minature lemon tree

The leaves of my mini citrus plant drop every winter and I end up buying another one. It is always kept on a south facing window sill, but come a cold spell, the leaves always drop. I stop watering regularly in the winter, do evrything I am supposed to, but it still happens. Help!

 

Ian I have the same problem with bought citrus plants, not had the problem with ones grown from seed. Cant explain why though.