Trees with catkins

What are catkins?

Catkins are slim clusters of flowers without petals. In most cases, all of the flowers in a single catkin are either male or female. Some species, such as oak, have both male and female single-sex catkins on the same tree (monoecious), whereas others, like poplar, bear male and female catkins on separate trees (dioecious). Other species have only male catkins, such as hazel and oak whose female flowers are smaller and less conspicuous. The sweet chestnut is unusual in producing both male catkins and bisexual catkins which have female flowers at the base.

Pollen-rich male flowers are often produced in pendulous clusters – like the fuzzy yellow hazel catkins that appear around mid-February. Most catkin-producing trees are wind-pollinated. The hanging catkins enable tiny grains of pollen to be picked up and carried by the wind. An exception to this is willow, which has evolved to be primarily insect-pollinated.

What time of year do catkins come out?

The best time of year to look for trees with catkins is in late winter and early spring before the deciduous canopy appears. Trees produce catkins at this time of year because their leaves have not yet developed, so wind-borne pollen is not impeded by foliage and the tiny grains are more likely to make contact with female flowers.

Catkins provide pollen and some also provide nectar at an important time of year for early pollinating insects. They can also be extremely useful to help identify trees in winter and spring.

Which trees have catkins?

Walnut (Juglans regia)

Walnut produces upright spike-like clusters of female flowers and pendulous male catkins on the same tree. The catkins release their pollen around May, at the same time as the tree comes into leaf, and then drop off. Once pollinated, the female flowers develop into green egg-shaped fruits that ripen in autumn. Inside the fruit, the edible kernel is protected by a hard outer shell.

English oak (Quercus robur)

English oak is monoecious, producing long, thin, yellow-green male catkins that hang down from the branches. Small, red female flowers grow on stalks ready to receive the wind-blown pollen when it's released in April and May. After pollination has occurred, the female flowers develop into acorns.

Sweet chestnut (Castanea sativa)

Sweet chestnut is also monoecious, with trees producing both male catkins and bisexual catkins which have strongly scented male flowers and female flowers grouped in threes at the base. The erect yellow catkins are around 15cm long and appear in clusters. They produce nectar and pollen, which attracts bees and other pollinating insects. After pollination, the female flowers develop into spiny fruits encasing the sweet chestnuts.

Beech (Fagus sylvatica)

In April and May, beech trees produce yellow male catkins that droop down on long stalks at the end of twigs and release their pollen into the wind. Female flowers develop in pairs, also on stalks, on the same tree as male catkins. After pollination, woody cups develop enclosing two small triangular nuts.

Silver birch (Betula pendula)

Silver birch is another monoecious species. Around April, hanging yellowy-brown male catkins start to release their pollen. The smaller green female catkins point upwards until after pollination, when they also hang downwards and turn crimson as the seeds develop. Many tiny seeds are released in the autumn and dispersed by the wind.

Alder (Alnus glutinosa)

Wind-pollinated alder has yellowy-brown male catkins that dangle from the end of branches and are 2-6cm long. They shed their pollen in February and March, before the leaves appear. Alder grows smaller, red female flowers in clusters on every tree in addition to the male catkins. Once pollinated, they develop into green fruits and then brown, woody structures that resemble miniature pine cones. They release their seeds in autumn and can remain on the tree for a year or more.

Hornbeam (Carpinus betulus)

Hornbeam is monoecious and wind-pollinated. It has pendulous green male catkins that grow up to 5cm long, with red anthers that release pollen in April and May. The female catkins are also green, but shorter at around 2cm. Pollinated flowers develop into small nutlets in the autumn. Each nutlet is attached to a large, three-winged bract which causes the nut to spin as it falls, aiding seed dispersal in the wind.

Hazel (Corylus avellana)

In late winter, male catkins resembling fuzzy golden lamb tails hang down from hazel trees and clusters of six tiny female flowers, each with two red styles that makes the cluster look rather like a sea anemone, open on the bare branches. Although hazel is monoecious, it is self-incompatible, so pollen needs to travel to female flowers on a different tree. But with every male catkin capable of producing around eight million pollen grains, hazel maximises its chance of successful pollination. When flowers are pollinated, they develop into hazel nuts.

White poplar (Populus alba)

Poplars and aspen are dioecious, meaning they produce male and female catkins on separate trees. White poplar catkins can be difficult to spot because both red male and green female catkins grow towards the top of the tree. They appear in March, well before the leaves, and once the female catkins have been wind-pollinated, they develop into seeds with lots of silky white hairs that are released back into the wind in summer.

Aspen (Populus tremula)

Aspen catkins appear in February and March. Male trees produce silky brown catkins before the leaves appear and green catkins develop on female trees. Once fertilised by the wind-blown pollen, they turn into fluffy white seeds which are released in late April and May.

White willow (Salix alba)

White willow has pendulous male catkins, up to 5cm long, and slightly shorter female catkins. Like poplars, all willows are dioecious. The catkins appear around the same time as the leaves and, once the female flowers are pollinated, they develop into seeds covered in a fluffy white down which helps them to be dispersed by the wind.

Pussy willow (Salix caprea)

Another spring favourite with pollinating insects, pussy willow (also known as goat willow) produces erect male catkins in March and April before the leaves appear. The furry male catkins are silvery-grey and turn yellow as the golden stamens become visible. Green female catkins develop on separate trees a little later, in April, and shed their seeds in May. Pussy willow catkins provide an important source of food for some solitary bee species, particularly mining bees, and queen bumblebees. Having just emerged from hibernation, queen bumblebees are hungry, and pussy willow catkins provide nectar and pollen at a time when there is little other food available.