I try to be as natural as possible, but heartworm is not one that I'm willing to mess around with. I don't do year round preventive. I'm in NW Indiana, not far from Chicago, and we have hard freezes in the winter. I only do preventive from about May until October. I do make sure I test every year before starting preventive again. I don't do anything for fleas or ticks, as they've never been a problem. Food grade diatomaceous earth is a good alternative to pesticidal products for intestinal parasites and for treatment of the yard. It can also be rubbed in the coat for fleas as long as you're careful that the dog doesn't inhale the powder.
April Nicole, how often do you have Sophie tested for heartworms? I would consider a product like you use for the other six months of the year, if your girl is testing clean every year.