Here is a great idea for filling the bottom of your large planters. You want something on the bottom to allow the water to drain out, otherwise your roots will rot. You might use rocks or pieces of broken pots, but those can be really heavy and you might not be able to move your planter once you add your dirt and plants.
Instead, cut up old pool noodles and put them on the bottom before you add your dirt and plants. Pool noodles are lightweight and easy to find. They will allow your water to drain and not weigh your planter down. You might even be able to move it around after you have planted it.