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Explanation: I would stress the design element here - after all, that is what they are doing - rather than any mention of gardeners, who would generally be the ones who get their hands dirty turning the design into reality.
However it's perfectly possible that paysagiste - which is nicely ambiguous - can cover both, at the risk of putting a landscape architect's nose out of joint. A bit like calling an interior designer a painter and decorator ...
of these people, have you tried Googling them to see what it is they do exactly, how they describe their work, and possibly if they have all the above in English?
I would go for landscape architects, but landscape gardeners might work too
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6 mins confidence: peer agreement (net): +3
landscape designers
Explanation: I would stress the design element here - after all, that is what they are doing - rather than any mention of gardeners, who would generally be the ones who get their hands dirty turning the design into reality.
The translation workplace 
peer agreement (net): +3
