My gardening pal, Rebecca, "Queen of Dahlias", gave me some of these multicolored dahlias years ago. I planted them along the stone wall by our driveway in the Cape garden and they flowered prolifically all season. By the fall, the right time dig them up and store them for the winter, we had rented the house and I just had to leave them.
Much to my surprise, the next spring, after tenants had departed, there they were, up months before the Concord garden ones that I had started in March indoors in pots.
They have come back every year since, increasing in size, and untroubled by even the snowiest or coldest winters. They bloom for months, starting in late June and keep going until early November.
The secret must be excellent drainage plus retained heat from the sun shining on the stone wall.
I am now trying another old variety bought from Old House Gardens, Mrs. I. De Ver Warner, which is reported to come back, plus another, just as a test, 'Bahama Mama" . Both were beautiful.
I also have some gladiolas on the other end of the wall that repeat every year too. They were bought, on impulse, from Costco and the picture showed them to be pure white and dark purple. They are instead, tropical sunset pink, and have survived the house being raised up and the wall being built beside them. I read on Margaret Roach's excellent website, A Way to Garden, that work is being done hybridizing hardy glads by Joseph Tychonievich . He should look at these.
In an environment that can be so harsh: severe salt desiccating winds, plant munching critters, dry, sandy soils, some plants thrive with little to no help from the gardener.