surface to trim. Newby question.

 From:  Rudy
1242.1 
Hello all,
happy 2008.

Is there a way to trim a surface...in this case (see JPG) I would like to trim it around the the flowery contour of this heighted Z surface, to get rid of the flat part, and keep just the flower, heighted part.

Any suggestion is appreciated,
Thank you,
Rudy