Stewart's right. We inherited some in the garden of our last house, and they took over everything they could reach. We were eight years there, and still never eradicated them completely.
There's a good article about them
here - which also confirms Granny Mike's comments about wind...
On the other hand, you're right, and they're the perfect survival food. I'm thinking they'd be a great candidate for
guerrilla gardening - they need no looking after, can thrive on any old soil, and will spread as much as you like. They don't even look 'food-like', so it's unlikely amateur foragers would spot them post SHTF. I think I'm going to get some tubers tomorrow!