To store green onions or scallions, set them in a jar, add several inches of water, and set the jar in a sunny location. Snip the vegetables as you need them, but leave the roots in the water. Change the water frequently so the plants continue to grow.
Green onions and scallions also keep well in the refrigerator when stood upright in a jar of water and covered with a plastic bag. Keeping them in water prevents wilting, a symptom of dehydration. When the roots are submerged in water, they continue to draw moisture up into the rest of the plant. This preserves the crisp, firm texture of freshly picked scallions and green onions.
Water also revives green onions and scallions that have already wilted, a technique called "shocking." To shock wilted scallions or green onions, submerge them in a large bowl of ice water. Shocking revives these vegetables in 15 to 30 minutes.
Water keeps green onions and scallions fresh for days. However, freezing is an effective option for long-term storage. To freeze these vegetables, rinse them in running water, and dry them thoroughly with paper towels. Chop the vegetables and store them in freezer-safe containers. Add them to recipes shortly before serving to prevent excessive softening.