what-decides-fall-coloursMany of us might know why the leaves of our favourite deciduous plants change colour in the fall. But have you ever wondered why some plants turn yellow and others red? Or perhaps you have a tree that turned orange one year and dark red another year. In this week’s blog post we’re going to explore the answers to these questions and hopefully satisfy the curious scientist in every gardener.

First off, let’s quickly discuss why leaves change colour.

During the growing season, a chemical called chlorophyll is present in the leaves of our plants. Chlorophyll is vital to photosynthesis, the process wherein a plant converts sunlight into usable energy. Chlorophyll is what gives leaves their green pigment. (As an aside, if you’ve ever planted a colourful plant in a shady area, you might have noticed that the leaves faded to a greener colour – this is because the plant produced more chlorophyll to compensate for reduced light levels.)

There are several other pigments present in the leaves, including carotenoids and anthocyanins. Carotenoids are responsible for yellow, orange, and brown pigmentation, while anthocyanins are responsible for reds and purples.

During the growing season, most plants produce so much chlorophyll that it masks the other pigments present in the leaves. However as the days grow shorter in the fall, chlorophyll production begins to taper off and we see other pigments reveal themselves!

With that out of the way, let’s delve into what factors determine fall leaf colouration. The fact of the matter is that most species of trees and shrubs tend to colour a certain way. Poplars and aspens, for example, are almost always a bright yellow in the fall. Oaks usually appear red or brownish, and often colour later than other species of trees.

That being said, seasonal variations can affect how intense (or not) fall colouration is. If we have a stretch of sunny, warm fall days with cool (but not frosty) nights, fall colouration is usually showier: bright, rich reds and deep purples will be more common. Conversely, if temperatures reach freezing early, days are particularly hot, or the weather is cloudy or rainy, yellows, oranges, and browns will be more prevalent. This is because anthocyanins are produced primarily in the fall and are produced in their largest quantity when we have sunny warm days with cool nights.

The amount of precipitation we receive over a given season can also affect fall colour. Dry summers tend to delay the onset of fall colours. If it is unseasonably warm in early fall, leaf colouration will be less intense.

The myriad of factors that can affect fall colouration means that no two seasons are likely to produce the exact same colours on our plants. This is part of the fun of gardening in the fall – you never know what to expect as our trees and shrubs give us one last colourful hurrah!

Looking for more information on trees and shrubs? Consult our Tree and Shrub Knowledge Base.