Whitethorn Acacia

Vachellia constricta

• Content: Debbie Bird
• Photo: Pods: Ned Harris, www.flickr.com/photos/ned_harris, Thorns and flower: Debbie Bird

For this month’s plant, I thought I’d go with a plant that we see often and perhaps lend a new perspective or appreciation of it. Whitethorn Acacia (Vachellia constricta) is ubiquitous in Sabino Canyon and most of Tucson. The term ‘constricta’ refers to the reddish brown, curved pods, which are constricted between each seed. They’re quite narrow too, only 1/8 inch wide. Vachellia constricta is in the Fabaceae or Pea Family. This family contains many familiar plants such as Mesquite (Prosopis velutina), Fairy Duster (Calliandra eriophylla), and Desert Senna (Senna covesii).

Vachellia constricta can sometimes be confused with other similar looking plants such as Cat-claw (Senegalia greggii). To identify Vachellia constricta, look for ‘V’ shaped straight white spines. Flowers are fragrant, golden-yellow, round clusters in conspicuous ball shapes arising from the leaf axils. The small leaves are even-pinnate (having leaflets arranged on either side of the stem, typically in pairs opposite each other), with each of the 3-9 pairs of pinnae made of 4-16 pairs of leaflets. The leaves are also drought and cold deciduous. Look for reddish-brown bark. Keep in mind that the spines or thorns may be absent in older plants. If there are no flowers, leaves, or spines on the plant, the persistent narrow and constricted seed pods are a good clue to the identification of this plant. Sometimes the old pods may be lying on the ground underneath the plant.

Vachellia constricta has a lifespan of about 70 years. Although the bright yellow flowers are attractive to many pollinators, the flowers offer little reward.

There is no nectar and sparse pollen. The plants are still pollinated nonetheless by insects and wind. Even so, Vachellia constricta has the redeeming feature of providing fodder in the form of seeds and protein for birds and rodents. As with many other plants in the ‘Pea’ family, colonies of nitrogen-fixing bacteria in the roots help Vachellia constricta to absorb nutrients in the soil. The decomposing leaves also add nitrogen to the surrounding soil benefitting nearby plants as well. The Seri (an indigenous group of people in Sonora) use the leaves and seeds in a medicinal tea for stomach problems.

I find it interesting that this plant, which blooms in the late spring, is blooming now as I write this. It’s mid-August and we’ve had some nice monsoon season rains. One of the adaptations of many desert plants is that they are opportunistic and can bloom when conditions are to their liking and become dormant when conditions are not. You can find Whitethorn Acacia (Vachellia constricta) along any trail or roadside in Sabino Canyon. Be sure to take time to enjoy the lovely fragrance.