You can now get to the LA Arboretum using the new Metro Gold Line and Foothill Transit Bus.
From the arboretum's web site...
Just take your favorite MTA bus or train to Union Station, and then take the Gold Line to the Sierra Madre Villa station.
From there you can transfer to Foothill Transit busses that will take you within short walking distance to the Arboretum.
For more information on the Arboretum, visit their web site: