A wonderful place to be probably throughout the year. I was there in late fall and got to see the fading colors of their beautiful Japanese garden. It was not at all a disappointment, but I am sure spring/summer would be more beautiful. Also, note that there is an entrance fee to get in and a nice cafe that serves some nice stuff to eat if you get tired running around the garden.
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There is parking and also the public transport is reasonably good in STL and you can take one to this place as well.