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  Noteworthy Plants For September 1999  

See Home Page For Wood Carver Alex Chitura

Trees & Shrubs With
Dense, Hard Wood

© W.P. Armstrong 28 April 2010

The term hardwood includes numerous species of trees and shrubs with dense, heavy wood. Depending on their density, hardwoods can be subdivided into "medium heavy," "heavy," "very heavy," and woods that actually sink in water called "ironwoods." Hardwoods are usually limited to flowering trees and shrubs because their wood contains water-conducting cells (tracheids and vessel elements) plus tightly-packed, thick-walled fiber cells which are lacking in the wood of conifers. Generally the cone-bearing trees, such as pines (Pinus), spruce (Picea), fir (Abies), and redwood (Sequoia), are considered to be softwoods because their wood is composed essentially of water-conducting cells (tracheids) without wood fiber cells. The weight and hardness of wood is attributed to the density of the cells, the amount of lignin in the cell walls, and the percentage of tiny air spaces or pores within the cell walls. In fact, some slow-growing conifers, such as pinyon pines and desert junipers (Juniperus), actually have wood with small, densely-packed tracheids that is actually harder and heavier than some of the so-called "medium heavy hardwoods." The classification and separation of these categories is based on specific gravity, and is discussed below and in Table 1.

Wood is composed of the dead cells of a tree trunk, particularly the inner xylem tissue when the bark has been removed. The weight of wood is essentially due to the cellulose and lignin in the cell walls around the billions of individual cells. Lignin, a brown phenolic polymer composed of benzene rings, imparts great strength and hardness to the wood. Since the cell water material has a specific gravity of about 1.5 and is heavier than water, the relative buoyancy of different woods is due to air cavities (lumens) within the cell walls, and the thickness of the walls and the amount of lignin they contain. This is why water-logged soft, porous woods will sink in water when all their air spaces become filled with water. Ironwoods are so hard and heavy because they contain numerous long, tightly-packed wood fiber cells with very thick, heavily lignified cell walls and little or no air spaces.

There are more than one hundred species of trees and shrubs in the world with the common name of "ironwood." As their common name suggests, the wood of these species is very hard and heavy. Depending on their native country, many of these species have dozens of other descriptive common names. For example, South American ironwoods are often referred to as "quebracho," which translated means "axe-breaker." Most of the hardest and heaviest ironwood trees grow in tropical regions, but there are also many temperate ironwood species, some of which are planted in southern California and Florida. Some of the showy ornamentals, such as yellow and pink trumpet trees (Tabebuia serratifolia and T. ipe), are known as ironwoods in other parts of the world. In South America, trumpet trees drop their leaves during the dry season and produce a profusion of blossoms. The crowns of these huge timber trees resemble gigantic bouquets in the midst of the forest. As a group, ironwoods represent many fascinating flowering trees in a variety of different and unrelated plant families. What they share in common is their dense, hard wood with some of the most remarkable properties of any trees on earth. Some of their amazing uses range from hardwood carvings and Venetian blind rollers to the propeller shaft bearings of submarines.

The pink-flowered South American trumpet tree or pau d'arco (Tabebuia avellanedae), a member of the Bignonia Family (Bignoniaceae). This species is often listed as T. ipe and T. impetiginosa. It is commonly grown in southern California for its showy clusters of pink blossoms. Each flower produces a long, slender, cigar-shaped seed capsule (bottom) containing numerous winged seeds. Like many other species of Tabebuia, the palmately compound leaves are divided into five leaflets. The powdered inner bark of some pink-flowered species of pau d'arco is sold as a popular immunostimulant herbal remedy.

Some South American species of Tabebuia are referred to as pau d'arco, including the pink-flowered T. impetiginosa and T. avellanedae. According to The New York Botanical Garden Encyclopedia of Horticulture Volume 10, 1982, T. avellanedae is a synonym for T. impetiginosa, and T. ipe "is so closely similar to T. impetiginosa that it can scarcely be more than a variety of that species." These attractive pink-flowered species are commonly used as landscape trees in temperate regions.

The powdered inner bark of pink-flowered species of pau d'arco is sold as a popular herbal remedy that reportedly stimulates the immune system. According to a book by Kenneth Jones (Pau d'Arco: Immune Power From the Rain Forest, Healing Arts Press, 1995), this valuable herb has been proven successful in the treatment of certain cancers, allergies associated with the Candida yeast syndrome, and in disorders involving a weakened immune system.

