Nature Notes: Creosote

For many people in the Chihuahuan Desert the smell of rain comes from creosote bush resins. Did you know it’s been used as medicine for centuries, is the ultimate survivor in southwestern American deserts, and is a great ornamental shrub for home landscapes in the region? 

Travelers in the Trans-Pecos soon meet the Creosote Bush (Larrea tridentata) (LARE-ee-a tri-den-TAY- ta).  It’s found in great numbers, almost to the exclusion of other plants, on gravelly soil.  It gets its name from the strong-smelling resin on its leaves. Its bright yellow flowers bloom any time but mostly from February to May.  It’s often called “Greasewood.” 

Larrea didn’t evolve in the northern hemisphere.  It came here during the last ice age from the deserts of Chile and Peru.  Its fuzzy seeds probably became attached in the plumage of migrating birds.  Different populations of Larrea live in the Chihuahuan, Sonoran and Mojave Deserts. They’re distinguished by their number of chromosomes and all have more than the normal number of chromosomes.  This characteristic, called polyploidy, enhances drought resistance, and plants which are polyploidic are common in deserts.

Larrea is probably the most drought resistant plant known. The xylem cells in its roots have thick walls and contain resin, enabling these cells to hold four times as much water per cell as the xylem cells in the roots of a forest plant.  In addition, the fungi which live in symbiotic association with the Larrea are efficient water collectors.

Each Larrea leaf has stomates or pores where oxygen and some water escape after photosynthesis. When drought occurs, the stomates on the leaves of Larrea begin to close for longer and longer periods during the day, until they’re open only in the early morning.  So, as the drought lengthens, photosynthesis slows.  This causes Larrea’s lower leaves and interior leaves to wilt and turn yellow but hang on the plant for a week or two.  Decay of leaves produces nitrogen, and the plant transfers the nitrogen from the dead leaves to the leaves still on the bush.

If the drought worsens, the remaining leaves go dormant.  The resin in the leaves protects them from drying out. When moisture returns, the resin prevents a sudden rise in osmotic pressure that would rupture the cells.  Modern science has found that this resin contains anti-biotic properties.  Native Americans had used Larrea as a salve for wounds for hundreds of years.

In a gravelly soil, Larrea are evenly spread apart.  For years it was thought that Larrea had a chemical in its roots to prevent the germination of other plants – but in other soils other plants readily grow near Larrea.  A different theory postulates that Larrea produces a chemical that prevents its own seeds from germinating under the adult plants.

Some Larrea in the Mojave Desert grow in large rings.  These plants have reproduced by offsets (like bulbs).  One ring in the Mojave has been carbon dated as being 11,000 years old.

Larrea’s reactions to rain are also fascinating.  On as little as 2 10ths of an inch of rain Larrea can grow; and a half inch rain causes Larrea to grow 2 inches!  But the plant contains growth inhibitors which prevent it from growing so much that it can’t survive the next drought.  

Native plant growers once reported poor germination rates for Larrea- but these plant growers learned to germinate a Larrea at 80-90% humidity and 150 degrees F – the air temperature at ground level in the desert summer! 

Larrea is the smell of rain in the Chihuahuan desert, for the plant releases its oils after rain and become very sticky. You can see them growing in pastures along our highways or, when you’re in Midland, see the ones growing at the Sibley Nature Center’s gardens. Why not plant one in your garden?