As I see it there are only two things you can do.  Either keep the cat indoors until you are ready to plant (which would involve getting and making the cat use a litter box), or invest in a "scat mat" which would keep the cat off of the desk.  An alternative to a scat mat would be to cover the desk with something your cat does not like (tin foil works for most of my cats).  I don't mean upholstering it .... just putting a few sheets over stuff when you aren't there to supervise.

And, you're right ... it does sort of look like the top of an old hot water bottle.

Could be worse, could be raining.