I agree that you should get a PBGC ruling - it is the only way to know for sure.
I think it is fairly clear that architects are considered professional service employers. The question is does the term "landscape" change the meaning. It will ultimately come down to fact/circumstances. Is this particular "landscape architect" just a glorified pool boy, or are they really a certified architect with a specialty in landscaping.
Engineers – yes, sanitation engineers – probably not.
This prior ruling may or may not fit your situation.
http://www.pbgc.gov/oplet/97-2.pdf