#1 - Why did you shoot this @ f/20? Should have shot @ a small f/stop value (larger aperture) to get the shutter speed up. Did you hand hold this shot? What was the intent of the shot?
#2 - It seems that this was shot with some decent light, why did you choose ISO 1600? For daytime outdoor shots you don't need to shoot with a high ISO, this looks like you could have shot it at ISO 200. High ISO will give you noise and noise will degrade image clarity and sharpness. I like the idea that you were trying to capture the curved benches but sometimes you gotta get higher to make the shot more interesting.
#3 - I like that you got low for this shot and you used a lower ISO, but I would have looked for a more interesting background to shoot against. Looks like if you moved to the right you could have used a bush (or something) as the background. To reduce the depth of field and put the background more out of focus, you could have used f/3.5 and zoomed out a bit from 18mm and stayed in close to the plant.