Specific Gravity

The relative density and weight (mass) of different woods
(or other substances) can be compared mathematically:

Probably the best way to appreciate the relative hardness of different woods is the concept of "specific gravity," a numerical scale based on 1.0 for pure water. Without getting too mathematical, the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). The brilliant Greek mathematician and inventor Archimedes discovered over 2,100 years ago that a body in water is buoyed up by a force equal to weight of the water displaced. Archimedes reportedly came upon this discovery in his bathtub, and ran out into the street without his clothing shouting "Eureka, I have found it." Since one gram of pure water occupies a volume of one cubic centimeter, anything having a specific gravity greater than 1.0 will sink in pure water. The principles of buoyancy and specific gravity are utilized in many ways, from scuba diving and chemistry to the hardness of dry, seasoned wood. Some of the heaviest hardwood trees and shrubs of the United States have specific gravities between 0.80 and 0.95; including shagbark hickory (Carya ovata), persimmon (Diospyros virginiana) and ironwood (Ostrya virginiana) of the eastern states, and canyon live oak (Quercus chrysolepis), Engelmann oak (Q. engelmannii), hollyleaf cherry (Prunus ilicifolia) and Santa Cruz Island ironwood (Lyonothamnus floribundus ssp. asplenifolius) of southern California. Although some of these trees are called ironwoods, their dense, dry wood will still float in water. Since the pure cell wall material (lignin and cellulose) of wood has a density of about 1.5 grams per cubic centimeter, even the world's heaviest hardwoods generally have specific gravities less than 1.5 due to tiny pores (lumens) within the cell walls. True ironwoods include trees and shrubs with dry, seasoned woods that actually sink in water, with specific gravities greater than 1.0. They include lignum vitae (Guaicum officinale, 1.37); quebracho (Schinopsis balansae, 1.28); pau d'arco (Tabebuia serratifolia, 1.20); knob-thorn (Acacia pallens, 1.19); desert ironwood (Olneya tesota, 1.15); and ebony (Diospyros ebenum, 1.12). To appreciate the weight of these hardwoods, compare them with tropical American balsa (Ochroma pyramidale), one of the softest and lightest woods with a specific gravity of only 0.17.

Wood That Is Very Soft & Light In Weight


Left: Tropical American balsa (Ochroma pyramidale).

Above: Hawaiian coral tree or wiliwili (Erythrina sandwicensis).

The limb on scale weighs only 10 ounces (284 g). Because of its light weight and durability, native wiliwili wood was used by Hawaiian sailors for outriggers on their canoes.

Tropical American balsa (Ochroma pyramidale) is one of the softest and lightest woods with a specific gravity of only 0.17. Although harder than balsa, the native Hawaiian coral tree called wiliwili (Erythrina sandwicensis) also has a very soft, light wood. In fact, it was highly prized by Hawaiians for the outriggers of their traditional canoes. Because of its buoyancy, it was also used for surfboards and fishnet floats.

Painted carvings from South America made from balsa (Ochroma pyramidale).

Tropical fish made of balsa (Ochroma pyramidale).

  See Flowers, Pollination & Dispersal Of Coral Trees  

The structure of wood is a fascinating subject with some amazing practical applications. By analyzing the cellular structure and annual growth rings of wood it is possible to determine the ages of trees and shrubs and even gather data concerning past climates. Ancient creosote bushes (Larrea tridentata) in the Mojave Desert have been found to be well over 10,000 years old. In fact, the cell structure of these slow-growing shrubs is so dense that the dry wood actually sinks in water. Comparative studies of the wood of living and recently deceased trees with logs found in abandoned Indian villages have been used to date extinct cultures. One of the most damaging pieces of evidence that led to the conviction of Bruno Hauptmann, the man accused of kidnapping Charles A. Lindbergh's son in 1932, was the ladder used in the crime. Microscopic examinations of the ladder revealed that it was homemade and from precisely which kinds of wood. One rail on the ladder was found to match a floorboard from Hauptmann's attic, and it was proven that the rail and board were originally from the same piece of wood.

  Creosote Bush: Oldest Living Thing?  

Lack Of Visible Annual Rings In Tropical Trees?

In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree (Miliusa velutina) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.

Poussart, P.M., Myneni, S.C.B., Lanzirotti, A., et al. 2006. Geophysical Research Letters 3: L17711.

