well I can't say we quite suffer from the same type of winter conditions...but our fall comes too early before potatoes are fully grown and frost can also be detrimental to the potatoes in the ground. As a kid we had lots of gardens and grew most of our own food and that is how we extended the potato growing season by tarping them like you are doing. I'm glad it's working out well for you too.
Hey my friend, happy days, if this works and it seems to be we can literally grow spuds all year round. Next to plant sweet potatoes ;) Cheer$;)