Balsa wood: Where does it come from and why is it so light?

OCHROMA is a genus of flowering plants in the mallow family, Malvaceae, containing the sole species Ochroma pyramidale or lagopus commonly known as the balsa tree.

Balsa trees grow naturally in the humid rainforests of Central and South America as well as in tropical seasonal thorn forests there. Its natural range extends from southern Mexico down through Central America, to the north and west coast of South America as far as Bolivia.

Balsa thrives in a warm climate with plenty of rainfall and good drainage, especially on higher ground. It is evergreen in wetter climes or dry-season deciduous in dry thorn forests to the south.

Balsa trees in a plantation.

Ecuador has the ideal geography and weather for growing balsa trees.

This small nation, on the north-western, Pacific coast of South America, is the leading source and exporter of balsa in the world. It has been introduced into other countries as a timber crop too – Papua New Guinea, Indonesia, Thailand, and the Solomon Islands.

The word balsa itself means ‘raft’ in Spanish, because of its excellent floatation qualities. In Ecuador it is also dubbed ‘boya’ – or buoy.

Balsa trees grow very rapidly. Six months after germination, the stem is already about 3 cm in diameter and 3–4 metres tall. In six to 10 years it is ready for cutting, having reached a height of 20–28 metres and a DBH of 55 cm.

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If left to continue growing, the new wood laid down on the outside layers becomes very hard and the heart wood starts to rot. Unharvested, a balsa tree may grow to a diameter of two metres plus, but very little usable timber can be obtained from a tree that large.

Balsa is one of the few trees in the jungle which has a simple leaf shape – a fact that makes the species stand out in the jungle.

The basal leaf is similar in shape to a grape leaf, only far larger. When the tree is young, these leaves measure as much as a metre or more across. They become progressively smaller as the tree grows older.

Balsa trees thrive naturally in the humid rain forests of Central and South America.

Flowers are produced from year three onwards, typically at the end of the rainy season when few other trees are in bloom. The large inflorescences open in the late afternoon and remain so overnight. Each may contain a tempting pool of nectar up to 2.5 cm deep. Daytime visitors include capuchin monkeys, but most pollination occurs at night. The main feeders and pollinators were thought to be bats, but recent evidence suggests that two small, nocturnal, arboreal mammals, the kinkajou and the olingo, may be the primary ones.

Balsa is classified as a hardwood despite the wood itself being very soft. It is the softest commercial hardwood.

WHY IS BALSA WOOD SO LIGHT?

The secret to balsa wood’s lightness is revealed under the microscope.

The cells are large and very thin walled, so that the ratio of solid matter to open space is as low as possible. Most woods have blobs of heavy, plastic-like cement, called lignin, holding the cells together. In balsa, lignin is at a minimum and only about 40 per cent of the volume of a piece of fresh balsa is solid substance.

To give a towering balsa tree the strength it needs to stand upright in the jungle, nature pumps each cell full of water until they become rigid or turgid.

Balsa is the softest commercial hardwood.

As green balsa wood typically contains five times as much water by weight as it has actual wood substance, it must be carefully kiln dried to remove most of the humidity before it can be sold. Kiln drying is a tedious two-week process that carefully drives off the excess water until the moisture content is only 6 per cent. The process also kills off any bacteria, fungi and insects that may have been in the raw wood.

In old money, 8–12 pounds per cubic foot is considered medium or average weight for balsa and is the most plentiful.

USES

Balsa wood is very light, soft and buoyant, with a coarse, open grain.

It is one of the lightest varieties of wood available, but not the absolute featherweight. It is remarkably sturdy however, and is often considered the strongest wood for its weight in the world. Like all timbers, it comes in various grades.

Over the years, mankind has utilised this manageable, light and versatile natural material in many products.

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During World War I, the US military sought out balsa wood as a substitute for cork but it soon proved more useful as a lightweight construction material for gliders and shipping containers.

Hobbyists began to work with it because it could be carved easily with standard woodworking tools and bent into a number of shapes without sacrificing strength.

A simple model glider.

Older FJ readers may remember craft lessons at primary school, cutting balsa wood using craft knives before those were frowned upon on safety grounds.

It is even less dense than cork and thus has long been used for lifebelts, fishing lures and buoyancy aids. Balsa wood is crafted into top-of-the-range surfboards as well.

It was famously pressed into service by the Norwegian explorer Thor Heyerdahl in his expedition raft Kon-Tiki in 1947, to prove that ancient peoples could have crossed from Peru to Polynesia on such flimsy-looking constructions. The main body of the float was nine balsa tree trunks, up to 14 m long and 60 cm (2 ft) in diameter, lashed together with hemp ropes.

Balsa wood has excellent insulating properties and is used for refrigerators and cold-storage rooms. It is great for soundproofing acoustics and electrical insulation too.

This wood is used to make very light, stiff structures in model bridge tests and for the construction of light wooden aeroplanes, most famously the World War II de Havilland Mosquito. In modern aircraft, it is good for constructing passenger compartments.

The fifth and sixth generations of the Chevrolet Corvette sports car (1997–2013) had floor pans composed of balsa sandwiched between sheets of carbon fibre reinforced plastic.

BROUGHT INTO CULTIVATION

In the wild, this transient tree is an opportunist, colonising gaps in the canopy when those arise and light penetrates to the forest floor. It acts as a short-lived nurse for other, more durable forest trees.

In nature, the numerous soft seeds with their soft tufts for parachutes are carried away on the wind with the hope of landing in a sunlit patch where they can germinate. If they touch down in the umbral under the dark forest canopy, the seeds lie dormant. Local farmers regard this tree as a weed where it germinates and grows quickly on their cleared food plots in the jungle.

Found at low densities in natural forests, the trees were traditionally hand cut and extracted by oxen. It was not thought suitable for growing commercially.

Only quite recently has this tree been domesticated in Ecuador and tended in large groves or stands as a crop with the plants harvested after 6–10 years of growth.

Ecuador supplies 95 per cent or more of commercial balsa. About 60 per cent is now plantation-grown in densely packed patches of around 1,000 trees per hectare (compared to about 2–3 per hectare in the wild).

It is yet another example of how indigenous trees can be domesticated and grown in plantations sustainably and in a way that is commercially viable, benefitting local communities and relieving the pressure on stocks in the wild.

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