  Anatomy & Grain Structure Of Wood  
Cell Structure Of Stems & Roots

There are many native hardwood trees and shrubs in San Diego County, California with wood that is very dense but still floats in water because its specific gravity is less than 1.0. At least two species, hollyleaf cherry (Prunus ilicifolia) and Engelmann oak (Quercus engelmannii), have wood that will barely float. The following table compares some of the native woods in San Diego County, California with woods from other areas, including insular and northern California, the eastern United States and tropical regions:

   Native To San Diego County   
   E. U.S., California & Tropical   
Trees With Dry (Seasoned) Wood That Sinks In Water
Olneya tesota
(Desert Ironwood): 1.15
Guaiacum officinale)
(Lignum Vitae): 1.37
Cercocarpus betuloides
Mountain Mahogany: 1.10
Diospyros ebenum)
Ebony: 1.12
Very Heavy Wood
Prunus ilicifolia
Hollyleaf Cherry: 0.98
Diospyros virginiana
Persimmon: 0.83
Quercus engelmannii
Engelmann Oak: 0.94
Carya ovata
Shagbark Hickory: 0.83
Acacia greggii
Cat's Claw Acacia: 0.85
Ostrya virginiana
Eastern Ironwood: 0.80
Quercus chrysolepis
Canyon Live Oak: 0.85
Lyonothamnus floribundus
Catalina Ironwood: 0.80
Quercus agrifolia
Coast Live Oak: 0.83
Robinia pseudoacacia
Black Locust: 0.79
Prosopis glandulosa
Mesquite: 0.77
Maclura pomifera
Osage Orange: 0.77
Heavy Wood
Cornus nuttallii
Pacific Dogwood: 0.75
Carya illinoensis
Pecan: 0.72
Arbutus menziesii
Madrone: 0.71
Betula alleghaniensis
Yellow Birch: 0.69
Fraxinus velutina
Arizona Ash: 0.68
Quercus coccinea
Scarlet Oak: 0.67
Umbellularia californica
California Bay Tree: 0.65
Cercis canadensis
Redbud: 0.63
Quercus kelloggii
California Black Oak: 0.64
Tectona grandis
Teak: 0.63
Juglans californica
California Black Walnut: 0.63
Acer saccharum
Sugar Maple: 0.63
Medium Heavy Wood
Chilopsis linearis
Desert Willow: 0.59
Liquidambar styraciflua
Sweet Gum: 0.59
Cercidium floridum
Palo Verde: 0.55
Prunus serotina
Black Cherry: 0.56
Psorothamnus spinosus
Smoke Tree: 0.55
Acer saccharinum
Silver Maple: 0.53
Celtis reticulata
Western Hackberry: 0.53
Swietenia macrophylla
Honduras Mahogany: 0.51
Acer macrophyllum
Big-Leaf Maple: 0.50
Magnolia grandiflora
Southern Magnolia: 0.50
Soft Wood
Pinus ponderosa
Ponderosa Pine: 0.46
Sequoia sempervirens
Coast Redwood: 0.40
Calocedrus decurrens
Incense Cedar: 0.40
Picea engelmannii
Engelmann Spruce: 0.35
Pinus lambertiana
Sugar Pine: 0.36
Quercus suber
Cork Oak Bark: 0.24
Abies concolor
White Fir: 0.36
Ochroma pyramidale
Balsa: 0.17

Table 1. Comparison of some of the native woods in San Diego County, California with woods from other areas, including California, the eastern United States and tropical regions. Note: Angiosperm woods are often classified botanically as "hardwoods," while gymnosperms are called "softwoods." Although balsa is very soft and light, it is placed in the hardwood category because it is an angiosperm.

The very distinctive, shredded bark of the Santa Cruz Island ironwood (Lyonothamnus floribundus ssp. asplenifolius), a beautiful endemic tree of the California Channel Islands. In Table 1 this tree is placed in the category of "very heavy wood."

Hollyleaf cherry (Prunus ilicifolia), a chaparral shrub in the mountains of southern California with holly-like leaves, bright red fruits and very dense wood. The dry, seasoned wood barely floats in water with a specific gravity of 0.98. This beautiful shrub belongs to the Rose Family (Rosaceae), like the Santa Cruz Island ironwood.

Left: Pacific madrone (Arbutus menziesii), an evergreen tree with smooth red bark that is native to forests of the Pacific coast of North America. It occurs in isolated (disjunct) populations in the mountains of southern California and northern Baja California. Arbutus peninsularis is endemic to mountains of the Cape region in Baja California. Another Mexican madroño (A. xalapensis) grows in mountainous regions of Sinaloa and Chihuahua south through Veracruz and Oaxaca to Guatemala. Right: The urn-shaped (urceolate) flowers of the European strawberry tree (A. unedo) are typical of madrones, manzanita (Arctostaphylos) and other members of the heath family (Ericaceae).

