Players who are level 6 or above, will find a pop-up when they log into the game, which will ask if they want to place a new Rose Garden in their Zoo. Roses can be found a number of ways -- three basic roses (yellow, red and white) can be sent as free gifts from friends, a blue one will only be available through Facebook feed and lavender, pink orange and green can be collected from the garden daily, acquired from a friend's Facebook feed or by combining (or cross-breeding) roses to get the desired color.
The entire garden will level up as well -- going from Silver to adamantium -- and each level will unlock new flowers. A new trophy related to rose gardens will roll out with these new gardens, which will be soon, though the exact release date is still TBD.
[Via Zoo World forums]