I know it's stating the obvious but.........don't go to Kew Gardens unless you're really interested in plants and gardening. I'm not and I have to be honest and say that I was a bit underwhelmed by the place and thought that the cost was too high.
Let me elaborate, there's a lot of open green spaces, I mean A LOT, which may be green and natural but are not gardens. 
I think I expected something more akin to the Chelsea Flower Show than Hyde Park hence my overall disappointment. I did enjoy the hot houses, Kew Palace and the pagoda (picture).
Ultimately though, I found it to be disappointing and not worth the entrance fee (currently £22) especially when there are some excellent parks to visit in London, for free.