  See Edible Berries of Strawberry Tree  

Many of the native hardwood trees and shrubs in the southwestern United States were used by Indians and early settlers in a variety of ways. Logs of the hardest woods, including desert ironwood (Olneya tesota) and mountain mahogany (Cercocarpus betuloides), were used as fuel, burning for hours and giving off intense heat before they turned into fine white ashes. The wood of native oaks, particularly the canyon live oak (Quercus chrysolepis), were used by early pioneers for wedges (with iron-rimmed heads) for splitting redwood ties, and for the heads of mauls (heavy wood mallets). During the days of wooden carriages and wagon building, canyon live oak was a prized wood for singletrees, axles and wheels. Bigleaf maple (Acer macrophyllum) was also popular for singletrees, a wooden bar hooked at either end to traces of the harness. Desert willow (Chilopsis linearis), a member of the Bignonia Family (Bignoniaceae) was highly prized by local Indians for making bows. Unlike the weak-limbed true willows (Salix), the wood of desert willow is very stiff and strong. Of course, one of the finest bow woods in North America was the osage orange (Maclura pomifera) of the eastern United States, a member of the Mulberry Family (Moraceae).

Ziricote (Cordia sebestena) is a native hardwood of the Florida Keys and Caribbean region. It belongs to the large Borage Family (Boraginaceae), a family that includes mostly herbaceous wildflowers like the forget-me-nots (Myosotis and Cryptantha). The beautiful, dark-grained hardwood is commonly used for wood carvings. The tree is also known as cericote and geiger tree. A similar Polynesian species is called "kou" (Cordia subcordata).

Ziricote (Cordia sebestena), an evergreen, hardwood tree native to the Florida Keys and Caribbean region. It belongs to the large Borage Family (Boraginaceae), a family that includes mostly herbaceous wildflowers like the forget-me-nots (Myosotis and Cryptantha). The orange-red, crinkled petals are fused into a funnel-shaped corolla that tapers into a slender tube (see detached corolla on leaf). Another species called "kou" (C. subcordata) is native the Polynesian region. The beautiful wood and orange blossoms are similar to the Caribbean species; however, the leaves are not as stiff and rough as in C. sebestena.

Milo or beach hibiscus (Thespesia populnea), a member of the Hibiscus Family (Malvaceae) that is widely distributed throughout the Indo-Pacific and Caribbean region. Like another sea hibiscus called "hau" (Hibiscus tiliaceus), the buoyant seeds are readily dispersed by seawater and may have colonized the Hawaiian islands prior to the remarkable voyages of ancient Polynesians. The hard, dense wood has a beautiful dark grain and is made in bowls, dishes and platters.

  See Beach Hibiscus: A Textile Plant  
See Flower Structure of Hibiscus

The Genus Eucalyptus

The genus Eucalyptus comprises about 95 percent of the forest trees of Australia, with over 500 described species--including some of the tallest trees on earth. Some of the species are called "ironbarks" because their bark superficially resembles the color and texture of black or reddish (rusted) iron. Although the dry, seasoned wood of most eucalypts will float in water, there is at least one species called "wandoo" (E. redunca) that is definitely an ironwood. California has literally hundreds of species of trees, but there were few forested regions in the vicinity of the booming coastal cities where railroads were being constructed. During the late 1800s and early 1900s several species of Australian Eucalyptus gums (including E. camaldulensis and E. globulus) were extensively planted in California for lumber, firewood, windbreaks and railroad ties. Although the species selected for extensive plantings grew into forests very rapidly, the wood proved very undesirable for lumber and railroad ties because of extensive splitting during the drying process. Today, these forests have forever changed the character of coastal southern and central California.

Severely cracked, sun-dried log of red gum (Eucalyptus camaldulensis), one of the most commonly planted eucalyptus in California. Although the wood is very hard, it proved to be undesirable for lumber and railroad ties because of extensive splitting.

The name "gum" can be traced back to the voyage of Captain James Cook to the South Pacific in 1770. Captain Cook discovered the east coast of Australia, called New Holland at that time. In one harbor, the ship's naturalists found so many unusual and beautiful plants that they named it Botany Bay. Eight years later, a fleet of eleven English ships reached Botany Bay with 1,530 people, 736 of them convicts. This marked the establishment of England's most important prison camp of the nineteenth century, and the European settlement of a vast land called Australia. The actual discovery of the genus Eucalyptus is credited to the ship's botanist, Joseph Banks (later Sir Joseph Banks). One of the newly discovered species "red bloodwood" (E. gummifera) had a reddish gum exuding from its trunk, and the naturalists called it a "gum tree."

