Guaiacum coulteri

Accession Count: 4
Common Name: guayacán
Family Name: Zygophyllaceae
Botanical Name: Guaiacum coulteri
Botanical Synonyms: Guaiacum planchoni
Family Synonyms: Caltrop
Sub Species:
Guaiacum coulteri is a slow-growing, evergreen, crooked branching shrub. It is rarely seen taller than 5 feet in residential gardens, and it usually takes over 60 years to reach that height, although it is known to grow over 25' tall in its native region if left undisturbed. The wood of the Guaiacum species is one of the hardest, toughest, densest woods known, and is heavier than water. The leaves are dark green, small, oval, and pinnately compound, with 2 to 6 pairs of leaflets. With insufficient water, the leaflets rotate their stems in high temperatures so that they catch the sun on their edges to minimize heat stress (see picture). The flowers have five vivid blue to purple petals, with a primary bloom of clustered flowers in spring followed by sporadic single blooms until fall. The ripe fruit are yellow-orange to brown, winged capsules containing seeds coated with a bright red aril. 

Compound: Gua cou
Geographic Origin: Mexico, Guatemala
Ecozone Origin: Nearctic
Biome Origin: MX
Natural History:
This plant is found in dry, tropical forests of Mexico and Guatemala. It is a vulnerable species, easily out-competed by faster-growing plants and endangered by encroaching human land uses. Bees and butterflies are attracted to the flowers. Birds assist in seed dispersal. Mammals browse the leaves and consume the seed capsules. Because of its slow growth, the plant may not achieve significant seed production until 25 years old or more.

Cultivation Notes:
Propagation may be done in vitro by sowing seeds that have been soaked and swelled in water into an agar or gelatin medium. This plant thrives in full sun to part shade in gravelly, well drained soils, and suffers freeze damage below 25 degrees F. It should not be pruned because of its slow growth rate, typically less than one inch a year. Fertilizing with composted manure mid winter may speed growth. This plant forms a taproot, making it difficult to transplant. 

G. coulteri is occasionally used as an ornamental shrub. Its primary drawback is a tendency to look sparse in dry conditions because of leaflet rotation to edge-on view. It has been harvested locally for its highly valuable dense heartwood, lignum vitae, which has led to a scarcity of mature trees. It is very similar to, and can easily be mistaken for, G. sanctum, a plant widely distributed throughout Mexico, Central America, the Caribbean and Florida, and renowned for its medical properties.

Height: 6 - 10 feet
Width: 6 - 10 feet
Growth Rate: Slow Growing
Grow Season: Summer
Flower Season: Spring
Color: Purple
Function: Accent
Spread: Non-spreading
Allergen: Non-allergenic
Invasive: Benign
Toxicity: Benign
Hardy: Semi-hardy
Water Use: Low water Use

  1. Arizona State University Virtual Library of Phoenix Landscape Plants. Retrieved December 29, 2016.
  2. Conservation Assessment of Guaiacum sanctum and Guaiacum coulteri: Historic Distribution and Future Trends in Mexico, Leonel López-Toledo, Constantino Gonzalez-Salazar, David F.R.P. Burslem and Miguel Martinez-Ramos, Biotropica, Vol. 43, No. 2 (March 2011), pp. 246-255 (10 pages), Published By: Association for Tropical Biology and Conservation, Biotropica,
  4. Missouri Botanical Garden Guaiacum sanctum
  5. Perspectives on the systematics and phylogenetics of Guaiacum (Zygophyllaceae): complexities in conservation of endangered hardwoods due to fragmentation, introgression, and intermittent gene flow

Guaiacum coulteri