There are 5 gardens around London, they are as follows: The Rose Garden, The Primrose Garden, The Rowan Garden, The Hawthorn Garden and The Marigold Garden. Each has its own building and its own special purpose, and is staffed by a large number of people.  

The Rose Garden
The original and -- some would say -- best, the Rose Garden has become a central hub for all others. Catering to all disciplines, this garden has offices of the higher ups allegedly secreted somewhere within it its walls. Despite its central location and importance, the Rose Garden is actually the smallest of the five buildings -- this is due to the small beginnings of the organisation. It is concealed inside of two neighbouring town houses, modified inside to make it one building.

The Primrose Garden
Primrose Walpergists specialise in the capture and removal of vampires. Located in the North of London, the building is an expansive bunker buried beneath the North-most corner of St. James's park. Its proximity to St James's Palace is no coincidence; this Garden was added to protect the reigning monarch. On special occasions -- Queen Victoria's wedding day in particular -- their security is out in force.

 

The Rowan Garden
Located beneath the Tower of London, the Rowan Garden is responsible for the imprisonment of all captured supernatural creatures. Bound demons are kept here, and the Department of Demonology is also conveniently located within. Most vampires and werewolves, when caught, are destroyed -- but there are exceptions to every rule, and some long-term prisoners reside here.

The Hawthorn Garden
The Hawthorn Garden, located to the South, specialises in the Undead, and retains a staff of expert exorcists. They have recently taken up residence in the Bryant and May factory building, making use of complicated glamour spells to produce an image of a working factory to those passing by on the outside. This is where the most talented witches are retained.

The Marigold Garden
The Marigold Garden is where the weapons are manufactured. They specialise in the study and elimination of werewolves, and the Tulip Garden is also the place you would go to stock up on stakes, swords, silver bullets, crossbows, arrows, and Holy Water. Located in and beneath a large, old Church in West London, this is the only Garden maintained on consecrated ground.