Howl's Secret Garden is a setting only found in the movie adaptation of the story. Like many things in the movie, this location seems to be a combination of several other prominent locations in the book, namely the Garden at the edge of The Waste (being a massive garden created by magic, used for the flower shop) and the Porthaven Marshes (a wetland where Howl catches Calcifer).
StoryUpon moving the castle, one of the doors opens now to Howl's Secret Garden, a surprise present Howl planned for Sophie Hatter. While he shows her, he explains that the cottage there was once his uncle's, who gave it to him when he was young to practice magic in. He often practiced in the garden, helping the flowers grow with his magic.
One day, there was a meteor shower, when falling stars rained down all over the garden, most either dying in the wetlands or drowning in the lake. Howl, having studied them, knew that if he could catch one, he could trade its heart for some of its power, and allow it to stay alive. It was that night that he caught Calcifer, who became his personal Fire Demon.Sophie, who had used the magical door to travel back in time to that date, called out to Howl, telling him to find her in his own future so that she could break their contract and free Howl from his fate of becoming a monster.
Back in the present day, a battleship on its way to war flew over the garden while Howl and Sophie were exploring it. Using his magic, Howl disabled its weapons and caused it to sink dangerously close to the ground. In response, Madame Suliman's henchmen were dispatched to fly after them. Howl transformed halfway into his bird monster and flew Sophie back through the door into the castle, leaving to take on the henchmen on his own.
For several days, the garden was used for flowers for the flower shop they opened in Sophie's old hat shop.