Which trees have catkins? And how to tell them apart

Have you spotted small flowers on a winter tree? These catkins have an important role in spreading tree seeds and show us that spring is on the way. Find out what they look like and why trees have them, along with where and when to spot them on five of our native trees.

What is a catkin?

Catkins are long slim clusters of tiny flowers that grow on some trees. They have small petals, or none at all.

What are catkins for?

Catkins allow the tree to reproduce. Catkins enable female flowers to be pollinated when pollen from male flowers is blown by the wind. Male catkins drop off after releasing pollen, whereas female catkins collect pollen and then release the seeds.

Once the seeds have developed, they are dispersed by the wind to avoid growing right below their parent. The exceptions are willow, which is pollinated by insects rather than wind, and sweet chestnut, which can be pollinated by either wind or insects.

When to see catkins

Many catkins appear over winter and are easy to spot on the leafless branches. They tend to flower during spring, but it depends on the weather.

You can typically see catkins in flower from January to May, though these dates can fluctuate. Differences in their timing may be linked with climate change. 

Which trees have catkins? 5 trees to look for

1. 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.

Appearance: Hazel catkins look like long yellow lambs' tails and house the male flowers. Female flowers are a small vase-shaped bud with red filaments sticking out. If pollinated, these buds become the hazelnuts in autumn.

When to see: January to March

2. Alder (Alnus glutinosa)

Each alder tree has both male and female flowers.

Appearance: The catkins which house the male flowers are up to 6cm long. Young catkins appear green before turning yellow and releasing pollen in February. The female flower is small and red, 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.

When to see: February to March

3. Silver birch (Betula pendula)

Silver birch is also monoecious.

Appearance: 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 red-brown. 

When to see: March to May

4. White willow (Salix alba)

White willow is dioecious, meaning male and female flowers grow on separate trees. Catkins appear in early spring.

Appearance: Male catkins are 4-5cm long while female catkins are 3-4cm long and a bit narrower than the male. After pollination by insects, female catkins lengthen and develop small capsules, each containing tiny seeds encased in white down. 

When to see: April to May

5. Pedunculate oak (Quercus robur)

Oaks hold both male and female flowers on the same tree.

Appearance: 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 and red, with fine filaments protruding to catch the pollen. These can be found on short stalks called peduncles and will develop into acorns by autumn.

When to see: April to May

Tell us when you see catkins

Catkins flowering is one of 69 wildlife events our amazing volunteers record for Nature’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. 

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!