Thorns:Whats the Point?
Thorns and spines are Nature’s way of saying back off or “no you don’t.” They are found everywhere in the biosphere, fish, lizards, insects, mammals, trees, and shrubs. These adaptations are deeply rooted in the genetics of these organism and reflect the environments in which they evolved.
In trees and shrubs, thorns start out as the same meristematic (growth generating) tissues that give rise to leaves and stems. Early in their development these tissues become shrunken and harden (sclerified) into thorns. In cacti they serve a dual purpose. Thorns make cacti less attractive to desert animals searching for moisture (thirsty Coyote’s and Javelin’s are still often undeterred). They also create a partial boundary layer of still air, immediately above the surface of the cacti pads, that reduces evapotranspiration like leaf hairs on more mesic plants.
With popular landscape tree species, like Mesquites, Palo Verdes, Acacias, we create distinctions between thorned and thornless selections. Thornless hybrid Palo Verdes, (‘Variety AZT,’ Desert Museum) offer designers and contractors’ trees with desert texture, stature, and flowers, along with an absence of thorns. Similarly, Thornless Mesquites can be used in virtually any landscape application (turf/non-turf, pedestrian, streetscape, desert or traditionally themed landscapes).
Thorns present the greatest concerns with those desert species that do not prominently display them. These would include Ironwood, Texas Ebony, Acacia schaffneri (Twisted or Wheat’s Acacia) or any other desert adapted landscape tree species where thorns are obscured by the leaf canopy or configuration of the stems and branches.
When is a thorn, not a thorn? Many trees, desert and mesic, can retain non-dehiscent petioles, flowers and fruit stems and branch terminals that may appear pointed but are not classically considered thorns. ‘Variety AZT’ Seedless Mesquite and Acacia berlandieri (Gaujillo) both have a small capsule at the base of most, but not all, petioles that do not extend beyond the axis of the branch and are not a hazard to pedestrians or landscape maintenance workers.
Desert tree species are fundamental elements of landscape designs that aspire to reflect and embrace the unique beauty of the American southwest. Given the challenging environment that many of these species evolved in, it should come as no surprise that many come armed with thorns. Thorns have never been a deterrent to planting roses, pyracantha, bougainvillea, and Pomegranate in the landscape. To thoroughly utilize the distinctive qualities of these desert-adapted and mesic trees and shrubs we must be judicious and mindful of their placement within the landscape.