Onions are incredibly easy to grow in a hydroponic setup. They take relatively little time to mature, need minimal help in the way of nutrients and sunlight, and come in a satisfying range of onion types.
Just about any beginner could grow an onion crop this way.
But if you want to learn how to grow the best hydroponic onions, you’ll need to follow a few simple rules for supporting the correct growing conditions, using the best hydroponic systems, as well as starting, maintaining, and harvesting your onions. All of which we discuss in this article.
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Benefits of Growing Onions in Hydroponics
Onions live in the Allium family that includes garlic, chives, and leeks. They are traditionally a biennial crop, which means they take two years to mature enough to create flowers (and seeds), but as a farmed plant they only need one year’s growth before they can be harvested.
In soil and outdoors, the onion bulb fattens up into a good size for eating when the right day-length is reached. At the end of the growing cycle, onions will lose their green foliage. This is usually autumn, or spring if you’re growing overwintering onions. At this point, they are ready for harvesting.
In a very general sense, onions come in three types. Long-day and short-day. Long-day onions (mostly the European species, but also grown in the northern US states thanks to their long summer days) need 14 or more hours of sunlight to make the bulb nice and fat. Intermediate-day onions need 11-12 hours of sunlight per day and are generally grown in southern Europe or northern Africa. Short-day onions (generally from the southern states of the US) only need 10 hours.
If you have a light rig on your hydroponic setup you can grow onions all year round, and you can speed up this process. That will mean a faster harvest and bigger crop. You can make use of succession planting practices and harvest all year round.
Also, onions are vulnerable to a lot of pests such as onion fly, onion eelworm, and various fungi. By growing them hydroponically you can avoid a lot of these problems.
Onions need a lot of nutrients to grow a nice fat bulb, and this is much easier to provide hydroponically because you’re in complete control of the nutrients in your solution.
Onions also suffer if weeds grow around them, and this is very unlikely to happen in a hydroponic setup.
Growing Conditions for Hydroponic Onions
Temperature
Onions grow in temperate conditions, and for the best crop, you should keep your temperatures in the 65-70F range.
Nutrients
As we mentioned onions need high levels of nutrients to make a good-sized bulb. However, it’s a good idea to starve the plants in the early growing stages. This will force them to grow longer root systems, which will give them a good stable base and increase growth later on.
Check out our recommendations for the Best Hydroponic Nutrients
Onions need regular feeding with phosphorus, nitrogen, and potash. Balancing the nitrogen feeds can be tricky as too much will give you an onion with a small bulb and large foliage, but too little will leave you with a small weak onion.
The last nitrogen feed should be at least four weeks before harvesting.
It is vital that the nutrient solution is changed regularly, around every three weeks.
Light
As mentioned this will depend on whether you have long-day onions (14 hours of light per day), intermediate-day onions (11-12 hours of light per day), or short-day onions (10 hours of light per day).
Short-day onions would be the most cost-effective to grow indoors as they need the least amount of light.
Short- and long-day onions reach around the same size when mature, while intermediate-day onions are generally smaller.
If you are growing outdoors then you will need to make sure your onions are in a spot with full sun, and that it is the right time of year, otherwise they won’t grow properly.
But one of the great joys of indoor hydroponic grows is that a good light rig can be preprogrammed and left to make its magic.
Here are our recommendations: Best LED Hydroponic Grow Lights & Best Hydroponic Grow Light Reflector Hoods
Best Hydroponic Systems For Onions
You can grow onions via a number of hydroponic systems.
If you use a simple wick system, you can grow smaller onions very successfully. These are great for pickling.
If you want to grow larger bulbs, then you can use a deep water culture system (DWC), ebb and flow, or nutrient film technique (NFT).
DWC System For Onions
The DWC system is great for beginner hydroponic growers or people with limited space. All you have to do is make sure that only the roots of the onions touch the water. The bulb should always be dry.
Our recommendation is a five-gallon bucket system like the PowerGrow DWC Hydroponic Bucket System.
Included in this package are the following:
- Five-gallon bucket with a net pot integrated into the lid
- Air pump
- Air tubing
- Air Stone
- Growing Medium
- And Rockwool seed starting packs
The above list is your basic requirements for a DWC hydroponic setup.
Setting up this system is pretty simple and can vary a bit depending on whether you’re growing outside or inside.
Just make sure to have a net pot that is big enough for the onion you intend on harvesting. You will be able to grow incredibly robust plants in this system, but it won’t give you a huge quantity of plants. For that you’d be better off with the ebb and flow or NFT systems.
