I think you'd be right about the greenhouses. There would be a lot of suitable scavengeable materials; not only did they have general famine in the early years but also the weather would have been very weird for a while (and quite possibly still is); and the early-years population would have been used to having access to a lot of warmer-climate foods and probably would have tried to keep growing them, if only enough for an occasional treat. Even an unheated greenhouse will significantly extend the growing season; and they can be heated in various ways, ranging from a hot composting heap to, as you say, geothermal.
And, on looking up cinnamon -- it is sometimes grown as a potted plant as a hobby, overwintering indoors. Not impossible that somebody, or even several somebodies, had a few plants in year zero; and very likely that they'd have tried to keep them alive.
