Dahlias need a large pot with lots of room for roots to spread. If you only have a small container, you might want to consider re-potting them in a larger pot when they are about six inches tall. They need plenty of light, so position them where they get direct sunlight for at least eight hours per day. Make sure you fertilize your dahlias regularly but don’t over-fertilize. If you fertilize too much, you can end up with stunted plants that never bloom.