Have you ever stopped to notice the small and interesting flowers that grow on many of our UK trees? Some of these are catkins: long slim clusters of tiny flowers, with small petals or none at all. Let’s take a look at which trees have catkins, why they grow and when you can see them.
What are catkins for?
Essentially, catkins allow the tree to reproduce. Catkins allow the female flowers to be pollinated as the pollen from the male flowers is blown by the wind. Once the seeds have developed they are dispersed by the wind to avoid growing right below their parent. The exception is willow which uses insects for pollination rather than wind.
Which trees have catkins?
Alder (Alnus glutinosa)
Each alder tree has both male and female flowers. The catkins which house the male flowers are up to 6cm long. Young catkins appear green before turning yellow. The female flower is a smaller red structure, about 1cm long and with many hair-like structures across the surface. Once this is pollinated, it turns into the brown alder fruit or ‘cone’ which houses the seeds.
Hazel (Corylus avellana)
Hazel shrubs are monoecious, which means they have male and female flowers on the same plant. They can’t self-fertilise though. Hazel catkins look like alder and also house the male flowers. The female flowers are a small vase-shaped bud with red filaments sticking out. If pollinated, these buds become the hazel nuts in autumn.
Silver birch (Betula pedula)
Silver birch is also monoecious. Male catkins are 4-5cm long and yellow-brown in colour. They hang in groups of two to four at the tips of shoots, like lambs' tails. Female catkins are smaller, short, bright green and erect. Once pollinated, female catkins thicken and change colour to a dark crimson. Masses of tiny seeds are borne in autumn – they have tiny ‘wings’ on each side of the seed to help them travel on the wind.
White willow(Salix alba)
White willow is dioecious, meaning male and female flowers grow on separate trees. Catkins appear in early spring. The male catkins are 4-5 cm long while female catkins are 3-4 cm long and a bit narrower than the male. After pollination by insects, the female catkins lengthen and develop small capsules, each containing tiny seeds encased in white down. These white feathery attachments help seeds to float on the wind like a dandelion seed head.
Pedunculate oak (Quercus robur)
Oak catkins appear less densely packed than those on other trees. Male catkins are yellow, each around 6cm long and grow in rows, hanging down from the branch in a curtain. The female flowers are tiny with fine filaments protruding to catch the pollen. These later become the acorns.
When to see catkins
Many catkins appear over winter and are easy to spot on the otherwise bare branches. They tend to flower during spring, but it depends on the weather. You can expect to see catkins in flower in the following months:
Hazel: January to March
Alder: February to March
Silver birch: March to May
Oak: April to May
White willow: April to May.
Recording our seasons
Catkins flowering is one of 69 wildlife events our amazing volunteers record forNature’s Calendar. This project tracks the effects of weather and climate change on wildlife across the UK – its records date all the way back to 1736!
We ask our recorders across the UK to keep an eye on catkins on their local hazel, silver birch and pedunculate oak trees. We want to know when they appear to swell and release pollen – this is their first flowering date. Though our table shows typical opening periods, these can fluctuate and any changes in their timing may be linked with climate change. That’s why it’s important that we monitor events like this.
By taking a few minutes to share what you see, you'll be adding to hundreds of years' worth of important data for studies worldwide. Every record is crucial and valuable. The data recorded helps us to understand the effects of climate change and other patterns in the natural environment. We couldn't do this work without you!