Excerpt from Valley 101: The Great Big Book of Life, a collection of Clay Thompson’s columns for The Arizona Republic. (Originally published December 14, 1999.)

Cups on cacti in the Town of Carefree. Photo Credit: Violet Killen

Q: My husband and I recently drove to Cave Creek and along the way we noticed many people had put plastic-foam cups on their cacti. I said this is for protection from the frost, but my husband said it was some sort of Christmas decoration. Who wins?

A: You know, your mother was right: You could have done better.

As entertaining as it would be to tell you that, yes, placing plastic-foam cups on cacti is a treasured holiday tradition dating back to the earliest Spanish settlers, we must tell the truth: Your husband is 100 percent wrong.

The cups are for frost protection.

We doubled-checked this with Patrick Quirk, a cactus horticulturist at the Desert Botanical Garden, which, by the way, is a swell place and a great spot to dump winter visitors for an afternoon.

Plastic-foam cups can trap just enough warmer air to protect the growing tip of a cactus. Plus, they won’t blow off, and they’re easier to use than the old sheet or whatever that you might use to cover your petunias on a chilly night.

And we have more chilly nights than you might expect. The dates of the first and last frosts vary depending on what part of the Valley you live in. Generally, the outlying parts of town are chillier than the closer-in neighborhoods. For instance, the average first and last frost dates for Laveen are Nov. 27 and Feb. 21, while the average season at Sky Harbor International Airport is Dec. 12 to Feb. 7.

Cactuses seem to have survived around here for ages without the benefit of foam cups. So why start now?

Because, Quirk said, the cupped cactuses you saw probably weren’t from around here.

Cactuses have adapted to cold weather as far north as Cape Cod, and our local varieties, such as the saguaro, aren’t especially at risk from frost.

But homeowners or landscapers who bring in cactus from farther south—Mexico or the Caribbean or South America—need to take steps to protect their imports.