Onion bundles are young onions that are popular eaten raw or in salads. They are often called scallions or green onions. The tiny onion require cooler conditions, making them popular in a greenhouse setting in early spring or during September or October in a cold-frame greenhouse. The ideal daytime growing temperature for onions should hover between 70 to 80 degrees Fahrenheit. Onion bundles often thrive in a garden setting as a transplant if started from seed in a greenhouse.
Place onion seeds in a mixture of washed fine sand and shredded sphagnum moss. The mixture should be 75 percent sphagnum moss and 25 percent washed fine sand. Place 1/2 inch rich potting soil into the bottom of the starter tray. Lay 1 inch of the sphagnum moss and sand mixture across the top of the garden soil. Place the seed-starting mixture in shallow starter trays.
Plant seeds 1/2 inch deep into the seed starter mixture. Sprinkle the seeds in rows in the trays. Cover the seeds with the seed starter mixture. Mist the soil until nicely moist. Cover the entire tray in clear plastic. This will help retain moisture on the seeds and keep the humidity level up. Place the tray into the greenhouse. The seedlings should receive at least six hours of sun per day to germinate.
Transplant the onion bundle seedlings into starter plugs or container seedling trays. Use a plant starter soil when transplanting.
Place the seedling trays in a greenhouse with 40-watt fluorescent tubes suspended above the onions to help maintain heat if the greenhouse is located in a northern region of the country where the weather dips below 45 degrees Fahrenheit regularly. Place the light tubes 8 inches above the seedlings. The greenhouse temperature should be maintained between 60 and 70 degrees Fahrenheit.
Place the onion bundles into a simple cold-frame greenhouse if they are being grown in the southern United States where freezing temperatures are infrequent. If the temperature remains above 45 degrees Fahrenheit, leave the lid of the cold frame slightly open to allow the onion bundles to receive ample air flow. If the temperature threatens to dip below 45 degrees Fahrenheit, close the lid of the cold-frame greenhouse to help the young onions maintain temperatures.
Mist the onion bundles daily and keep the soil moist to the touch. Apply fertilizer that is high in nitrogen three weeks after the onions have germinated. Use a 10-20-20 mixture. Apply the fertilizer according to the directions on the label. Fertilize the onions every two to three weeks for successful growth.
Harden greenhouse onions for planting outside in the garden by gradually lowering the temperature in the greenhouse by turning the lights off. To harden onions grown in a cold-frame greenhouse, leave the lid open on the cold frame for longer stretches of time. Withhold misting and limit water while hardening the plants. Hardening will normally take 10 days before the onion bundles can be transplanted into the garden.