Paper milk and juice cartons are great for starting seeds because they are waterproof and strong enough to hold soil. You can cut the carton in half or make a window on one side, fill it with potting mix, and plant your seeds. Home Baked Bliss recommends cartons for bigger or fast-growing plants. When you're ready to move your seedlings, just cut off the bottom and plant the whole carton in the ground to protect the roots from pests.