I'm not sure, but I believe it's the Asarina erubescens that forms tubers, not Asarina scandens. So it may depend on the species as to whether you'll find tubers or not. I have overwintered A. scandens plants indoors and they just continued to grow and bloom on my enclosed back porch. But I never even considered looking for tubers, so maybe they do form them! I also read that you can propagate them from cuttings. Definitely going to have to try that this year!