The crisp, sharp taste of fresh onions and radishes jazzes up salads, soups and sandwiches. You can grow root vegetables hydroponically as long as you provide enough depth for the roots to grow. Both of these crops grow well with ebb-and-flow systems, in which you grow hydroponic veggies in a tub or tray filled with sterile growing medium and flushed periodically with nutrient solution. Your system should be at least 6 inches deep to accommodate the bulbs of the onions and radishes with some room to spare.
Fill your ebb-and-flow system with rockwool, perlite or expanded clay pebbles. Mix and test the nutrient solution and flush the growing bed.
Plant radish seeds 1/4 to 1/2 inch deep in the growing medium. Space seeds 1/2 inch to 4 inches apart, depending on the cultivar and the anticipated size at harvest. Winter radishes generally require more space than spring radishes.
Plant onion sets 1 inch deep in the growing medium. The spacing between sets depends on whether you intend to harvest the onions as green onions or as bulb onions. Space green onions 1 inch apart, but allow 4 inches of space between large bulb onions. If you want to grow both, space the sets 1 inch apart and harvest enough plants as green onions to allow for proper spacing for your bulb onions.
Plant onion seeds 1/3 inch deep in the grow bed. Because onion seeds are tiny, you may want to dig a shallow furrow, sprinkle the onion seeds into it, cover them with a thin layer of growing medium and thin young seedlings to the proper spacing once they emerge.
Rinse the soil from the roots of any transplants before placing them in the growing medium, as garden soil can contain pathogens and pests that can infect your hydroponic system.
Harvest radishes, green onions and bulb onions by gently pulling the plant from the growing medium. Radishes and green onions typically require about a month of growth before harvest. Harvest bulb onions when the tops have fallen over, after about three months.