The Rosedal Garden is located within the 3 de Febrero Parks in Palermo (better known as the Bosques de Palermo).
There are almost 8,000 roses of 93 different species over 4 hectares (that's almost 10 acres for you fellow Americans). All of this is completely free to visit and in Spring it is a beautiful spectacle.
Note that while it is free to visit, it is fenced in and has opening hours:
Summer: 8 am to 8 pm
Winter: 9 am to 6 pm
All things floral and garden in Buenos Aires are attributed in some way to epic French landscape architect Carlos Thays and while this was not his design, it was the project of his greatest protege - Benito Carrasco. This garden is a way of continuing Thays' legacy in the city.
Along the entrance you'll find busts of famous poets and just to the right of the entrance is a beautiful Andalusian patio.