A good rule of thumb is to reach out and hug the tree (we know, we know, it sounds… tree-huggy). If you can get your arms around the tree (at the height of your chest), it’s likely smaller than 30cm (unless you play Center for the Toronto Raptors). If you can’t get your arms all the way around the tree, it’s more than 30cm and you’re probably going to need a tree permit from the City of Ottawa (see above for exemptions). To be absolutely sure, use a body measuring tape to measure the tree at chest height.