Take a look at the above photos. Both photos were shot with my Canon 7D and the Canon 100mm-400mm L lens at 200mm and 1/350 shutter speed. The only difference was the f-stop. Even an f/5.0 f-stop can produce a great blurred background (bokeh) when the lens magnification is high enough.
So what about an f/2.8 f-stop? Well, an f/2.8 alone will not produce a significantly blurred background. Take a look at the above photos. Both photos were taken with the Canon 15-55mm EF-S lens at 55mm. The background in the f/2.8 photo was blurred, but not as much as in the 200mm photo. The background is simply too close to produce a significant blur even at f/2.8.
Spring flowers are just starting to bloom, so experiment with this simple formula:
magnification + wide open f-stop + distance = blurred background
You can produce a pleasant blurred background even with a f/5.0 f-stop when the magnification is high enough. Now you might be asking, what would happen if you had a faster lens like the Canon 85mm f/1.2 L? The wider f-stop would produce a smoother blurred background. (Take a look at the bokeh comparison in my review of Sigma’s 50mm f/1.4 lens.) But, don’t let that stop you from being creative with the lenses that you already have.