Black locust

Black locust is a member of the legume family; it is able to “fix nitrogen” in the soil.  It is native to the Appalachian Mountains, from Pennsylvania to Alabama.  However, in the last century, it has spread to almost every state. With a widespread, shallow root system, it is ideal for thin soils where it will prevent soil erosion; it is often used for strip-mine reclamation projects that are barren due to mining debris and acid soils. 

The tree has long thin bean-like pods.  (Do not confuse black locust with honey locust.  They are not related.) The flowers are very sweet smelling during the early spring and the pollen and nectar are used by bees to produce excellent honey. 

High natural decay resistance of the wood has resulted in frequent use of black locust for fencing.  It also was the popular species for the pins that held glass insulators on the cross-arms of electric and telegraph/telephone poles.  It also was the prized species for wheel hubs for wooden wheels, such as used on the western “covered wagons.” It also is prized for xylophone keys.  Today, this wood has fallen into neglect within our industry (lumber prices are often low, especially for cabinet grades; dry firewood is a common use). It deserves better treatment.

The tree is short lived, so does not grow to large sizes; a large tree would be 24” diameter at the base and 50 feet tall.  This means that wide clear pieces of black locust lumber are not commonly produced.  A wood boring insect often invades the tree and kills them at an early age.  A leaf miner often turns entire hillsides into brown-leafed, “dead-looking” trees for a week or so in the summer, but this damage does not affect the trees overall.

Processing suggestions and characteristics

Weight. This is a fairly heavy wood, over 10% heavier than red oak.  The weight, when dry, is 50 pounds per cubic foot or about 4 pounds per board foot.

Strength. Due to its high density, black locust is one of the strongest, hardest native American species.  For dry wood, the ultimate strength (MOR) is 19,400 psi, stiffness (MOE) is 2.05 million psi and hardness is 1700 pounds.  Comparative oak values are 14,300 psi, 1.82 million psi, and 1290 pounds. The high hardness makes it a good choice for flooring, table tops, desk tops and other impact areas.  Handles stock is also a reasonable use.

Mechanical fastening is difficult due to the tendency for splitting.  Predrilling holes for nails and screws is advisable.

Drying. The wood dries slowly with some risk of checking and end splitting.  Warping is also a  major risk.  Slow shed air-drying should be followed by kiln drying, as most kilns cannot dry it green-from-the-saw slow enough without high costs.  End coating is also required to prevent end cracking.  All-in-all, good drying practices are essential for top quality.

Shrinkage in drying is moderately high, which accounts for the high risk of checking and warp.  Overall shrinkage in drying from green to 6% MC is 7.2% tangentially (the width in flatsawn lumber) and 4.6% radially (the thickness of flatsawn lumber).

Stability. Once dried, the wood will move slightly if there are large RH changes or if the MC is not matched to the environment’s EMC conditions.  A typical final MC range is 6.0 to 7.5%, unless used in a humid location.  It takes a 4% MC change to result in 1% size change tangentially and 6% MC change radially.

Machining and gluing. This wood machines with difficulty due to its hardness.  It does require very sharp tools with fairly large hook angles; dulling is rapid.  It ranks similar to hickory in machining.  The surface is smooth and polishes well, especially with fresh sandpaper.  Due to the heavier chips, dust systems must have adequate power.

This wood glues with some difficulty.  As is typical of dense species, surfaces must be flat; the best joint is developed if gluing follows the surface preparation within 30 minutes.  The wood is not very forgiving if gluing procedures are not perfect.

Grain and color. The heartwood is often is quite green when first cut, but ages quickly to a russett brown color.  The grain is moderately fine, but the annual rings are obvious and add character to the appearance.

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About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.