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Onions are fairly easy to grow in outdoor hydroponic systems. They can also be successfully grown in an indoor set up.
Starting Onion Plants for Hydroponic Gardens
Onions can be started from either seed or set. An onion set is an immature small onion bulb planted instead of seed. I prefer to start mine from seeds in oasis cubes and later transfer them to a hydroponic setup.
They can also be started in rockwool, composite plugs or even conventionally started in soil and then transferred to the hydroponic system.
Regardless of whether you are using oasis cubes, rockwool, or composite plugs the starting scenarios is similar. Wet the seeds, soak them for a short while if you want, or simply sprinkle water on them. Onion seeds are very small so are easily washed away. Place your cubes in a warm dark area for a few days which will help stimulate germination.
Freshly germinated seeds don't need nutrients, in fact even a modest amount could potentially hurt them.
The seeds utilize the energy and nutrients of the seeds embryonic system. Fertilizer in the embryonic stage could stunt it's natural growth or chemically burn the seedling, and being that they are too young to smoke it could even kill them.
Most onion seeds germinate in about a 6 - 10 days in temperatures of 65-70 degrees F. You should expect to harvest your onions 3- 4 months after starting from seed. Starting from sets, expect to harvest in about 80-90 days.
Environment for Hydroponic Onions
It is best that you have no nutrients in the water for the first few days that you place the seedlings in. By starving the onions in the formulative days the roots are forced to grow strong and vigorous in search of nutrients. When you do incorporate nutrients to their regimen, they will have a stable root base to absorb those nutrients and optimize growth.
With Onions, care should be taken to use a balanced nutrient solution. Too much nitrogen will create excessive foliage at the expense of the Onion bulb itself. 
Too little nitrogen will lead to a deficiency characterized by reduced growth and stamina. Follow directions on the packaging of the nutrient supplier. Monitoring and maintaining pH and Electrical Conductivity are important in ensuring the proper synthesis of available nutrients.
See - TDS in Hydroponics
Checking pH. After adding the nutrients, wait at least an hour prior to checking pH.
Aeration. You should aerate the solution continuously not only to maintain hydroponic nutrients and discourage anaerobic bacteria bu to oxygenate the roots as well.
Chlorinated Water. if you are using Chlorinated tap water in your hydroponic set up, it should be allowed to sit for 48-72 hours in order to dissipate the chlorine.
Nutrient solution should be one designated for root vegetables. General purpose nutrients will suffice, but one specifically designed for root crops is best.
In nature Onions do best in full sun. In a hydroponic system indoors they need about 12 hours of light daily, this varies depending on the variety . Short day varieties will get by on slightly less - see Onion Varieties.
Cold Hardiness - Temperature
If growing outside - Onions themselves in nature can tolerate some frost and snow, but be aware that if your solution freezes, kiss them goodbye. The optimal temperature range for Onions is 55 to 75 degrees Fahrenheit. When grown out of this temperature range they sometimes bolt to seed. Bolting Onions should be harvested ASAP.
Ultra simplified startup system for Onions
An ultra simplified startup system for Onions can be made with some polystyrene, some flower pots with an adequate depth and soilless growing medium. They can even be grown on a window sill.
For a reservoir, a Tupperware or storage plastic bin will suffice, depending on how many Onions you plan to grow. Your reservoir should be a minimum of 6-8 inches deep.
Trim the styrene to fit within the reservoir, allowing for a small clearance around the circumference of the styrene or Styrofoam. The styrene will float to the surface of the solution.
Cut holes through the styrene to allow only the botton of your pot to sit in the hole. Pots are narrower at the base, so the top will be above water.
Insert the pots in the holes and fill them with a lightweight medium. Perlite Vermiculite blend, I feel, is best. Your onions will grow in this medium so long as they are provided with the adeqaute nutrients
Green onions can be harvested in as little as 3 weeks using this system. The solution should be replaced at least monthly .
1 - Nutrient Solution and Solution pH Influences on Onion Growth and Mineral Content