TOP TIP: Flower Pot Drain Hole Cover: Just lay a coffee filter in the bottom of the pot covering the hole before filling the pot. The soil will stay in the pot and the water will still drain out because that is what coffee filters are designed to do; let water through. They last for a long time, too; by the time they fall apart, the potting soil is well packed and the plant has roots that hold the soil together.
Cut the bottom off a plastic pot to contain invasive plants & bury in the ground. I have been doing this for a few years & works great for containing plants that tend to spread, but that I still want to plant in the garden. This also works good for planting a plant in an area that tends to get filled with roots etc. from other plants. I will use a larger pot so that the plant can spread it's roots and grow deeper roots, but keeps other tree roots from choking out the one I planted.