Obviously starlight is most likely not enough for photosynthesis. (Of course, there has been no
need for such a plant to evolve in the real world…)

What we know: There were plants everywhere during the Spring of Arda, thanks to the Lamps. After their destruction, Yavanna grieved because their growth was “stayed”, and she “set a sleep
upon many things that had arisen in the Spring, so that they should not age”.

There is, however, a mention of growth in dark Beleriand – Nan Elmoth where Elwe and Melian
met and “the trees of Nan Elmoth grew tall and dark before they […]”. Hmm. Of course, that could be an exception due to Melian’s presence, seeing as it’s also mentioned how Doriath is a place of “life and joy” because of her, in contrast to most of Beleriand, and Niphredil blooms there when Luthien is born.

So, the explanation is that the vegetation is under the Sleep of Yavanna, but that doesn’t mean there aren’t a few things to consider:

– Yavanna kept the plants from dying, and if that means they were frozen in time, the Elves could presumably eat them, though they wouldn’t grow back. If “Sleep” means they were like our plants in winter, nutrition becomes more problematic. Their diet might have to be mostly carnivorous, maybe supplemented with roots and nuts or fruit remnants. (Again, since none of it grows back, they’d have to forage and put themselves in danger more and more.)

– Also, there’d be funghi. Lots and
lots of funghi, possibly everyone’s main food source.

– I’m also worried
about the oxygen – the ‘easiest’ explanation I can think of
is that the trees in Aman produced it and Manwe made sure
it was distributed to Middle-Earth, too. (Sounds better than ‘the
Elves didn’t need oxygen’, anyway.)

– Some plants could be getting glucose from somewhere else and transforming
it into other chemicals they need, more like heterotrophs. Maybe the
soil is just full of glucose at the time, courtesy of Yavanna. They’d have pale or unusually colored leaves.

– Any exceptional plants that managed to adapt to the darkness could have died out soon after the Sun rose. Which could have been
hard to get used to for the Elves and possibly Dwarves of
Middle-Earth. Imagine desperately missing your favourite plant or mushroom that used to be really common, went extinct because of the Sun and may or may not exist in Aman but the damn Noldor can’t tell you because they had so much other stuff to eat they never paid attention to the thing you’re describing (this could be part of Eöl’s villain origin story…)