There have been several thistles in the challenge and this is a nice one. Your blurred background and the darkness behind the purple are both excellent. It's also nice and sharp where it needs to be.
The colours work well together. The depth of field works well. The composition really pushes the subject matter forward adding a bit of drama to a fairly simple subject.
The clarity is wonderful and the tension between the soft pinky purple and the hard green spikes is very effective. I do feel, however, that it would be a stornger photo if it were either cropped tighter (less top) or larger (more lower half)--probably the first.
as you say, ouch! ;) Don't wanna get to close to that one. Nice capture, nice light and good DoF. The diagonal also works good and i like how filled the frame.
I never submitted to this challenge but this was something that I considered going for...you`ve done an excellent job.. it suits the macro approach..good lighting also.