This was my first year growing them, but from what I have found, they need to be kept warm (above 16*C.) to survive the winter. They were easy to start from seed.

Odd, Wylie. Around here they take our light frosts just fine, though they don't bloom until it is pretty hot.

I know they come from the prairies, but mine have died all the way back and I am waiting to see if in the Spring they return. I had Googled "Eusotma care", and the sites all said the plants needed to be kept warm.

Growers probably keep them warm to speed growth. Around here, the flowering stems die down in late fall, but dense tufts of foliage develop at the base over winter and early spring, and elongate into flowering stems in late spring.