I grew Dahlia imperialis from seed for 3 years back in the 1980s; specifically, I started it from seed, and it survived in my garden over winter for 3 summers and 2 winters. A winter with 10 F temperatures killed it, but it survived light frosts with short term temperatures to around 27 F in the first two winters. The third summer it was spectacular, 10 ft tall and covered with flowers. Then the third winter came along . . . If I'd dug the tubers up each winter, I might still be growing it. Kathleen PNW coast, mild wet winters (most of the time), cool dry summers