Witch hazels are the first class shrubs that cannot be mistaken for any other plant. Their spidery, cold-hardy flowers open in the coldest weather and are not just showy, they are fragrant too. Preferring an acid, moist soil but not tolerating waterlogging, they have an attractive, spreading habit and pay their way all year.
But why do they bloom at this coldest time of the year? Most winter flowers have catkins so they are pollinated by wind. Why would a flower that is fragrant and colourful and so obviously adapted to pollination by insects, bloom in the depths of winter? Of course there is no grand plan and the common (medicinal) witch hazel (H. virginiana) from the east of the USA actually flowers in autumn. You may know this if you bought a witch hazel and the rootstock suckered (they are all grafted onto this species) and you found that part of your plant flowered in October. It is thought that they have evolved from spring-flowering plants, blooming earlier and earlier, into winter. The obvious disadvantages of flowering when there are so few pollinators about is offset by the fact that when they do bloom there is so little competition that the any insects are bound to visit them. The colourful flowers produce nectar and attract very early bees but it is thought that moths and flies are the main pollinators, especially large blow flies.
I have written a piece on these plants for The English Garden, out now (or soon).