Why do trees wake up in spring?

Young ash leaves bursting out of their buds

Bursting buds!

Trees bursting into leaf is one of the wonderful signs of spring we all enjoy, but how much do you know about the budburst? Read on to find out!

close up of leaf buds on a twig

It started last summer…  

The new growth of leaves we see each year in spring is due to a complex programme of changes in a tree. You’d think all the action would be in spring, but if you look closely at twigs during autumn and winter, you’ll see they already have tiny leaf buds! These buds are formed at the end of summer while they have energy to grow, before the days get cooler and shorter. The buds then lie dormant over the winter, waiting for spring to arrive.

Trees have hormones too!  

As the seasons change, and the warmth of spring comes, the buds swell. Hormones within each bud, such as cytokinin and auxin, play critical roles in bud bursting by promoting growth and encouraging cells to divide. These minute changes happening within the buds have a spectacular impact within our forests.

close up of ash tree buds on the tip of a twig
Tree bud in front of the sun

Bursting into leaf 

New spring leaves burst into life to help the trees catch as much sunlight as they can during the long summer days. Trees are in a race against time to unfurl their leaves before summer arrives. The leaves use sunlight, water and carbon dioxide to turn into sugars, which feed the tree while it grows, and the oxygen that we breathe. This process is called photosynthesis.

Timing is everything 

The first leaves will start to emerge from their buds between March and April each year. However, this can differ with weather and climate. Fluctuating temperatures can then damage the bud, or vulnerable young leaves. A warm winter can confuse the tree to come out of dormancy and buds may break through too early.

Westonbirt Arboretum recorded our earliest ever budburst last spring, a hazel tree on 29 November 2021!

Beech tree leaves close up
Close-up of Sitka spruce buds bursting

Conifers join in too 

Not only broadleaved trees form buds, most conifers do too! In fact, one of the first trees to turn green is the Dahurian larch, a deciduous conifer at Bedgebury Pinetum. This rarely seen tree from north-east Eurasia comes into leaf in January in the milder English weather.

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