Other species of eucalyptus with persistent bark fall into five additional groups, called ironbarks (bark hard and deeply fissured), peppermint barks (bark finely fibrous), stringy barks (bark long and fibrous), boxes (bark rough and fibrous), and bloodwoods (bark rough, cracked and scaly on trunk and large limbs). Another group of large trees, called ashes, have rough bark on the trunk but smoother bark on the branches. In fact, the mountain ash (Eucalyptus regnans) rivals the California redwoods as the world's tallest trees. With about 500 described species dominating more than 80 percent of Australia's forests, it is convenient to categorize them within different groups based upon their bark type. In fact, one of the most striking species with thick, deeply furrowed, persistent black bark is the red ironbark (E. sideroxylon), commonly planted at Palomar College. In addition to tree forms, there are numerous drought resistant, shrubby eucalyptus called mallees. Some of these resprout from subterranean lignotubers like many of our chaparral shrubs. One of these (Eucalyptus macrocarpa) produces spectacular red blossoms and the largest seed capsules of any eucalyptus. Some mallees of parched desert regions store water in their roots, a fact well-known to Australian aborigines.

The dark, deeply fissured bark of red ironbark (Eucalyptus sideroxylon), one of the most commonly planted trees in San Diego County. Unlike the true "gums," the bark does not exfoliate annually.

A polished bowl made from the beautiful, hard wood of red ironbark (Eucalyptus sideroxylon). Made by Kenneth R. Roth, M.D. of the San Diego County Wood Carvers Guild.

Go To San Diego County Carvers Guild
See Eucalyptus Macrocarpa in Full Bloom
  Fifteen Species Of Eucalyptus In California  

One of the most interesting uses for Eucalyptus wood is the sacred Aboriginal wind instrument called a "didgeridoo." Developed by indigenous people of northern Australia, this is one of the oldest known wind instruments, dating back at least 1,500 years. It is traditionally made from Eucalyptus trees which have their interiors hollowed out by termites. Finding the right tree requires diligent searching. If the hollow is too big or to small, it will make a poor quality instrument. Generally, the longer the instrument, the lower the pitch or key.

A didgeridoo photographed at Seaport Village in San Diego.

  See San Diego's Didgeridoo Guy  

Chemically the eucalyptus "gums" are rich in tannins (kinotannic acid) and are similar to another phenolic compound called catechu. They are known in the trade as kinos or gum kinos and are used as tannins to convert animal hide into leather. One of the main Australian sources of kino is the common red gum (Eucalyptus camaldulensis), naturalized throughout San Diego County. Kino gums are also used medicinally as astringents to relieve throat irritation, dysentery and diarrhea. True polysaccharide gums, such as locust bean gum from the carob tree (Ceratonia siliqua), and chicle, a terpene gum from the latex sap of the sapodilla tree (Achras zapota), are chemically quite different. They all probably serve to seal off wounds and prevent bacterial and fungal infections.

Gum kino oozing from a fissure in the trunk of a sugar gum (Eucalyptus cladocalyx), one of the most common large gums planted throughout San Diego County, California.

Oil of eucalyptus (eucalyptol) is a volatile terpene compound (called an essential oil) which is distilled from the leaves of several species. It is used for flavorings, dentifrices, cough drops, and for the synthesis of menthol. The lemony fragrance from the leaves of lemon-scented gum (E. citriodora) is due to another volatile terpene called citronellal. One of the reasons that few plants will grow well beneath naturalized gum forests in southern California is that volatile terpenes from fallen leaves are leached into the soil, thereby inhibiting seed germination and growth of competing species.

The hills overlooking downtown San Diego are dominated by tall eucalyptus, especially the sugar gums (E. cladocalyx).

The wood of different species of eucalyptus varies considerably, from wood as soft as pines to very hard, close-grained wood as dense as oak and hickory. Eucalypts constitute most of the forest vegetation of Australia and are one of the most important hardwood timber resources in the world. There are a number of species that provide excellent lumber for furniture, wood-carving and construction, including karri (E. diversicolor), spotted gum (E. maculata), blackbutt (E. pilularis), and jarrah (E. marginata). In fact, jarrah is stronger and more durable than oak and resistant to termites and marine borers.

