients. This will give you an insight into your nutrient solution level and let you know when it’s time to add more.

Light supply

Onions need a good amount of light for health, as discussed previously. Outdoor onions need at least 12 hours, depending on the variety. Indoor hydroponic setups require artificial lights such as LEDs or fluorescent bulbs to maintain growth.


Onions require cool temperature levels between 65 and 70 degrees F for optimal growth. As with all hydroponic crops, supplying hydroponic onions with the perfect temperature level will create happier, healthier plants.

Air circulation and aeration

Air circulation is extra important for hydroponic plants because they’re growing in so much moisture. This moisture can cause root rot or mildew if not properly aerated.

Some solutions are air pumps and air stones, depending on your hydroponic system. You can also use different growing medium to provide extra aeration. For example, perlite is great for aerating plants.

Ceiling fans can also help!

Chlorinated water

While chlorinated water is’t recommended, be sure to get rid of the chlorine if you are using it. Let the water sit for about an hour to let the chlorine dissipate. Then it should be all set to use in your system.

pH Levels

Hydroponic onions need pH levels between 5.5 and 6.5. Check the pH about an hour after adding nutrients, and periodically besides that.

How To Harvest Hydroponic Onions?

harvested onions

Hydroponic green onions are ready to be harvested in less than a month after planting. On the other hand, bulb onions take closer to three months.

It’s best to harvest hydroponic onions in the morning. All you need to do is pull them up! It’s easier than harvesting conventional onions because there’s no digging involved.

One sign that hydroponic onions are ready to be harvested is if flower stalks appear. This indicates that the onions are no longer growing. Another sign is if the foliage is starting to turn yellow and wilt.

Once you harvest your onions, you need to let them dry out to cure. Lay them out on a flat surface that has good air circulation and low temperatures.

Afterwards, you can store them in an onion sack away from sunlight and they should be good for many months!

Indoor or Outdoor Growing?

There are pros and cons to both indoor and outdoor growing for hydroponic onions.

Indoor Growing Space

With an indoor growing space, you can completely customize the light and temperature that your onions receive. However, unless you like to keep your home chilly, it might be hard to reach those cooler temperatures.

Indoor growing spaces are also less susceptible to bugs, pests, and diseases.

Lastly, you can grow onions indoors all year long.

Outdoor Growing Space

With an outdoor growing space, you can’t fully control the temperature or hours of light per day that your onions receive. But it’s easier to get those cool temperatures that onions need if you have your setup outdoors during the right season.

As long as you measure how much direct sunlight each area of your yard gets, it’s possible that your onions can get adequate light.

You have to worry about bugs, pests, and diseases when growing outdoors.

Unfortunately, you won’t be able to grow onions all year long if you’re growing outdoors.


Now let’s go over some frequently asked questions.

[H3] Can you grow onions in just water?

You can grow onion roots and sprouts in a jar with water, but your onion can’t reach full maturity unless it’s in a hydroponic system with a growing medium and nutrients.

To grow onions in just water, use toothpicks to hold an onion in a glass of water. Soon the roots will start to grow and green shoots will develop on top.

Is hydroponic better than organic?

Although organic growing is more eco-friendly and healthier than conventional farming, hydroponics uses less resources and produces higher yields than organic farming. Hydroponic crops can also be grown year-round, anywhere in the world, regardless of soil fertility and available space. You can also grow hydroponic crops organically!


Imagine going to your hydroponic batch of onions the next time you need an onion while cooking! Hydroponic onions will definitely come in handy considering how often they’re used in the kitchen. They’re also just a fun, rewarding crop to grow. Who needs regular onions when you can grow them hydroponically instead?

If you want to know more about hydroponic gardening, browse the rest of our blog!