Autumn Leaves: Why and How They Turn Colors (podcast)

Sep 8th, 2017 | By | Category: Featured Articles, podcast, the show
Big Tooth Maple

Colored leaves of a Big Tooth Maple, image by Sally and Andy Wasowski. From wildflower.org.

Autumn is right around the corner, and with it comes the promise of sustained cooler temperatures, rainfall, and a more colorful landscape.

Certain deciduous tree species’ foliage will turn from summer green to glorious shades of yellow, orange, red and even purple before falling to the ground where we’ll rake them up to add to our compost.

Why and how do leaves change color in autumn.

Damon Waitt, Director of the North Carolina Botanical Garden in Chapel Hill, and formerly Senior Director and botanist at the Ladybird Johnson Wildflower, says: “That’s a lot like the ‘why is the sky blue’ kind of question.”

He says, in truth, the color change “serves no purpose.” It’s a function of chemical processes in the trees.

Processes triggered by shorter days.  And that’s about as romantic a notion of why leaves change color as you will get from a botanist.  Romancing the leaf aside, there is some cool biology going on with leaves and trees to get from everyday green to: “Darling, red is so your color. Love it.”

Damon says when you think about being a leaf — and who doesn’t? — you realize winter is not your friend.

“Especially if you’re a thin, flat one,” he said, “because cold temperatures are going to kill that leaf during the wintertime.”

Fall isn’t good for thin, flat leaves, and not just because of the chill in the air. As days become shorter, trees start squirreling away nutrients for the winter months  — taking those from the leaves. Wait says: “What deciduous trees with large, flat leaves do, is they capture all the good chemicals out of the leaves before winter and put them back into the tree and store them in their roots.”

Botanist, Damon Waitt, says a consequence of that is the leaf is no longer making chlorophyll — and chlorophyll, as you know, gives leaves their green color. As chlorophyll levels drops, other colors begin to appear. Some of the colors already exist, masked by the presence of  chlorophyll, and others are produced after chlorophyll production stops, and the leaf seals itself off from the rest of the tree.

The leaves will continue clinging to the branches until a mechanical action like a gusty breeze knocks them to the ground. But they don’t take their leave, so to speak, until the tree has recycled the majority of nutrients they harbor. But once the tree has had it’s way with the leaves, it’s away with the leaves. And they become our “problem.”

And we add them to our compost, or put on the curb for brush pickup, where — in Austin, Texas — they become an ingredient in a product called Dillo Dirt.

Yet, in an ideal world, says Damon Waitt, we’d leave the leaves on the lawn as nature intended.

Looking for trees to provide color in your yard? Here are some of Damon Waitt’s favorites:

  1. Acer grandidentatum
  2. Liquidambar styraciflua
  3. Platanus occidentalis
  4. Populus deltoides
  5. Quercus falcata
  6. Quercus muehlenbergii
  7. Quercus shumardii
  8. Quercus texana
  9. Rhus lanceolata
  10. Taxodium distichum
  11. Ulmus crassifolia
  12. Ungnadia speciosa
  13. Viburnum rufidulum

 

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