Choose a firm onion bulb with a dry, intact skin and no soft spots or mold
Place the onion in a cool, dry area until roots begin to form if it is not already sprouting
Cut off the bottom of the onion (the root end) to expose the base; leave the remaining onion intact
Fill a shallow container with water so the bottom of the onion touches the water
Set the onion in the container with the cut/root end down
Place in bright light (near a window) and keep at room temperature
Refresh the water every 1–3 days to prevent rot
Wait for roots and green shoots to develop from the top
When shoots are several inches long, transplant into soil for better growth
Use a pot with drainage holes filled with well-draining potting mix
Plant the onion so the base is covered with soil and the top remains above the soil surface
Water thoroughly after planting, then keep soil lightly moist but not waterlogged
Provide 6–10 hours of sunlight daily or use a grow light
Fertilize lightly every 2–4 weeks with a balanced fertilizer (optional)
Harvest green tops by cutting as needed once they are 6–8 inches long
For a full onion bulb, continue growing in soil for many months and ensure consistent sunlight and watering
Expect that an onion grown from a store-bought bulb may produce greens rather than a full new bulb in many cases
Remove any rotting or foul-smelling sections immediately and discard affected plants to prevent spread
Keep an eye out for pests and disease; improve airflow and avoid overwatering