What’s That After-Rain Aroma in the Valley?
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 September 3, 2000.)
Q: What exactly causes that fresh/earthy scent when it rains in the Valley? It’s a real distinct scent, not flowery or sweet, but more like a fresh, clean smell.
A: Your question worked its way to the top of the pile at just the right time — Tuesday, when we had that delightful morning rain. As soon as the skies cleared we leapt into the Valley 101 mobile research lab, and set out to find the source of the scent.
You are right. There was a truly remarkable aroma in the air, which we soon traced to a half-eaten Big Mac under the floor mats in the backseat of the Valley 101 mobile research lab. While we would not describe this as “fresh/earthy” it was definitely distinctive.
After a stop at the car wash, we sought advice from the estimable Carolyn O’Malley, executive director of the Desert Botanical Gardens and an honorary member of the Valley 101 faculty.
Said she: “It’s the creosote bush. It’s a native and it’s all over the Valley. It is very pungent.” And it smells wonderful after a rain.
In fact, O’Malley knows of a couple who had a sprinkler system installed specifically to rain upon their creosote bushes on evenings when they entertain so their guests could enjoy the aroma.
Creosote bushes are members of the evergreen family and are named for the tar-like aroma of their resin. An interesting thing about creosote bushes: The crown of the plant splits into lobes, which bend over into the soil and send out their own roots and branches, making them clones of the original.
One group of creosote clones found in the Mojave Desert is believed to be about 11,700 years old, perhaps the oldest living plant life we know of.
That “fresh” smell you refer to is in actuality the Greasewood or creosote bush. The smell used to be much more distinct in decades past, so I supect it’s a little of wet asphalt thrown into the mix now. I used to go out and gather up handfuls of leaves that are easily stripped off of the branches and put them in baggies. When I would lament the smell of the desert, I would put them into sections of pantyhose that my wife wanted to throw out due to “runs” in them, and then smash the leaves between my fingers inside the hose and hang it up on the shower-head. Voila, the scent of a summer desert.
“Greasewood” (Sarcobataceae) and “Creosote” (Larrea tridentata) are 2 separate species and really don’t look alike. The leaves of creosote are waxy (a sort of resin protection from heat and water loss), its branches tough and unfriendly, and virtually nothing will eat it even in the worst drought or famine. Some animals will browse on greasewood but it can cause kidney failure, especially in sheep.
Greasewood is usually found east of Arizona, from N. Dakota down into Texas and Mexico. Creosote is native to Arizona and is, indeed, the source of that particular refreshing after-rain scent. When wet, tt actually does smell like the creosote we used to use to preserve fence posts, but its scent not nearly as strong.
A creosote bush with a plentiful water source can be a lovely thing. Its dark green leaves will be thick and shiny, its spring flowers vibrant yellow and plentiful, and I have seen them 12-15 feet tall.
Perhaps one of the nicest things about creosote is that, unlike so many other green things in the desert, it has no thorns, spikes, spines, or stickers.
Back when I lived surrounded by desert instead of city, I could tell when rain was coming because the scent of wet creosote traveled ahead of it. Now, I rarely smell creosote in the city. The scent is masked by other city smells such as wet lawns, asphalt, and concrete – more’s the pity.