The most cold hardy citrus root stock is

The cold hardy citrus rootstock is Trifoliate orange.
The trifoliate orange, also known as trifoliate lemon and Poncirus, is extremely hardy, surviving in areas colder than USDA Zone 6. It can tolerate moderate frost and snow, making a large shrub or small tree 4 - 8 m tall. It is a common rootstock. This small fruit is very seedy and bitter.