The Bundesgartenschau, or BUGA, runs from 16 April to 17 October and was expected to reach this milestone only in its final days.
Some 3,000 events have been organised across the show's 185 days alongside a selection of flower and garden arrangements.
Koblenz's 48ha of outdoor displays can also be admired from the town's new cable car.
The biennial British garden festivals of the 1980s were based on Bundesgartenschau as an initiative to boost private investment in run-down industrial areas.
BUGA was founded in 1951 in Hanover and has been held biennially in a different German city ever since. The 2013 event is scheduled to be held in Hamburg.