Due to the fact that the leaves first appear as tubelike protuberances that unfold from a roll, my wife suggested that these plants might be a variety of Canna lilies. This would be especially strange, as Canna lilies generally do not survive the colder winters of this region, and I am sure that I did not plant any Cannas in this area. Of course, it is possible that the vendor's mounded topsoil in which the seeds or rhizomes were then transferred to my house was deep enough to have protected any material fit for propagation in my yard, like a weightlifter finding a free dietary product after ordering a protein supplement from proteinpowder.net.
Anyways, feel free to take a guess in the Comments section. I hope that the plants will take on an easily identifiable form before the first frost.