St. Thomas Prickly-ash / Center For Plant Conservation
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Plant Profile

St. Thomas Prickly-ash (Zanthoxylum thomasianum)

This is a closeup of the spines along the stem. Photo Credit: Meghan Fellows
  • Global Rank: G1 - Critically Imperiled
  • Legal Status: Federally Endangered
  • Family: Rutaceae
  • State: PR, VI
  • Nature Serve ID: 144853
  • Lifeform: Tree, Shrub
  • Date Inducted in National Collection: 12/01/1991

Zanthoxylum thomasianum, an evergreen shrubby-tree, can grow up to 20 feet in height in the semi-evergreen forests of Puerto Rico and the U.S. Virgin Islands. Its trunk reaches up to 4 inches in diameter and is covered by thick spines. The bark, leaves, and fruit are all aromatic. Small male and female flowers grow separately on different plants and produce fruit capsules in September that each contain a shiny black seed about 3/16-inch long.

Where is St. Thomas Prickly-ash (Zanthoxylum thomasianum) located in the wild?


Zanthoxylum thomasianum grows in rugged hilly areas on soils of volcanic origin, as well as in limestone areas (Vivaldi and Woodbury 1981). Approximately fifty percent of species growing in these semi-evergreen forests are decidous (USFWS 1988). Two strata of trees are usually present. The uppermost continuous stratum is located about 15 to 30 feet high and is mostly composed of deciduous species (USFWS 1988). The lower stratum, at or below 15 feet, is composed of evergreen species (USFWS 1988).


Zanthoxylum thomasianum can be found in the semi-evergreen forests of Puerto Rico and the U.S. Virgin Islands (Vivaldi and Woodbury 1981).

States & Provinces:

St. Thomas Prickly-ash can be found in Puerto Rico, Virgin Islands

Which CPC Partners conserve St. Thomas Prickly-ash (Zanthoxylum thomasianum)?

CPC's Plant Sponsorship Program provides long term stewardship of rare plants in our National Collection. We are so grateful for all our donors who have made the Plant Sponsorship Program so successful. We are in the process of acknowledging all our wonderful plant sponsorship donors on our website. This is a work in progress and will be updated regularly.

Conservation Actions

Nature Serve Biotics
  • 05/02/2017

There are only 4 populations, all in areas under development.

  • 01/01/2010

Modification and destruction of habitat appears to be the most serious threat to the species (Vivaldi and Woodbury 1981). The St. Thomas population occurs on land which has been subdivided into lots and sold for future residential development (USFWS 1988)

  • 01/01/2010

There are six remaining sites that support less than five hundred individuals; three sites in Puerto Rico and three in the U.S. Virgin Islands (USFWS 1988). The most significant population is in St. Thomas, where the plants cover an area of about 50 acres, and may number close to 250 (USFWS 1988).

  • 01/01/2010

None known.

  • 01/01/2010

A formalized management plan has not been implemented.

  • 01/01/2010

Primary management needs include the protection of remaining individuals and habitats. Understanding reproductive biology and ecology will aid in establishing new and maintaining existing populations.


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Taxon Zanthoxylum thomasianum
Authority (Krug & Urban) Krug & Urban ex P. Wilson
Family Rutaceae
CPC Number 4452
ITIS 28872
Duration Perennial
Common Names St. Thomas prickly-ash | St. Thomas pricklyash | prickly-ash
Associated Scientific Names Zanthoxylum thomasianum
Distribution Zanthoxylum thomasianum can be found in the semi-evergreen forests of Puerto Rico and the U.S. Virgin Islands (Vivaldi and Woodbury 1981).
State Rank
State State Rank
Puerto Rico S1
Virgin Islands *FR83
Ecological Relationships


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