Roses only last like a couple weeks and that's if you leave them in water. They really only exist to be pretty. So that's like saying, "My love for you is transitory and based solely on your appearance."

But a potato! Potatoes last forever! In fact, not only will they not rot, they actually grow stuff even if you just leave them in the sack. That part alone makes it a good symbol. But there's more! There are so many ways to enjoy a potato! You can even make a battery with it! That's like saying, "I have many ways in which I show my love for you."

And potatoes may be ugly, but they're still awesome. So that's like saying, "It doesn't matter at all what you look like, I'll still love you."