Why leaves change colours in the fall

... and why it's a good idea to rake them, among other facts about fall's golden cloak.

The coming of bitter winter weather is always a bit easier to take when nature puts on a fall show, first.

For the Edmonton area, that means a fluttering cloak of gold as the leaves of trees native to Alberta's parkland, like aspen and Manitoba maple, start to turn in August and September, said a University of Alberta tree expert.

"In Eastern Canada you'll get maples with red and orange leaves, but that show of golden yellow is special to this part of Canada," said Justine Karst, an assistant professor in the Faculty of Agricultural, Life and Environmental Sciences.

Whether you love their beauty or hate to rake them, there's more to the fall leaf than meets the eye. UAlberta experts explain.

Why do leaves change colours?

Leaves contain several pigments that could be yellow, red, even purple depending on the tree species, Karst said.

"The reason we don't see them in summer is because they are masked by green pigment known as chlorophyll. In the fall that chlorophyll starts to degrade and when that happens, those other pigments can be seen."

She added the season for fall foliage usually peaks in mid-September, "so get out and enjoy it!"

Do some leaves take longer to change than others?

Yes. "Ash trees are among the first to drop their leaves, while oak is one of the last to go," Karst said. Weather can also determine how long the fall show is. Wind can end the season sooner than usual, as can a cold snap that instantly turns leaves dry, brown and down on the ground.

What are good places in the city for viewing fall leaves?

Edmonton's river valley, with its heavily wooded areas, is one of the best places to view the colourful autumn landscape, Karst said.

"My favorite spot in Edmonton is up on top of the stairs that are in the Wolf Willow neighbourhood, going into the river valley."

The U of A north campus, with its selection of more than 400 trees-40 different native and ornamental species-is also a nice place to take a scenic stroll. The campus also includes eight hectares of forested area along the North Saskatchewan River with walking trails and some boardwalks.

How are dead leaves important to the ecosystem?

"The leaf litter fuels an entire ecosystem below the ground," Karst noted. Tiny invertebrates like mites and earthworms eat and convert the leaves to organic matter which eventually turns into nutrients that feed plants.

What's that tangy smell in the air?

"There's that nice smell when we think of fall," Karst said. That spicy fragrance is caused by damp leaves fermenting on the ground. And while it sounds gross, it has a purpose. "That decomposition is important because it means nutrients are being recycled."

Do we have to rake up the leaves in the yard?

It can be a big job, but yes, the leaves have to be gathered up somehow, said Ken Willis, head of Horticulture at the U of A Botanic Garden. Besides raking them up from hard surfaces like sidewalks, patios and eavestroughs, it's important to also clean up leaves that land on existing garden plants, particularly evergreen shrubs, to avoid burns on the foliage. The good news is, those leaves can be put to good organic use by investing in a mulching type lawn mower with sharp blades.

"Mulching leaves into turf is highly beneficial, adding very valuable organic matter. Best of all, it's free," Willis said. When the weather cools down, reduce your mowing height from the summer height of four inches to 2.5 inches. Continually mow over any fallen leaves until freeze up, mow in one direction, then change perpendicularly.

Leaves can also be added to compost piles or green recycling bins. Compost piles comprised only of leaves will decompose over time into a wonderful soil supplement-leaf mould.