One of the most common naturalized species of eucalyptus throughout central and southern California is the red gum (Eucalyptus camaldulensis), also known as E. rostrata. This species is a prolific seeder and has literally taken over canyons and river valleys where sufficient moisture can support groves of these large forest trees. Red gums seemed immune to any insect predator--that is until 1998 when the leaves of a tree in Los Angeles County were found to be infested with the red gum lerp psyllid (Glycaspis brimblecombei), an Australian insect that severely defoliates this species of eucalyptus. Psyllids are also called jumping plant lice and belong to the Order Homoptera, along with scale insects, aphids and cicadas. In fact, they superficially resemble a miniature cicada except they are the size of aphids (less than 4 mm long). Both immature and adult psyllids feed by sucking plant juices out of the leaves, causing serious leaf drop which weakens the trees and makes them more susceptible to other pests, such as wood-boring beetles. The wingless, yellowish immature form (nymph) secretes a waxy protective cover called a lerp. "Lerp" is a term derived from an aboriginal Australian language describing this cover. The nymphs also secrete large amounts of sticky honeydew on the leaves which results in blackened foliage due to the growth of sooty mold. This insect infestation has rapidly spread throughout Los Angeles, Orange and San Diego Counties, and at the time of this writing (March, 2000) is causing major defoliation of red gums. A tree that once thrived in this region without any major insect predators is now facing a serious threat to its survival.

See Red Gum Lerp In Southern California

The Rain Tree Of Tropical America

The tropical American rain tree or monkey pod (Samanea saman) has been introduced throughout tropical regions of the world. A member of the legume family Fabaceae, it is also listed in some references as Pithecellobium saman. Introduced to the Hawaiian Islands in the 1800s, this rain forest tree reaches heights of 80 feet (24 m) with a symmetrical, spreading crown. The beautiful, porous, dark-grained wood has a specific gravity of 0.5 to 0.6, similar to Honduran mahogany.

A monkey pod tree (Samanea saman) on the island of Hawaii.

A bowl made from the monkey pod tree (Samanea saman).

The Muhuhu and Mukwa Trees Of Africa

Athough the sunflower family (Asteraceae) is the largest plant family, with approximately 24,000 species, very few of these are classified as trees. But there is one African species with a dense, brownish-yellow wood and a specific gravity of nearly 1.0. Some references give a value of 1.1 which would place it in the category of an ironwood in this report; however, my samples appear to be less than 1.0 because they float in water. The tree is called muhuhu (Brachylaena huillensis) and it is indigenous to Angola, Kenya, Mozambique, Tanzania and Uganda. It occurs in dry lowland forests and semi-deciduous dry upland forests. In Kenya 50% of the trees harvested for wood carving are muhuhu. In addition to carving, this beautiful wood is used for flooring, furniture and bowls. A fragrant essential oil is also extracted (distilled) from muhuhu wood. Because muhuhu trees have been overexploited throughout their range, carvers have been encouraged to use other imported trees, such as neem (Azadirachta indica), jacaranda (Jacaranda mimosifolia) and silk oak (Grevillea robusta).

Muhuhu wood is commonly used for animal carvings in Kenya.

Another heavily expoited and endangered African hardwood is known in Rhodesia and Zimbabwe as muninga or mukwa wood (Pterocarpus angolensis). This fine-grained, beautiful hardwood is commonly used for furniture and wood carvings. It is comparable in hardness to walnut, ash and pecan in this article, with a specific gravity of 0.6 to 0.7. Like other bloodwoods of the genus Pterocarpus, the sap is deep red. In fact, native Rhodesians use the sap as a permanent red dye for their clothing.

A hippo carved from South African mukwa wood (Pterocarpus angolensis).

This beautiful giraffe stands eight feet tall and is carved from
a single piece of muninga wood (Pterocarpus angolensis).

  See Another Species Of Pterocarpus In Dominica  
Pterocarpus With A Strangler Fig In Dominica

But of all the so-called "hardwoods," the most spectacular species are the "ironwoods" that actually sinks in water. Fifteen of the world's heaviest ironwoods with specific gravities between 1.11 and 1.37 are listed in the following Table 2. All of these seasoned, dry hardwoods will sink in water, with a specific gravity greater than one:

Common Name
(Scientific Name)
Plant Family
(Scientific Name)
Lignum Vitae
(Guaiacum officinale)
S.E. U.S. &
(Piratinera guianensis)
(Krugiodendron ferreum)
S. Florida
and Keys
Burma Ironwood
(Xylia xylocarpa)
(Schinopsis balansae)
S. America
(Swartzia leiocalycina)
(Combretum imberbe)
South Africa
(Eusideroxylon zwageri)
Pau d'Arco
(Tabebuia serratifolia)
S. America
(Acacia nigrescens)
Brazil Ironwood
(Caesalpinia ferrea)
S. America
Desert Ironwood
(Olneya tesota)
S.W. U.S.
Ceylon Ironwood
(Mesua ferrea)
(Diospyros ebenum)
Mountain Mahogany
(Cercocarpus ledifolius)
S.W. U.S.
Black Ironwood
(Olea laurifolia)
Olive (Oleaceae)

Table 2. Fifteen of the world's heaviest ironwoods.

