Was this Japanese maple this gorgeous last year? It can’t have been.

Now that I think of it, a freak hailstorm last October blew the leaves to bits before they had a chance to change color, and we woke up to a foot-deep carpet of shredded green leaf mulch all over the garden. On the plus side the leaves came down in one heavy flop, and after a strenuous day of raking we were pretty much done for the season.

Not this year. Yesterday I spent a nice quiet hour with a couple of three year olds pulling leaves out of flower beds by hand and making waddle-y trips up and down the steps to the compost bin. Some of the younger ones got disoriented and would lose their destination on the way. Turning around and around, at last they’d remember what they were looking for, their eyes would find the bin, and they’d say, “Oh!” with the delighted intonation of an adult bumping into an old friend.

The Japanese Maple is still holding onto its leaves, so last week’s lesson with the Kids Crew kids was all about the chemical pigments in fall foliage. Many of them had learned in school why trees shed leaves: the costs of heavy snows and freezing temperatures outweigh the potential benefits of making food during a dry, low-light season. As the chlorophyll starts to degrade, the kids knew, the other colors get to show off.

But as it turns out, that’s not the complete picture. With yellow and orange trees it’s true, the pigments have been there all season long waiting for their turn under the blinding green chlorophyll, and the changing colors are just pleasant byproduct of the tree cutting its losses. But trees with red leaves must actually expend a fair amount of energy to create the red anthocyanin pigments during the fall, and scientists haven’t quite figured out why trees would spend so much energy on a flashy display when they should be growing roots and storing food for the winter.

I try to present the kids with science’s unsolved mysteries whenever I can. I think it makes science seem more accessible, to show them that there’s room for them in a field where the adults don’t have everything figured out.

Maybe the trees turn red because people like them, so people plant them, one kid thought. (Artificial selection, exactamundo, I said). Maybe red is just the tree’s favorite color. Maybe red trees grow in redder soil.

Scientists think that the red pigment might deter aphids from overwintering there, or it might attract birds to come help distribute seeds. In 2005, two biologists at Colgate came up with the reigning hypothesis, which is that the anthocyanins might have an allelopathic benefit — pigments in the leaf litter might actually suppress the growth of other plants under the canopy of the tree, the way garlic mustard exudes a chemical that messes with other plants’ nutrient uptake, or the way Alianthus inhibits neighbors’ growth. They extracted anthocyanins from maple leaves and added them to the soil of lettuce seedlings. The anthocyanins “dramatically reduced germination and growth compared to all other treatments” (Frey & Eldridge, 2005).

We practiced extracting the anthocyanins by using rocks to pound them into thick drawing paper, and now I’m tempted to try to replicate the Colgate experiment. The question is, if we compost the red leaves with all the others, will the anthocyanins in the compost inhibit next year’s growth?