Just to see the shinzen garden 2-3 hours is more than enough time, the garden is not big, however its located inside a huge park, with different playgrounds, a lake for fishing, if you wanted to explore the park as well more time might be needed. Have fun! Its beautiful
Make sure you bring cash! It costs extra (under $10, can't remember exactly) to visit the Japanese Friendship Garden, & they don't take cards...
3 hours is about the right amount of time to enjoy the garden for an avid gardener
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