Some readers might wonder why I placed the South African black ironwood (Olea laurifolia) at the bottom of my list, while each year the Guinness Book of World Records lists this tree as the world's heaviest wood with a specific gravity of 1.49. In addition to the fact that most wood references do not concur with the Guinness Book, I studied a sample of this hardwood and the highest specific gravity I could obtain was 1.11, far below many other ironwoods. In fact, Acacia nigrescens (Acacia pallens), a common tree of the Kruger National Park region of South Africa, has a much heavier wood with a specific gravity of 1.19. Called knob-thorn acacia, the trunk is covered with woody, conical knobs each tipped with a thorn. Like other African acacias, the knob-thorn acacia is a painful tree to climb with bare hands. Some remarkable swollen-thorn acacias of Africa and Central America have hollowed-out thorns inhabited by symbiotic stinging ants that protect the trees from herbivores and epiphytes. According to Wood Density Phase 1--State of Knowledge, Australian Greenhouse Office, Technical Report No. 18 (Oct. 2000), there are several Acacia species in Australia that could be classified as ironwoods. Acacia xiphophylla has a specific gravity of 1.3 and another species called waddy wood (A. peuce) has a density of 1425. If this value is in kilograms per cubic meter, its specific gravity would be 1.425.

See Koa Wood In Hawaii
See Acacia Koa Tree In Hawaii
See Article About The Amazing Acacias
Giraffe Browsing On Spiny South African Acacia
  Central American Thorn Acacias With Symbiotic Ants  

Actually, the heaviest and hardest wood I have ever seen is a block in my laboratory labeled "snakewood," an unknown hardwood species presumably from South America. [This may be Piratinera guianensis of the mulberry family, although its specific gravity is higher than published sources for this species.] The block has a specific gravity of 1.43 which is about the same density as vegetable ivory. On the 10 point scale of mineral hardness, the block of leadwood is similar to vegetable ivory with a rating of 2.5. Compare this value with 3.5 for a copper U.S. penny and 10 for diamond. There is a species of Condalia called tropical ironwood with a reported specific gravity of 1.42, which makes it one of the heaviest woods on earth. Interestingly enough, we have a native species of Condalia (Ziziphus) in San Diego County called desert jujube (C. parryi). This spiny shrub occurs on the dry slopes along the western edge of the Colorado Desert. It belongs to the Buckthorn Family (Rhamnaceae) along with Florida black ironwood or leadwood (Krugiodendron ferreum), perhaps the heaviest wood in the United States. Another rare ironwood tree in the Buckthorn Family called "kauila" (Alphitonia ponderosa) is endemic to the Hawaiian Islands. The hard, heavy wood was used for a variety of tools and objects by early Hawaiians, including clubs, beaters, anvils, digging sticks, spears, and weights for fishing lines.

Assorted ironwoods: A. Desert Ironwood (Olneya tesota), B. African Black Ironwood (Olea laurifolia), C. Knob-Thorn Acacia (Acacia nigrescens), D. Lignum Vitae (Guaiacum officinale), E. Desert Ironwood (Olneya tesota), F. Ebony (Diospyros ebenum), G. Creosote Bush (Larrea tridentata).

Two of the hardest and heaviest native woods in southern California and adjacent Baja California are desert ironwood (Olneya tesota) and mountain mahogany (Cercocarpus ledifolius). The dried wood of both species readily sinks in water, and can quickly dull a "smoking" saw blade. In fact, chain saws have been known to break while cutting through large trunks of mountain mahogany. Mountain mahogany belongs to the Rose Family (Rosaceae) along with Santa Cruz Island ironwood, but the wood of mountain mahogany is much harder than its island relative. The dry fruits of mountain mahogany are easy to spot in the summer and fall, with their silvery, feathery styles glistening in the sunlight. The distinctive heartwood of desert ironwood is almost black and takes a high polish. It is similar in color and hardness to ebony (Diospyros ebenum), and is made into beautiful wood carvings sold in gift shops throughout the southwestern United States and Mexico. Some of these native shrubs can be seen at the Wild Animal Park in the Baja California and Native Plant Gardens.

Mountain Mahogany (Cercocarpus minutiflorus), a native shrub in the chaparral of southern California. The one-seeded fruit (achene) has a persistent, feathery style that glistens in the sunlight. Although they usually don't travel very far, the achenes are blown into the air by strong gusts of wind during the dry, fire season of late summer and fall. This species is not related to the West Indian mahogany (Swietenia mahagoni) or the Honduran mahogany (S. macrophylla), members of the true Mahogany Family (Meliaceae). Mountain Mahogany actually belongs to the Rose Family (Rosaceae) and produces very hard wood that sinks in water when dry (with a specific gravity greater than 1.0).

