I have never heard of that, nor have I seen it, They do have small nodules on the roots which are high in nitrogen, this is why the roots are best left in even in cold areas as they give so much to the soil.
Runners beans were one of the first veggies I grew when I was a child and have done so now for 60 years. I collect heirloom varieties.
They definatley will self seed if conditions are right but they do not grow in cold soil anymore that French beans do, and don't form huge tubers.
I have often grown two varieties of Achocha, it grows very easily as an annual but have never grown it as a perennial. It is an interesting veggie ti grow and rambles like crazy, tasty too. I think I still have seeds.
XX Jeannine
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