Registering a botanical garden , What i think is , you have to contact local govt office for details information. I was visiting a botanical garden in Shanghai, China that is Shanghai Botanical Garden , details bellow :
http://photo.makorsha.net/thumbnails.php?album=20
They are registered in Shanghai local govt.
Thank you
Benalla Botanical Gardens was created in 1886.
Ballarat Botanical Gardens was created in 1857.
Botanical Gardens of Tirana was created in 1964.
Gisborne Botanical Gardens was created in 1874.
Auburn Botanical Gardens was created in 1977.
Aarhus Botanical Gardens was created in 1875.
Arenal Botanical Gardens was created in 1991.
Aburi Botanical Gardens was created in 1890.
The Botanical Register ended in 1847.
The Botanical Register was created in 1815.
Royal Tasmanian Botanical Gardens was created in 1818.
Dothan Area Botanical Gardens was created in 1996.