Who owns a tree growing on or over the property line?
The answer to this question greatly depends on the province in which you live. In some provinces, the common law appears to allow a property owner to simply cut any part of the tree that crosses the property line. 
In Ontario, however, the Ontario Court of Appeal has recently clarified that an obscure statutory provision, found in the Ontario Forestry Act , means that a “boundary tree” (a tree whose trunk straddles a property line) is jointly owned by both property owners.  It therefore cannot be injured or destroyed without the consent of both owners.
The key provision of the Act is short, but very broadly worded:
Trees common property
(2) Every tree whose trunk is growing on the boundary between adjoining lands is the common property of the owners of the adjoining lands.
(3) Every person who injures or destroys a tree growing on the boundary between adjoining lands without the consent of the land owners is guilty of an offence under this Act.
The penalty is a fine of up to $20,000 or imprisonment for up to three months, in addition to any civil remedies. Most notable however is what this provision does not address. This provision makes no distinction between trees whose trunks emerge from the ground on a property line, and trees whose trunks emerge from the ground entirely on one side of a property line and cross the line in the air. It also makes no distinction between trees that have been intentionally planted or that have grown spontaneously, or between 100 year-old oaks and tiny weed-tree saplings. This provision appears to apply regardless of any of those facts, although there may still be creative solutions available, depending on specific circumstances.
In the urban development and real property context, this has the potential to be very problematic for developers. Root systems and canopies can extend a great distance, and it appears that one boundary tree co-owned by someone interested in preventing redevelopment or simply holding a developer for ransom, will in many cases have a powerful tool to do so. As a result, resolving boundary tree issues in Ontario can no longer be considered a trivial afterthought, and will more likely require the assistance of an experienced lawyer in addition to an arborist.
adjoining lands arborist boundary canopy developer imprisonment property statutory tree urban development