One of the things that many of us love about autumn is seeing the leaves on our trees burst into gorgeous shades of amber, crimson, maroon, and burnt orange. This magical, natural process is a profound reminder that change in life is inevitable – but can be beautiful.
Year after year, we wait for the leaves to signal that summer is ending and winter is on the way. Yet, few of us are aware of the science behind this change.
So why do leaves change colour and fall off the trees in autumn? And why is autumn colour better in some years than others? Below, we’ll explore the answers to these questions and offer five ways to use fallen leaves.
Why do leaves change colour in autumn?
Like most plants, trees use a green pigment called chlorophyll to carry out photosynthesis (and it’s this pigment that gives leaves their green colour).
As you may know, photosynthesis is the process whereby plants and trees convert sunlight, water, and carbon dioxide into oxygen and energy, so they can grow and live. The leaves of plants and trees play an important role here because they provide the largest surface area for sunlight absorption. However, in autumn, as the weather cools and the days get shorter, the leaves get less light and photosynthesis slows down.
At this point, plants and trees begin to prepare for winter by slowly moving nutrients out of the leaves and into the branches, trunk/stem, and roots, where they’re safely stored and protected from the cold – ready to be used to grow new leaves when the days become lighter and warmer again.
The absence of these nutrients in the leaves means they can no longer make chlorophyll, and the existing chlorophyll also begins to break down. This strips tree leaves of their green colour, revealing other hidden pigments of different colours underneath. (Note: this can happen to houseplants that aren’t getting enough sunlight too).
Aside from chlorophyll, there are two other pigments that give plants their colour – carotenes (yellow and orange) and anthocyanins (reds, pinks, and purples).
Yellow and orange pigments are present under green pigment throughout the spring and summer, and this is what we see when the chlorophyll is stripped back – while red, pink, and purple pigment can only be made during the autumn when the days become cooler.
Below, we’ll look at how this happens, and why variations in weather can affect displays of colour, year to year.
Why is autumn colour better in some years than others?
You may notice that displays of autumn colour are more impressive in some years than others, and this is usually influenced by the weather. For example, if the weather is particularly rainy or windy, leaves may be knocked off trees prematurely, before their full colour potential is reached.
The most brilliant leaf displays also tend to appear after a period of sunny days and cool nights. Sunny days enable trees to use the remaining chlorophyll to continue photosynthesising and producing sugars; while cooler nights cause the veins in the leaves to narrow to conserve heat, meaning the sugars become trapped.
A build-up of sugar in the leaves increases anthocyanin production, which makes leaves appear redder. The more sugar there is, the redder the leaves will be.
Leaves will also tend to appear brighter and more colourful during a dry autumn, as drenching rain can disrupt anthocyanin production, and make them more likely to decay ahead of schedule.
Why do leaves eventually turn brown?
As the days become shorter and colder, another way that trees prepare for the winter is to develop a seal at the base of leaves, which reduces the amount of water and nutrients that can enter them.
This happens after all the nutrients and water have been reabsorbed by the tree, and the leaves stop producing chlorophyll altogether. Without a supply of water and nutrients, leaves will eventually begin to die, which starts with the breakdown of any remaining pigment – and this is when we see leaves go brown.
Why do trees lose their leaves?
After leaves have changed from bright green to yellow, pink, purple, or orange, they may be swept off trees by gusty weather conditions – or they may first turn brown and crispy as they die, and then drop off.
The reason that leaves fall more easily in both scenarios is that the cell walls become brittle and break apart once they stop receiving water and nutrients.
5 ways to use fallen autumn leaves
Although we know the reason that most leaves have fallen is because they’re either dead or dying – this doesn’t mean that they’re no longer useful.
And while leaving fallen leaves where they lie is one of the best ways to recycle them (as, here, they’ll eventually break down and return to the soil food web), there are a few other ways you can use them if they’re starting to clog up your outdoor space.
Below, we’ve pulled together five ways you can make use of fallen autumn leaves.
1. Create a leaf pile for wildlife
By creating a leaf pile, you can provide essential food and shelter for wildlife. Many insects, including butterflies and beetles, as well as amphibians, like toads, and small mammals like hedgehogs, use leaf litter as breeding sites, and refuge from predators and harsh weather conditions.
Decomposing leaves and the insects within leaf piles also provide a crucial food source for various animals, including birds and amphibians – as well as providing organic matter which enriches the soil, improving its fertility and overall health.
Another perk of leaf piles is that, while they’re incredibly valuable, they’re also easy to make. Simply rake leaves into a pile somewhere they won’t be disturbed – for example, in the corner of your garden – and leave them be.
2. Make an autumn leaf wreath
With leaves in so many beautiful warming shades falling from the trees this autumn, you could consider bringing some of that vibrancy into your home.
DIY autumn wreaths are fun to craft and they’re a wonderful way to welcome the changing of the seasons. They can be hung on your front door, in your kitchen, or anywhere you want to add a splash of colour.
You can just use leaves to decorate your autumn leaf wreath or you may want to add other foliage, like dried flowers and grasses. To get started, check out this tutorial from Country Living.
3. Make leaf mould
Leaf mould is a dark, crumbly, and nutrient-rich organic material that forms as leaves break down over time – mostly due to the action of fungi, bacteria, and other microorganisms.
Leaf mould is highly beneficial for gardening and horticulture because it enriches soil by improving its structure, moisture retention, and aeration. It also enhances soil fertility by adding essential organic matter, and nutrients, promoting healthier plant growth – plus, it’s teeming with beneficial microorganisms, which can suppress pathogens that may damage plants.
If you’re struggling to control weeds, you can use leaf mould as a natural mulch too – sprinkling it on flower beds to block sunlight and smother weeds.
Though making leaf mould isn’t a quick process, it’s easy to do and the benefits are certainly worth it. Simply collect fallen leaves, shred or chop them, and create a pile with good aeration. Keep the pile moist but not waterlogged, and turn it regularly. Within six months to two years, the leaves will decompose into nutrient-rich leaf mould, ready for use in gardening.
To find out more about making leaf mould, check out this guide from BBC Gardeners’ World Magazine.
4. Mow over dead leaves
Another way to make the most of fallen leaves, without allowing them to take over your garden is to mow over them with a lawnmower.
This chops them into smaller pieces, which can allow them to break down into the lawn without blocking sunlight from your lawn altogether. As the leaf pieces break down, they’ll also act as a free soil conditioner – helping your grass to remain healthy throughout the winter.
5. Use them to create fun drawings with grandkids
As well as having many practical uses, leaves can also be great fun for kids to stride through and play in – and, as we’ve seen from the autumn wreath above, they can be great for getting creative too.
However, if you think that an autumn wreath might be a bit too sophisticated for any young kids in your family to get involved with, why not use fallen leaves to create some fun drawings instead?
Check out the video below to see how to make an owl, butterfly, fox, and clown.
The changing pigments in autumn leaves create a vibrant display of red, orange, and yellow hues – one that many of us look forward to every year.
The sound of fallen leaves rustling and crunching underfoot is also something that many of us relish during autumn walks in forests, woodlands, and other pretty natural spaces. So, we hope this article has helped to shed a bit more light on the reason behind this fascinating seasonal change.
Do you look forward to autumn? Do you enjoy watching the leaves change colour? We’d love to hear from you in the comments below.