Arbol Santo Guaiacum coulteri – Native Plant of the Week

Introduction

Guaiacum coulteri is called Arbol Santo in Mexico which translates to Holy Tree. Sometimes it is also referred to as Palo Santo, but that common name is also used for a Bursera species. Arbol Santo is a gorgeous slow-growing tree native to the seasonally dry forests of Mexico. When you see it growing in the wild it is a real treat because of its rarity. If you are lucky enough to find it in flower, as I did, take lots of photos of its gorgeous purple or blue flowers. I was even lucky enough to find a large old-growth tree once, and will likely never see one of those again due to their extreme rarity. It has incredibly dense wood and combined with its slow growth this has resulted in population declines throughout its entire range. It is truly a magnificent tree!

Because it is a Vulnerable species whose population is in serious decline due to overexploitation and exceeding slow growth rates, please do not ever harvest or damage the trees or shrubs. Instead, take pictures to enjoy at a later date, or if you are lucky enough to live in a suitable climate try growing it in your yard. Or if you are into bonsai, grow it as a bonsai tree. Its a beautiful slow-growing tree that makes a gorgeous bonsai. Seeds for this species seem exceedingly difficult to find for purchase, but I did find one site mentioning that they have them, I will list them in the Resources section below.

Description of Guaiacum coulteri Arbol Santo

Stem & Leaves

Arbol Santo is a slow-growing evergreen shrub or tree of the Zygophyllaceae family. Sometimes it only grows as a densely branched and scandent shrub, especially in rocky habitats. Other times it grows as a tree 10 – 15 m tall and 30 – 40 cm diameter. Sometimes it can grow to 25 m tall and 100 cm diameter when conditions are ideal and it is left undisturbed to grow for long periods of time. However, over-exploitation and slow growth has made individuals of this size exceedingly rare.

Its trunk bark is whitish and somewhat vertically fissured while the branches are usually greyish.

The dark green leathery leaves are paripinnate (compound with an even number of leaflets) and oppositely placed on the branches. There are 2 – 6 pairs of leaflets that will rotate on their stems in hot weather to put their edges facing the sun instead of their entire leaf blade in order to minimize heat stress.

Flowers & Fruits

The purple or blue flowers appear in small fasciculate inflorescences on the tree branches. They are 1 – 2 cm across and have 5 spreading petals on twisted claws. I only found reference to it having 5 petals but I have found flowers with four petals on the same tree as those with five petals, it was accompanied by 4 sepals and 8 stamens (see photo above). Its sepals are purplish and obovate in shape with slightly inrolled margins. The tree blooms first in spring and then sporadically throughout the year until fall.

The reproductive organs are tightly grouped in the center of the flower. It has twice as many stamens as petals. Usually 10 (8) stamens with somewhat translucent looking filaments topped with yellow anthers are often in a somewhat spiraled arrangement around the pistil. This pistil is composed of a syncarpous ovary with a single filiform style.

Its fruit is a yellow-orange ovoid capsule with black seeds. The seeds are covered in a bright red aril. Due to its slow growth, it may not achieve significant seed production until it reaches 25 years of age.

Similar Species Guaiacum coulteri is Frequently Confused With

The unique flowers make it hard to confuse with other genera, other than occasionally Ironwood, but only in their non-flowering states. Otherwise they can be confused with the other four species of Guaiacum which can be differentiated as follows:

