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Growing Guayacan: Guaiacum coulteri

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Description
Form: A shrub, or with pruning, a multi-trunked, spreading to weeping tree.
Lifespan: Perennial.
Leaf retention: Evergreen, but drought and freeze deciduous.
Growth rate: Very slow, possibly only 2.5"/year in the wild.
Mature Size: Usually 6' high and wide, but may eventually reach 16-20' high and 20-50 wide under favorable conditions.
Flowers: Five vividly dark blue to purple curved petals surround prominent yellow anthers. The flowers may occur singly or in clusters.
Bloom: Primary bloom occurs mid to late spring. Sparse blooms appear sporadically throughout the rest of spring and summer.
Fruit: Small, greenish-reddish-brown, narrowly winged capsules with 2-4 lobes, containing bright red seeds.
Leaves: Small green, oval leaflets grouped in pairs of 3-8. With insufficient water the leaflets rotate their stems in high temperatures to so that they catch the sun on their edges to minimize light and heat exposure.
Stems: Grey, crooked, no thorns. The branches on top of the shrub can be sparsely leaved with insufficient water. The wood of Guaiacum genus plants is called Lignum vitae and is one of the hardest, toughest, densest woods known, being heavier than water.
Wildlife: The flowers attract bees. The red seeds attract birds. The leaves are browsed by deer and livestock. The plant is a host to the larvae of at least one species of sulphur butterfly.
Toxic / Danger: No.
Origin: Regions within Mexico, and possibly Guatemala, that have high summer rainfall and dry winters.

Cultivation and Uses
USDA hardiness zones: 9b-11. This plant is damaged below 25°F when mature. It is frost-sensitive when young and should be protected from freezing in its first three years.
Heat tolerant: Yes.
Drought tolerant: Yes, but this plant may lose its leaves in severe summer drought.
Sun: Full sun. Part shade causes the plant to grow leggy and sparsely leaved.
Planting: Locate this plant in all day full sun in well draining soil. The site should be large enough to accommodate an eventual 6x6' size, with small shrubs or perennials used to fill in the sides until this plant becomes large enough to spur their removal. It will grow in a container.
Soil: Well drained, gravelly, dry, low in organic content. Apply composted manure in winter to fertilize.
Water after becoming established: Deep water every 4-6 weeks in winter. Deep water every 1-2 weeks in spring and summer for improved growth.
Mulch: In its first three years, mulch the root zone in winter in regions with freezing temperatures.
First Year Care: Do not fertilize. Do not prune. Protect from freezes.
Prune: Trim lightly in winter to control shape. This plant grows very slowly. Remove weeds by hand under and within one foot of the canopy.
Litter: Fallen flowers.
Propagation: Seed, less than one month old and soaked in water; woody cuttings. Dried seed over one month old is non-viable.
Uses: Ornamental.

Comments
This plant is a member of the Caltrop family (Zygophyllaceae). Its slow growth does not compete well with faster growing species. Other related species are Guaiacum officinale and G. sanctum, both small, slow-growing trees valued for their wood, and like G. coulteri and all other species in this genus, harvested to endangered status in their native region.



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Latest update: June, 2021.

José Eugenio Gómez Rodríguez, CC BY-SA 3.0 httpscreativecommons.orglicensesby-sa3.0, via Wikimedia Commons

José Eugenio Gómez Rodríguez, CC BY-SA 4.0 httpscreativecommons.orglicensesby-sa4.0, via Wikimedia Commons

Guaiacum coulteri: Guayacan leaves folded
These leaflets are rotated to minimize light exposure
during the heat of the day when water is insufficient.

Guaiacum coulteri: Guayacan
The sparsely leaved branches on this plant
suggest insufficient water.