Willows (Salix genus) are one of the earliest flowering tree species in the UK and are a fantastic nectar source for early pollinators such as bumblebees and butterflies. These trees are dioecious which means that there are separate male and female trees which can be distinguished by their flowers. Another more commonly known example of a dioecious tree would be the holly – only the female trees bear the red berries. One of the traditional names for willow flowers was ‘vegetable goslings’ which seems a perfect description to me!
Male willow flowers – the bright yellow pollen is on the end of the stamens and this brushes onto the pollinators when the come to drink from the nectar.
The flowers are quite unusual when compared with a simple flower such as a buttercup which follows the classic textbook diagram. Willow flowers are catkins – these are spikes of numerous tiny flowers rather than each catkin representing a single flower. Each of the yellow-tipped spikes in the male flower is one of the stamens and there are generally two or more of these to each individual flower within the catkin – the number varies with species. The same is true, although less easily illustrated, for the female flowers which have two or more stigmas per flower.
Female willow flowers – these are much less showy and do not have the yellow pollen of the male flowers. They also provide nectar to attract pollinators with the hope that the previous flower visited will be a male willow of the same species and thus the pollen will be transferred and the female flower fertilised.
The male and female flowers appear at the same time in order that the pollen from the male flowers is able to fertilise the female flowers. The flowers are quite different from one another in appearance and, side by side, it would be easy to assume that a male and a female willow tree were two different species.
Development of male willow flowers. On the left you can see the red outer scale to the bud which breaks and the catkin emerges from beneath. The first flowers on the catkin begin to open – the red tipped stamens can be seen. The yellow pollen then begins to be produced and finally the bumblebee comes to drink from the nectar and incidentally collect the pollen whilst doing so. This is an early bumblebee (Bombus pratorum) and the willow species is Salix x laestadiana which is a hybrid of goat willow and downy willow.
The willow flowers are an excellent source of nectar for early pollinating species, such as queen bumblebees which have emerged from hibernation and are establishing nests, or the early Nymphalidae butterflies which hibernate through the winter.
Small tortoiseshell (Aglaise urticae) butterfly on female willow flower. Note the presence of the nectar source in the centre of the catkin whilst the yellow-tipped stigma is higher. The stigma is where the pollen must reach in order to fertilise the female flower and this design encourages successful pollination which is an incidental rather than intentional act on the part of the pollinator which is only interested in a free feed!Unidentified solitary wasp on female willow flower – any ID tips would be most welcome! This demonstrates the effectiveness of the design of the female flower – see how the wasp must bend low into the flower to reach the nectar source, so bringing its body (which will hopefully be dusted with pollen from a male flower) into contact with the female stigma.
Grantham Ecology 
Lovely photos! I never knew that willows were known as ‘vegetable goslings’. Though looking at your pictures it is a very apt name for them.