These grow bags are constructed using UV stabilized black or white polythene. The first 10 mm of the bag is perforated with drainage slots and plants can be started in the hole. Once the roots have penetrated, which is usually within three weeks, they can be planted outside the bag. As the plants grow to fill space inside the bag, you can cut a hole at intervals of approximately 25 mm (1 inch) and plant your vegetable seedlings further away from light source for faster growing times.