Now I thought to get this shallow depth of field I had to use the smallest aperture setting on my lense, which is 3.5. and stand as close to the plant as my autofocus would allow. The result was that the plant looked small and insignificant in the frame, and the background not as blurred as I would have liked. Then I experimented with standing back, zooming in with a larger aperture setting. This was the most successful at f/9.
Don't you just love it when you make progress?