Nutshell: tidally locked earth has been taken over by mobile carnivorous plants, and humans are devolved to stupid arboreals, one-fifth our size. Moon is also tidally locked with earth, so giant vegetable spider-things go between earth and moon. They are more menacing in theory than practice, and generally serve as trains for the less dumb protagonists--though, for the most part, the narrative here affirms the proposition that it's better to be lucky rather than smart.
So, yeah, it's all exceedingly unlikely. But: that's the point of the post-apocalyptic genre. As I noted in my review of Wilhelm's Where Late the Sweet Birds Sang, the post-apocalytpic subgenre erases setting and has story limited to individual v. environment. So, basically, the genre sucks all around. This text is likely the archetypical statement of the generic principle: the entire world crawls with omnivorous plants, and characters die every other page or so.
It's got some nifty bits, including intelligent fungus and intelligent fish-things, and the interactions with those species are the most engaging parts of the novel (or collection of novellettes, to be proper).
Some rhetorical defaults, though: narration proceeds referring to "today" or "day" or "night," which are concepts that should be unavailable on a tidally locked planet.
Think of it this way: on a science fiction scale that runs from Heinlein/Asimov/Clarke future history of spacefaring, through the far future weirdnesses of Wolfe (and now Lawrence), to the terminus represented by the horrifying bit in Wells' "The Time Machine" about the end of the world, this one is somewhere bewtween Wolfe and Wells, heading toward Wells by the end, when the smarter plants begin the process of evacuation insofar as "Green rose up into the sky and the shaft pointing finger stretched into the canopy of space and the tip of it was lost to view" (181).
Some cool ideas here and there, but mostly a mess of plant-things eating the fuck out of the protagonists, who are mostly minor children.
Sought this one out, after some productive books-in-print research, specifically because one of my own projects has a similar astronomy. I am pleased to see that my drafts replicate neither the story nor the setting, such as they exist here. If any'all know of other tidally locked planet settings, I am now actively soliciting that knowledge, evaluations immaterial.
Not sure about the Hugo award (for short fiction), and can't really opine, as my edition is the US abridgement of the UK text that actually won.
Recommended for extraordinary ancestral compost heaps of the unconscious mind, those who fail to distinguish between past and present and future, and ones who head to deep space, expanding all the time as pressure drops.