A smaller aperture (larger number) is one way, which would have to be offset with more light (flash, higher ISO or slower shutter). It's also possible you may have been a bit too close and the near petals were closer than the lens' minimum focus distance - I can't really tell which one.
If you are a beginner, "Understanding Exposure" by Bryan Peterson is a good book to learn how the exposure triangle (shutter speed, aperture & ISO) work together.