Excerpt from Valley 101: A Slightly Skewed Guide to Living in Arizona, a collection of Clay Thompson’s columns for The Arizona Republic. (Originally published October 5, 1999.)

Q: Why do people paint the trunks of their citrus trees white?

A: HA! At last, a question we actually knew the answer to without having to look it up or ask somebody. It’s to protect them from the sun.

We are soooooo smart.

To celebrate, we asked an actual newcomer in the office if she knew why citrus trunks are painted white, and she said it was to repel insects. These comical newcomers.We were going to laugh at her until we remembered she is much higher up the food chain than us and holds what passes for our career in her elegant and well-manicured hands. So we didn’t laugh.

Just to double-check, and to look busy, we called Ralph Backhaus, a professor of plant biology at Arizona State University. “It’s to prevent sunburn,” Backhaus said. “It’s really important when the trees are young.”

White paint helps protect trees from the sun. Photo credit: Andrea Aker

Citrus trees have relatively thin bark. Left to their own, they grow more like a shrub than a tree, with shoots growing up at the base and covering the trunk.Without that shading, they need the protection of paint.

Once the canopy of the tree is thick and broad enough to shade the trunk the paint isn’t necessary, Backhaus said, but most people keep doing it anyway because they like the way it looks.

You can buy white trunk paint, but just plain old latex house paint will do. Don’t use oil-based paint — it will seep into the wood and poison the tree.

You can also, if you choose, buy trunk wraps—burlap or woven polyethylene that will protect a young tree.

Speaking of trees, you know what really frosts Backhaus’ keister? Well, he didn’t actually say this frosts his keister, but he did say it’s one of the biggest landscaping mistakes people make around here — big, cheap trees.

Bargain hunters often buy large trees that have been growing so long in containers that the roots are all jammed up and form “corkscrews,” Backhaus said. The reason they’re big and cheap is that nobody bought them when they were small and more expensive and they just kept growing in the container.

After they’re transplanted, the roots begin to grow, but because of their corkscrew shape they eventually just strangle themselves and the tree slowly dies. (This doesn’t apply to palm trees, which have different kinds of roots.)

“They’ll look good for two or three years and then they’ll just die,” Backhaus said. “People think it’s air pollution or some kind of mystery disease, but it isn’t, and these are the ones that tend to blow over in the storms.’