Any discussion of ironwoods would not be complete without mentioning the Caribbean tree called lignum vitae (Guaiacum officinale), one of the world's hardest and most famous woods. A similar species (G. sanctum) is native to the Florida Keys. Both species are members of the Caltrop Family (Zygophyllaceae) along with North and South American creosote bush and the ubiquitous puncture vine (Tribulus terrestris). The name lignum vitae means "wood of life," owing to the medicinal properties of the heavy, resinous wood. During the days of masted sailing ships, the wood and sweet-smelling resin globs were sought after for treatments and cures for a variety of human ailments, including gout, syphilis and rheumatism. Today the resin is still used for expectorants and as a dye to detect the presence of occult (hidden) blood. During an annual physical examination, gum guaiac (also called guaiac) is the reagent used to check for blood in a stool sample. Peroxidase enzymes in the blood cells oxidize chemicals in the resin resulting in a characteristic blue-green color change. The raw resin, called gum guaiac, contains about 15 percent vanillin (artificial vanilla), resulting in the sweet aroma. The density and high resin content of the wood make it extremely resistant to friction and abrasion and account for its remarkable self-lubrication properties. In fact, under certain conditions the wood wears better than iron. Because of this, the wood has been highly valued for pulley sheaves, bearings, casters, food-handling machinery, and especially for end grain thrust blocks which line the propeller shafts of steamships. During World War I, attempts were made to use other ironwoods such as Tabebuia guayacan from Central America for propeller shaft bearings, but the wood lacked the oily resin of lignum vitae.

Lignum vitae (Guaiacum officinale) is one of the world's hardest and heaviest woods. The natural "gum guaiac" has several medicinal uses, and produces the remarkable self-lubricating properties of the wood.

Foliage, fruit and bright red seeds of lignum vitae (Guaiacum officinale), one of the world's hardest and most famous ironwoods.

Africa has several different species of ironwood trees that belong to entirely different plant families. One of these is called leadwood or wawra (Combretum imberbe), a member of the Combretaceae. It is native from Tanzania south through Mozambique and Zimbabwe to South Africa. Wawra is a slow-growing deciduous tree, with some individuals in South Africa documented at over 1,000 years old. The dark wood is very hard with a specific gravity of 1.23. It is reportedly difficult to saw and very resistant to termites. Wawra wood is highly prized by woodcarvers. African artist Alex Chitura from Zimbabwe carves beautiful African mammals from this wood including elephants, rhinos and hippos. He also has huge scupltures of these exotic animals in wood and stone. His work is for sale at the San Diego Wild Animal Park and San Diego Zoo.

  See Home Page For Artist Alex Chitura  

A hand-carved hippo made from African wawra wood (Combretum imberbe), a member of the Combretaceae. The artist is Alex Chitura of Zimbabwe. He has made many beautiful carvings of animals from wawra wood and serpentine stone, for sale at the San Diego Wild Animal Park and San Diego Zoo.

There are so many fascinating stories about ironwoods that it would be impossible to condense them all into one short article. At least one species has probably touched your life in one way or another--perhaps as a beautiful ornamental tree, a lovely carved figure, an appliance or clock with wooden gears and roller bearings, or perhaps a voyage on an old ocean liner.

References About Ironwoods:

  1. Armstrong, W.P. 1995. "Ironwoods." Zoonooz 68 (1): 26-31.

  2. Armstrong, W.P. 1978. "Native Hardwoods In San Diego County." Environment Southwest No. 482: 6-10 &19.

  3. Constantine, A., Jr. and H.J. Hobbs (editor). 1975. Know Your Woods (Revised Edition). Charles Scribner's Sons, New York.

  4. Jones, K. 1995. Pau d'Arco: Immune Power From the Rain Forest. Healing Arts Press, Rochester, Vermont.

  5. Kelly, S. 1978. Eucalypts (Volumes 1 & 2). Thomas Nelson Ltd., Melbourne, Australia.

  6. Meyer, Hans. 2000. The World Book of Wood Names. Linden Publishing Co., Inc., Fresno, California.

  7. New, T.R. 1984. A Biology of Acacias. Oxford University Press, Melbourne.

  8. Palmer, E. and N. Pitman. 1961. Trees of South Africa. A.A. Balkema, Cape Town.

  9. Peattie, D.C. 1953. A Natural History of Western Trees. Bonanza Books, New York.

  10. Tame, T. 1992. Acacias of Southeast Australia. Kangaroo Press, Kenthurst, Australia.

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