Business question for the designers

Hi - I am an avid organic gardener and I write and host an online organic gardening show. A friend of mine has asked me to help her design her garden and shop for plants for her, and I'm trying to figure out what an appropriate hourly rate would be to charge her. I don't have experience designing any garden but my own (have done several large mixed beds), but I do know a lot about plants, and I read a lot on the topic. I need to cover babysitting costs ($10/hour) so I can have time to work on this, and then make a little on top of that, but since she is a friend and this is my first "real" job, I don't want to gouge her. Any ideas on what an appropriate rate would be? Thanks so much!