Most 20-foot flagpoles should be buried at least three feet deep in the ground. This will give the pole the stability it needs to stay upright, even in strong winds. If you live in an area with very sandy soil, you may need to bury the pole even deeper. Six feet is often enough to prevent the pole from toppling over. Another option is to install a base plate, which can help to distribute the weight of the pole and provide additional stability. No matter what method you use, it's important to make sure that your flagpole is securely anchored in the ground. Otherwise, you risk damaging your flag or causing injury to yourself or others.