• Guaiacum unijugum – a rare microendemic species only found on a 70 km stretch in the Cape region of Baja California Sur. While it is closely related they can be differentiated based on their smaller pinnate leaves and their size as this species is a shrub, not reaching more than 2 m in height. Furthermore, while Guaiacum coulteri grows in Baja California Sur it seems to be absent from the Cape region where this rare species is only found.
• Guaiacum angustifolium – separated geographically as an eastern species endemic to south Texas, USA and the states of Coahuila, Chihuahua, Nuevo Leon, and Tamaulipas in eastern Mexico. It has 8-16 leaflets that are 1 – 3 cm long and its capsules are obcordate, flattened, 2-4 lobed, winged, and hairy. Its petals are long and sometimes twisted but appear less clawed than G. coulteri.
• Guaiacum officinale – also separated geographically as an eastern species native to the Caribbean islands and northern South America. Its pinnate leaves are 2.5 – 3 cm long and 2 cm wide, much more ovate in shape than any of the other Guaiacum species. The claws on its purple or blue flowers are not twisted.
• Guaiacum sanctum – also separated geographically as an eastern species native to the Caribbean islands, Yucatan Peninsula, Central and northern South America. It has 6-8 leaflets that are 6–10 cm long. Its capsules are obovoid, 4–5-lobed, 4–5–winged, and glabrous.
• Olneya tesota – Palo Fierro or Ironwood – as a tree it is sometimes mistaken for this tree that grows in a similar range to Arbol Santo. They have similar paripinnate leaves. However, they can easily be distinguished upon flowering as the Ironwood has pink pea-like flowers since it is of the legume family.

Distribution of Arbol Santo Guaiacum coulteri

Arbol Santo is endemic to western Mexico. It is found in the states of Oaxaca, Baja California Sur, Colima, Sonora, Sinaloa, Nayarit, Michoacán, Jalisco, Chiapas, and Guerrero.

Guaiacum coulteri is sometimes listed as being native to Guatemala, but I was not able to definitively confirm this. Reference was found to it possibly hybridizing in this location with G. sanctorum, a much more widespread species, so this could be adding to the confusion. Other sources say that it is not found outside of western Mexico. It is, however, occasionally cultivated in the Caribbean and the southern USA.

Habitat & Growing Conditions of Arbol Santo, Palo Santo

Guaiacum coulteri is endemic to western Mexico where it thrives in the dry subtropical forests along the Pacific coastal states of western Mexico. It is a late succession species rarely found in young forests, further adding to its rarity.

It can sometimes be found in poor rocky exposed soils where it exists more as a shrub. In these conditions it is still able to flower and set fruit. On the other hand in forests it is found in more moderate soils. In all cases it prefers areas with low disturbance.

Growing Arbol Santo in Your Garden

Growing species native to your area is a great addition to your garden. Once established they require little to no maintenance of any kind. They already grow in your area without water or fertilizer, so they will easily grow in your yard if you live in their range. They also provide important wildlife and biodiversity values as well.

Due to its rarity this tree may be difficult to find in nurseries. The more common Guaiacum officinale is much more common and easier to find in nurseries. It would make a lovely garden specimen as well. If you want Guaiacum coulteri I did find one seed source online listed in Resources below. Seeds should be started and allowed to grow for a few years at least before transplanting them into your garden.

Once ready to plant in your garden simply plant it in full sun to part shade in any well-drained soil. If your soil is dry and poor amend it with a little compost upon planting to provide it with food and extra moisture during the establishment phase. You can also top-dress with some compost for extra moisture retention. Once established however it will require no maintenance as it is a drought-tolerant species adapted to dry forests and poor soils.

If you live too far north and are into bonsai, try growing it as a bonsai tree. Its a beautiful slow-growing tree that makes a gorgeous bonsai.

Wildlife Values of Arbol Santo

Bees and butterflies are attracted to the flowers and birds assist in seed dispersal when they eat the fruits. Mammals browse the leaves and will also eat the fruits.

Status of Guaiacum coulteri

Arbol Santo is considered Globally Vulnerable. It is endemic to western Mexico and is found nowhere else in the world. Its status was last assessed in 2016 and reclassified as vulnerable with its population noted as decreasing in terms of the number of mature individuals seen in the wild in its native habitat. In 2010 only 1.3% of Guaiacum coulteri habitat was protected so this is one of the biggest problems with their current decline in population.

The populations of the entire genus are in decline and most are now protected. Population decline is believed to be a result of a combination of overexploitation for its wood, habitat loss, and its slow rate of regeneration.

Traditional or Other Uses of Arbol Santo

Arbol Santo as Wood

Arbol Santo has one of the hardest woods so it was used extensively for various tools and furniture. It is still used for its wood today, despite its declining populations. The entire genus is often referred to as lignum vitae for its wood which is one of the hardest and most dense woods there is. It is said to have a self-lubricating quality that makes it popular for mechanical uses such as wooden bearings.

