Why are trees blooming early?

How do trees know when to flower? 

Flowering is a collection of seasonal cues: temperature, light and a plant's internal clock. 

Many temperate trees go dormant over winter. They need a stretch of winter cold to prompt them to wake up slowly, so that their buds don’t awaken too soon.

As spring nears, warmth gradually builds up in what scientists call a ‘heat sum,’ prompting buds to grow and then bloom.

Different species, and even different cultivars (plant varieties that humans have created within a species), may need different low or high temperatures in order to flower. 

Because these required temperatures can be measured, scientists can link changes in flowering dates directly to shifts in winter and spring temperatures.

The trees that put on a spectacular blossom display at Kew Gardens and Wakehurst every year – such as magnolias (Magnolia spp.) and trees in the Prunus genus like cherries, plums and apricots – are particularly sensitive to these balances. 

Should we be worried that spring is starting earlier?

Blossoms are beautiful, but they’re also an important food source for birds, insects and other animals. Earlier flowering usually means that the window for these food sources shifts earlier in the season. 

Ecosystems depend on delicate timing. When the timing of plants’ life cycles shifts out of step with pollinators, animals that eat the plants, or animals that migrate, it can set off a chain reaction that causes long-term damage to an ecosystem.

Just like the trees, some plants and wildlife are adapting to the changing climate. Wild British bees are coming out of hibernation around 6 days earlier for every 1°C of warming. 

But not all species move at the same pace. European honeybees respond differently than bumblebees. Migratory insects and birds can’t always adjust their patterns quickly enough.

As the climate warms, sensitive ‘specialist’ species, like the high brown fritillary – a rare UK butterfly whose caterpillars feed only on violets – may struggle the most.

What Kew is doing to help

At Royal Botanic Gardens, Kew, our collections are not only curated for beauty, but for knowledge. Botanical gardens like Kew and seed banks like the Millennium Seed Bank can help by maintaining genetic variation in the plants we grow and the seeds we collect.

The dried plant specimens in our Herbarium are often plants in flower that were collected and preserved centuries ago. This means we know the exact time of year these plants were flowering when they were collected. With millions of specimens in our Herbarium, that’s a lot of historical data.

Kew’s experts are still collecting data from our living collections that shows steady advances in flowering over time. Put together, all this data creates a long-term series of records that allows us to detect and understand these changes. We can also use these historical records to identify which species are most sensitive to change.

Using this information, we can decide where to put the most effort into conservation and learn how to plant resilient gardens. We’re passing on that knowledge to community gardens with programmes like our community nature partnership, Grow Wild.

Kew is a charity and all donations, memberships and shop purchases go towards our vital research in climate change and biodiversity loss.