It is like a small oasis in the middle of Marrakech- nice for a stroll, but small, you couldn't stay there for more than an hour (I think they don't allow it anyway). However, I couldn't help noticing that some of the information of the plant names and origins in the gardens are not quite right!
There is a small museum which is nice to visit (although they charge you for it), and there seems to be a café too, though it was closed when I visited).
Entrance fee is 60 dirham, if I am not mistaken.
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