Acacia

When you hear the word acacia, do you picture umbrella-shaped trees on the African savannah? Or perhaps shrubs in the dry American South-West? Or the Australian outback? Acacias are a group of trees and shrubs numbering more than 1,000 species, with a very wide distribution. Let us take a quick tour of the most interesting facts about these trees.

Wrongly called "acacia" because of its thorns, black locust belongs to the Papilionaceae family. Its botanical name is Robinia pseudoacacia. This species, native to North America, is also known as "black locust" or "yellow locust". The first specimen was planted in Paris, on the Place Dauphine, by Jean Robin, gardener to King Henri IV, in 1601.

Its range today covers a large part of France, its natural spread having continued unabated since the 18th century. This area accounts for almost 1% of French forests, or 130,000 hectares, mostly on private land.

Black locust is fairly rarely found in extensive stands, occurring in pine woods and mixed forests. A light-demanding species, it enjoys rapid growth and adapts to all soils and climates. It does, however, prefer light soils that are reasonably rich in water, and it is not found growing above 1,000 metres. It can reach 20 to 35 metres, with a maximum diameter of 1.2 m.

It is worth noting that it spreads mainly by vegetative reproduction (stump shoots and root suckering). The harvesting age varies between 30 and 70 years. Its lifespan can reach 300 years or more. The trunk is often straight but also forked, yielding irregular, short logs 30 to 60 cm in diameter.

Appearance of acacia

The heartwood of acacia is easy to distinguish: greenish yellow when fresh, it takes on a light golden brown tone under ultraviolet light. The sapwood, for its part, is very thin and yellowish white in colour. The grain is generally straight, apart from the frequent knots, which are often large.

A flame figure appears on flat-sawn faces. Its structure is highly uneven and the vessels are visible to the naked eye. The annual rings are very pronounced. With a density ranging from 0.72 to 0.90 at 12% moisture content (depending on the growing region), acacia is classed as medium-heavy to heavy.

Main characteristics of acacia

The heartwood of acacia is very durable and meets use class IV. The tyloses blocking the vessels make it a barely permeable material. This wood is considered prone to movement, even very prone (with total shrinkage of around 16%). It is also a wood with very high strength in tension, in bending and in compression, and it withstands impact very well. Its low resistance to splitting makes it a cleavable species.

Applications of acacia

The applications of this acacia stem from its hardness and durability. Fence posts, poles and above all vineyard stakes and shellfish-farming stakes are the most common uses. High-quality parquet flooring and furniture are also produced from it, including steam-bent chair components.

Garden landscaping, decking, trim trails, tool handles, ladder rungs and various machine parts can be made from acacia. So too can dowels, stave wood, masts, oars and steam-bent frames for boatbuilding. Marquetry and veneer are other possible uses.

Advice on use and finishing of acacia

To avoid splits during conversion, it is recommended to saw acacia logs whose moisture content has remained high, and to use a tooth pattern suited to sawing hardwoods. Drying is slow because of the tyloses blocking the passage of water through the vessels. It is also delicate: splits and various forms of distortion can occur if the schedule is too fast.

Drying temperatures should therefore range between 35 and 60°C from the start to the end of the operation. Bending gives good results, as does turning. Slicing is possible. Gluing presents no particular difficulty, but the pressing force must be higher the greater the thickness.