They're perennial, springing up from their small bulbs in January or February here in Britain. They'll grace our garden for several months, with individual plants (two leaves and a flower) lasting a few weeks each. Once the flowers fade, the leaves die back as well, and all signs of the plant vanish until next winter. In addition to the cheeriness they bring to winter gardens, snowdrops have one more gift for humanity. They contain an active substance called galantamine which has proved useful in the treatment (but not cure) of Alzheimer's disease and other forms of memory impairment.