52 days β As its name implies, 'Purple Top White Globe' turnips develop round roots that are bright purple on the upper part, and white below. This is probably the most common turnip sold in produce stands and grocery stores throughout North America. The globes grow four to five inches in diameter, but are best when harvested a little smaller when they are sweet and tender. Each packet containsΒ 1 gram, which is approximately 200 seeds.