Guaiacum coulteri Medicinal Uses

Guaiacum coulteri has a long history of medicinal use in Mexico. The Seri people who are native to Sonora, Mexico, used the berries as an unspecified medicine. The resin of the Caribbean species of Guaiacum was adopted for medicinal use by the Europeans by the late 15th century where it was used as a popular treatment for syphilis. Guaiacum coulteri was also later used for the same purpose.

Guaiacum resin was also used to stimulate menstruation and was sometimes used as an abortifacient in the 1700s. In the 1800s and 1900s herbalists and physicians alike used it as an expectorant, for sore throats, gout, and rheumatism.

It has been tested more recently against other popular Mexican herbal remedies and was found the most effective medicine for treating diabetes due to its ability to reduce hyperglycemia. It has been used to treat cancer and it has also been used as a diuretic, laxative, and as an anti-inflammatory for arthritis.

A phenolic compound derived from the entire genus has been commonly used to test for blood in the stool.

Arbol Santo as an Ornamental

Due to its lovely purple flowers, and its bright green evergreen leaves Arbol Santo is occasionally used as an ornamental. Sometimes it is used as a bonsai due to its slow growth and beautiful branching structure. With its declining populations, it would be nice to see this tree be propagated and more widely used as an ornamental, particularly in its native habitat. It is not going to become invasive outside its natural habitat due to its slow growth and length of time it takes to reach reproductive maturity. Ornamental plants are rarely seen exceeding 2 m in height but make beautiful shrubs for your yard if you live in a warm enough area. Other species of Guaiacum have become popular as ornamentals in Florida and California.

Ethical Wildcrafting of Guaiacum coulteri

Due to its vulnerable status, and lack of adequate protection of remaining wild stock, wildcrafting is not recommended. The wood should never be harvested anymore for this reason. Do not harvest the seeds as these need to remain in the wild for them to reproduce. See above section on Status. To learn about ethical wildcrafting principles check out my blog on Ethical Wildcrafting.

Alternatively, take lots of pictures if you find them in the wild. Also, you can grow it in your garden for its lovely growth form and beautiful purple flowers.

References and Resources

Arid Zone Trees – Possible Source for Guaiacum coulteri seeds http://www.aridzonetrees.com/guaiacum-coulteri.html

Dictionary of Botanical Terms – by Lyrae’s Nature Blog https://lyraenatureblog.com/blog/dictionary-of-botanical-terms/

Eflora Plants of North America http://www.efloras.org/browse.aspx?flora_id=1

Gordon, James E., Marco Antonio González, Jesús Vázquez Hernández, Roberto Ortega Lavariega and
Alberto Reyes-García (2005). Guaiacum coulteri: an over-logged dry forest tree of Oaxaca, Mexico. Oryx Vol 39 No 1 January 2005.

iNaturalist Plant Search https://www.inaturalist.org/home

IUCN Red List on Guaiacum coulteri https://www.iucnredlist.org/species/30847/68085761

McCauley, Ross A., Cortés-Palomec, Aurea C., & Oyama, Ken. (2010). Distribution, genetic structure, and conservation status of the rare microendemic species, Guaiacum unijugum (Zygophyllaceae) in the Cape Region of Baja California, Mexico. Revista mexicana de biodiversidad, 81(3), 745-758. Recuperado en 26 de diciembre de 2021, de http://www.scielo.org.mx/scielo.php?script=sci_arttext&pid=S1870-34532010000300016&lng=es&tlng=en.

Native American Ethnobotany http://naeb.brit.org/

University of Arizona Campus Arboretum on Guaiacum coulteri https://apps.cals.arizona.edu/arboretum/taxon.aspx?id=888

USDA Plants Database https://plants.sc.egov.usda.gov/home

Wikipedia on Guaiacum https://en.wikipedia.org/wiki/Guaiacum

Willis, Lyrae (2021).  Plant Families of North America.  Not yet published. 

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