bxl-forever
I guess there is only an office there, right?  Then it will be probably be office=* and it will render on the map as a small blue dot.

office=garden_architect exists.
Taginfo has 4 occurrences of it in the whole world.
And there is one office=urban_garden_design too.

Maybe you can go with the first option.
And since it’s a fairly uncommon business, filling the description tag can be useful.