According to Official Guinness Records,
The earliest documented public horticultural show was the Exhibition of Fruits, Flowers and Plants, which was hosted by the Pennyslvania Horticultural Society (established 1827), and held at the Masonic Hall, Pennsylvania, USA on 6 June 1829. The show continues to be held annually (except for 1918, 1943-46, when the USA was involved in WWI and WWII) and became known as The Philadephia Flower Show. The 2006 show will be the 172nd one (considering the war years).
For a complete list of 1829 records, please visit 1829 Guinness Records in United States.