Green onions, also called spring onions, are among the first fresh vegetables available in the spring. Harvested from immature bulb onions, they require only six to eight weeks of growth until harvest. Because green onions taste best fresh from the garden and because you can grow a large crop in just a small amount of space, they are an excellent vegetable to grow in hydroponic systems, even by beginners. As cool weather vegetables, green onions prefer spring and fall temperatures and grow best in a cooler environment. Because onion seeds are tiny, you may want to start your plants from transplants or sets rather than seed.
Things You'll Need
Ebb-and-flow hydroponic system
Onion seeds, sets or transplants
Starting from Seed
Fill a flat plant tray with seed starter or another sterile medium so that you don't transfer any soil diseases into your hydroponic garden.
Carve a small furrow into the soil surface, about 1/3 inch deep.
Plant the seeds in the soil, about 1/4 inch apart. Sprinkle soil over the seeds to cover them. You should see seedlings emerging in about 10 days.
Transplants and Sets
Remove and gently rinse any transplants grown in nonsterile medium before moving them into your hydroponic garden. Soil can contain disease organisms that can infect your system.
Transplant the seedlings into your hydroponic grow bed or plant onion sets in the grow bed. Select larger onion sets for the best green onions. Since you will be harvesting green onions in just a few weeks and won't allow the bulbs to grow to maturity, you can place the plants close enough that the bulbs at the roots nearly touch.
Harvest green onions when the tops reach 6 to 8 inches in length by gently pulling the entire plant from the growing bed.
Onions grow best in a pH ranging between 6 and 6.5, so check the pH of the nutrient solution regularly and make adjustments as needed.
Provide adequate light for your plants. Although green onions don't require the abundance of light needed by fruiting plants like tomatoes, they still prefer medium to high amounts of light. Keep them in a sunny spot or provide them with fluorescent lighting or horticultural lighting designed for growing plants. Hydroponics.com reports success growing onions in rockwool in an ebb-and-flow system. Because green onions produce so little tissue under the ground, you can grow them in a shallow system only 6 inches deep.