My understanding is that most species with spring-growing leaves survive to -25°C or so, while those with winter leaves are less hardy. I've never gotten *L. radiata* to survive even a mild winter, and it's usually said to be Zone 6 hardy. The species Jim listed, as well as *L. incarnata*, are perfectly hardy here, but I'm clearly doing something wrong as few of them flower for me. Sean Z Zone 6a SE Michigan