In the book To Reign In Hell (third and final part of The Rise and Fall of Khan Noonien Singh series by Greg Cox), they survived by eating what they could grow in caves beneath the surface.
A year after the destruction of their crops, the colony was barely
getting by. The cyclonic winds and UV radiation made farming on the
surface impossible, even if all the arable soil hadn’t already dried
up and blown away. Furthermore, in a fiendish irony, the most
successful survivors of the disaster—the Ceti eels—were too
indigestible to eat. The castaways’ only hope for sustenance came from
growing limited quantities of hand-pollinated Terran crops
underground, using Starfleet-provided “plasma lights” in lieu of
sunlight. A battered portable generator provided just enough
electricity to keep the subterranean gardens viable, while the
colony’s few surviving protein resequencers allowed them to satisfy
their most basic nutritional requirements.
Thank the Fates, he thought, that we managed to find enough seeds
beneath the burned-out fields to keep going. He and many others had
dug beneath the charred crops and volcanic ash with their bare hands
in search of scorched kernels of corn and seedlings of rice, while
every available man and woman had carted armloads of dead wildlife and
flora back to the caves for composting. It is a miracle that we have
managed to cultivate any fresh food at all, Khan reflected. He doubted
that mere ordinary humans could have done the same.