The botanical garden in Troja is situated near the Prague ZOO. It is open throughout the year and in my opinion is the prettiest in spring and autumn.
The outdoor exposition is a large area of 25 hectares in total. It consists of several interesting parts including eg. an ornamental garden, a Mediterranean flora area, a vineyard (with a wine cellar), a peat bog with a small lake, a peony meadow and much more – it depends on what you like and what you are in the mood for :-).
As the season changes, there are also topical exhibitions. There was a pumpkin exhibition, which just ended. During the year there are many more – my other favorite is the bonsai one.