True Narcissus serotinus is a lovely, autumn-flowering species usually bearing just one, lightly fragrant white flower on each of its miniature stems. These flowers have a shallow, yellow cup which is not incurved and which can clearly be seen to consist of 6 smaller segments.
Flowering is best achieved by growing the plants under alpine glass, with a hot, dry summer rest. We find that this is a bulb that really does benefit from a hot summer rest, the sort of rest we advise against for most other bulbs! We leave ours under glass all summer, at a time when we move the other glasshouse inhabitants outside into the comparative cool of covered frames. We leave them dry, then re-water from mid-September.
It can be shy to flower, needing this little bit of trickery to tip it into bloom. After flowering, the flower stems take the place of leaves (which are not produced on flowering plants), the stems elongate and fatten and perform all of the photosynthesis that the plant needs to produce a larger bulb.
We would suggest that it needs to be kept above 10ºC at night and preferably closer to 12º-15ºC during flower bud formation and flowering, to ensure success (low temperatures are perhaps most likely to occur at night). It just doesn’t seem to like being chilled at bud/flowering time. Warm it up and the flowers will pick up, keep it too cool and it may well abort or not develop flowers inside its buds.