[header] How to grow tomatoes upside down [title] Fill your seed starter cell with moist potting soil. [step] Continue watering the tomato, keeping it warm, and providing it with plenty of sunlight as it grows. The plant should be about 6 inches (15 cm) before you transplant it to the upside-down planter. 
This will ensure the plant and the root system are established enough to take root in the new location. [substeps] Don't let the tomato grow any bigger, otherwise the roots may get damaged when it's transplanted.