According to Official Guinness Records,
The tallest thistle measures 2.4 metres (8 ft 0 in) and was grown by Christine Sadler (Canada) in Ottawa, Ontario, Canada who measured it on 26 August, 2010.Christine found the thistle in her back garden and decided to leave it to continue growing to see how tall it could grow. It is believed to be a Bull Thistle, Cirsium vulgare, although it does have some characteristics of a Scotch Thistle, Onopordum acanthium.
For a complete list of 2010 records, please visit 2010 Guinness Records in Canada.