Ebb & Flow and NFT Systems for Onions
Both ebb and flow and NFT systems allow you to fit more plants into less space. The only difference is that ebb and flow won’t constantly expose your onions’ roots to the nutrient solution.
An ebb and flow system like the VIVOSUN Hydroponic Grow Kit, which is one layer with 36 plant sites, is exactly what you want and can be used for a variety of crops.
And it is easy to set up, has a pump timer, and nest baskets/net pots.
It’s very important that you space your onions correctly when using ebb and flow or NFT systems. Too close and your onions will be fighting for light when their leaves get big. It’s recommended that you grow two onions in adjacent pockets, but leave at least 4 feet on either side of the two onions.
In a standard NFT system, you can just leave a pocket empty on either side.
A great system to start with is the LAPOND Hydroponic Grow Kit is easy to use and set up. It is also created from food-grade PVC, so no plastics contamination.
The other consideration is the size of the net pot. If it’s too small the onion will break through the pot and get stuck in the NFT pipe.
The last system that is often used in classrooms is the Kratky method. Essentially you’re just growing an onion bulb suspended above a glass of water. While this is fun and educational for children, it’s not going to give you a large or useful crop.
For more hydroponic system recommendations check out our full guide: Best Hydroponic Systems
Starting Your Onion Hydroponic Crop
There are two ways to start your onion crop: by seed or by set. A “set” is just a small bulb.
If you’ve decided to start with seeds it is recommended that you sprout them in a growing medium and then transfer them to the hydroponic setup when they’re big enough. Just remember not to feed them with nutrients so that they grow bigger, stronger roots. It should take around 10 days for the seeds to sprout.
After about 30 days, when the sproutings are big enough, you can move them and their sprouting material directly into your net pots.
If you go with sets then you can place the bulbs directly into the net pot and growing medium. Rockwool, composite plugs and perlite all work well.
You’ll generally find the sets at your local nursery, but often you get a much smaller variety than you would by browsing the seed selection.
How to Set Up an Onion Hydroponic Unit
If you are setting up outdoors, find a spot that will give you the right amount of sun. Make sure it is not too windy. Onions will tolerate lower temperatures, even snow or frost, but if the nutrient solution freezes they will die.
If you are setting up indoors, make sure you have good airflow in the room you’ve chosen or even invest in a fan. Onions are vulnerable to the kinds of funghi and mold that grow in very humid environments.
We highly recommend making sure your setup is out of earshot. The pumps can become very noisy if you’re hearing them day in and day out.
Now that you’re happy with the environment, follow the instructions for the hydroponic system you have chosen.
Maintaining Your Hydroponic Onions
Onions are considered to be a cool-weather crop. If they get too hot they will ‘bolt”. That means they’ll start to flower, and diminishes the quality of the bulb.
For the best onions, you should try to keep your water pH between 6.0-6.7. So, ever so slightly acidic. If you are using chlorinated water when you change your nutrient solution, you should let it sit for 48-72 hours to allow the chlorine to dissipate.
We definitely recommend investing in a fertilizer that is designed for root crops. And you should test your water an hour after adding the nutrients to make sure that the pH is correct.
Each leaf on an onion plant corresponds to a layer on the bulb. And onions grow in two phases, leaves and then bulbs. So you want to encourage a good amount of leaf formation in the first phase, but less in the second phase so the plant is putting all its energy into plumping up the bulb.
When the leaves get long enough to bend, prune them at the breakage point. This will stop the onion from putting unnecessary effort into the leaves, and it will then transfer more energy into building a fatter bulb.
Learn more about maintenance here: Hydroponic Maintenance
Harvesting
Green onions are the quickest, needing only 21-30 days before they can be harvested. But onions grown for their bulbs will need 80-90 days of growth after planting the sets.
If you start to see yellowing foliage or even flower stalks, you’ll know the bulbs are ready for harvesting. Simply pull them out of the net pots and let them cure somewhere dry, cool, and well ventilated. Once they’ve cured you can store them in sacks or onion bags and use as needed.
What We’ve Learned About Growing Onions In a Hydroponic System?
Onions grow extremely well in hydroponics systems. And most of the systems work well for onions, so it’s really up to you to choose, based on your space, budget, and needs.
You may find it tricky, initially, to get the nitrogen feeding regime right, but once you’ve mastered it your onion harvest will be very satisfying.
Onions are obviously a root (bulb) vegetable and the bigger the root the better. So invest in a bigger net pot than you would for a leafy green.
And finally, for a lower-cost onion, you should stick to growing short-day varieties, as that will cost you less in electricity.
If you are looking for more information about growing onions hydroponically, please feel free to post your questions below and we’ll answer them as thoroughly as possible.