Why do evergreen trees stay green?
Evergreens stay green while their deciduous cousins reveal bare branches for the winter months. But why? Is it to conserve water? Will it help trees survive winter? What makes the leaves drop at that time of year?
The reason why your evergreen tree will stay green all year is simple. Whether it has needles or tropical palm tree foliage, retaining the green colour helps the plant survive best in its native conditions.
Find out how it works and why other trees lose their leaves below.
Deciduous vs evergreen
All plants can be split into two categories:
- Evergreen trees – leaves, or needles, stay on the branches all year round regardless of the weather.
- Deciduous trees – leaves grow and drop in a yearly cycle, putting on wonderful colour displays in autumn.
Technically, you might spot evergreens losing needles. The tree will gradually drop needles as it grows. The key, however, is that the evergreen doesn’t drop all of its needles at once like a deciduous tree! To understand why, it helps to learn about the purpose of needles and leaves.
Foliage is required for photosynthesis. The chlorophyll in leaves will turn carbon dioxide, water and sunlight into oxygen and glucose. The oxygen is released and the glucose is stored or used for energy.
Chlorophyll absorbs sunlight (every wavelength except green, hence the colour of most foliage), but in deciduous species, chlorophyll production stops when the days grow shorter and the temperature drops. The leaves, therefore, lose their green colour and eventually fall in the autumn.
Meanwhile, the narrow needles of evergreens will continue photosynthesis all year round, which is why they stay green and don’t drop their foliage. Let’s explore this in further detail.
Evergreen trees retain their needles all year. They will grow new needles at a slow rate to replace old ones as needles age, but otherwise, you will only see evergreen needles drop if the tree is dying or damaged.
Beautiful evergreen trees have developed to never drop their needles because it is advantageous to photosynthesise all year round. In locations that experience cold temperatures at freezing points or lower, thin needles with a waxy coating enable the tree to survive subzero temperatures without damage. Of course, narrow conifer needles can’t absorb as much sunlight in the summer as wide leaves, so evergreens remain green all year so they conduct photosynthesis at the level they require for growth.
But it’s not just cone-bearing trees with needles that are evergreens. Evergreens growing in tropical climates have evolved this way to take advantage of the steady sunlight supply, rather than to avoid ice damage. Trees that stay green tend to have narrow leaves and branch tips too, as with so much sunlight all year round, there’s no need to grow large leaves to absorb more light.
Examples of evergreen trees
Growing at high elevations in the northern hemisphere and in locations with year-round sun, some of the most interesting evergreens include:
- Bristlecone pines – the needles remain attached longer than most conifers, up to 16 years! Most evergreens in this family of pine can live to the ripe old age of 4,800 years.
- Strawberry tree – this is an evergreen tree from the Mediterranean with strawberry-like fruits. The Strawberry Tree does well in the south of England with our moderate climate and wet weather.
- Cotoneaster – a family that’s native to Palaearctic regions. They thrive in high altitudes and rugged coastal locations. Our Willow-Leaved Cotoneaster Tree is a best-seller.
- Holly tree – growing in cold climates, some varieties like the Golden Variegated Holly Tree get even more colourful as the temperatures drop.
While evergreen trees stay green, their deciduous counterparts change seasonally. Typically having broad leaves rather than needles, deciduous trees adapt to the sun. During spring and summer, when there is more sunlight to be absorbed, the tree will rapidly grow leaves to create and store a vast amount of energy and nutrients.
However, when the days shorten and the temperatures drop, these changes trigger hormones in the tree that cause it to drop its leaves. This is advantageous for the tree, as it isn’t exerting energy to grow new leaves when there isn’t much sunlight available for photosynthesis. It reserves energy instead. Furthermore, the cold temperatures of winter would easily damage and kill off any delicate green leaves if they remained.
So, the best course of action is to stop producing green chlorophyll, drop those leaves, and reserve as much energy as possible rather than staying green like evergreen trees.
Deciduous varieties are typically native to areas where the summers provide plenty of sunlight but the winters are dark enough that retaining leaves is pointless. Like evergreens, these varieties grow all around the world.
Examples of deciduous trees
With large leaves rather than year-round evergreen needles, deciduous trees will drop their leaves to survive temperatures at freezing point. As a result, you get to enjoy the wonderful colour changes in autumn. Some of the most spectacular deciduous varieties include:
- Birch trees – with distinctive peeling bark in old age, this family produces deciduous varieties with gorgeous fluttering foliage like the Silver Birch.
- Maple trees – again with distinctive bark, trees like the Snake Bark Maple Tree put on a show with bright red leaves in autumn.
- Fruit trees – this includes Apple Trees, Plum Trees, Peach Trees and Cherry Trees.
Don’t forget to sweep up those fallen leaves to use as a good mulch layer in your garden. You can find many other trees to grow at Gardeners Dream!