If temps are going to be below 30F regularly, you'll need an auxiliary source. My parents got one last year, if they don't switch off it'll run constantly to keep temps at ~55F and drive their electric bill through the roof. It works well for them during the day in the winter most of the time and during the fall and spring.
It's always good to have a backup heat source if you live in a cold climate, but heat pumps have progressed a fair bit with running at lower ambient temperatures. Many manufacturers have models that can run down to, or below, 0°F. However, those models usually are a fair bit more costly, so it makes sense to evaluate how often it dips below the low ambient cutoff for the heat pump and the cost and type of your backup heat to determine the most economical route.