Maple

A maple was a type of tree native to Earth which was used to make maple syrup. Maples had "flame-coloured leaves". (PROSE: The Ship of Painted Shadows) Its wood was used to make a variety of objects. According to Alan Turing, the Eighth Doctor once had an umbrella with a handle made of maple, shaped like a duck's head. (PROSE: The Turing Test) Maple trees were known to exist in England in at least the 1930s, as at least one provided shade for the cemetery at St Cuthbert's in the village of Marpling. (PROSE: Eater of Wasps)

Maples were particularly associated with the state of Vermont. Fitz once joked to an unamused American historian named Bleney that a better name for a publication called Famous Crimes in Vermont might be Murder Under the Maples. (PROSE: The City of the Dead)

The maple leaf itself was a symbol for the country of Canada, as evidenced by a Green Park memorial to Canadian military forces that fought alongside the British in World War I and II. As Anji Kapoor and her boyfriend Dave once noticed, the memorial had many maple leaves carved into its stone surface. (PROSE: Hope)

Other references
Maple Seed was a business active in London at the time of the March of the Adipose. (TV: Partners in Crime)

Behind the scenes
At the end of The Bells of Saint John, a leaf was prominently featured that appeared to be that of a maple. However, as revealed in The Rings of Akhaten, the leaf came from suburban Blackpool and was thus more likely to be that of a plane tree (genus Platanus). In fact, the leaf prop in The Rings of Akhaten, where the leaf is central to the story, is clearly different to the prop in Bells. Thus, the prop in Bells, since it is the more minor usage, is likely best viewed as a production error.

In any event, the leaf type was never actually stated on screen.