Yes, they are. As the story points out, they're also nearly the only thing that would grow in the muddy, cratered morass of a WWI battlefield. Nobody really knows why.
It's a lovely custom, and when it comes up I feel like a bit of a party poop in pointing out that the rest of the original poem is, in fact, a call to "take up the torch", carry on the mission, and defeat the Germans, and not quite the lament against the overall wastage of war that it is sometimes portrayed as. It is an entirely understandable emotion given the circumstances and I don't fault McCrae one iota for expressing it. But let's not misrepresent it too much.
We too need “to take up the torch” aka mission of defending our brethren and countries against threats and enemies today as was being done by the Allied Armies in WWI. Today the primary enemy is the radical Moslems of ISIS and Al Qaeda that wish to destroy Western culture and society.