Saints take on Wigan later in one of the biggest derby games in Rugby League. St Helens enter the clash having won just one of their previous five meetings with the Warriors.
More top news
People are being encouraged to leave tributes and messages on the trees, which are on a trail in the city close to Manchester Arena.
It will be Harry and Meghan's first royal duty as husband and wife after they were married in a ceremony at Windsor Castle on Saturday.
David John Woods and Leslie Peter Baines are said to have carried out “a savage and brutal attack” on Matthew Cassidy, in May last year.