What is Diffraction?
Diffraction in photography is the bending of light as it passes by the small sharp edged aperture blades which form the aperture opening. The light gets squeezed together, or blends together. Diffraction is worse with smaller aperture settings like f/16 to f/22. You don't want the light to blend because it causes blurriness (my word for it). I like my images as sharp as possible so I don't use apertures smaller then f16.
f/16 provides the maximum amount of Depth of Field without much diffraction. Using apertures smaller then f/16 (such as f/22) creates a problem with diffraction. But if the aperture is much less then f/5.6 it will decease the Depth of Field so hardly anything is in focus.
So, use the smallest aperture you can to increase the Depth of Field, but try and keep it between f8 and f16. Also try and have a fast enough shutter speed to stop any action, like a breeze blowing the flowers around. See what I mean about having a lot of opportunities to overcome obstacles? That’s why I like taking macro pictures in sunny white windowsills with plenty of light. I can use smaller apertures for greater Depth of Field, a faster shutter speed, and my subject is well lit.