May 19, 2022

Blackgum: water-loving tree produces honey and wood products

Blackgum, also called black tupelo, tupelo gum or simply tupelo, is a tree that likes to grow in water and waterlogged soils. In fact, the genus Nyssa is the name of a water nymph. The tree grows in every eastern state, from Maine to Texas. Tupelo, Miss., (Elvis’ birthplace) was named after this tree. A close relative is the aquatic tupelo (Nyssa aquatica) which has almost the same characteristics. Water tupelo honey is highly prized. The tree in the forest is often 3 feet in diameter and 80 feet high. Some trees are over 400 years old. Unlike most trees, tupelo trees are either male or female, with flowers being abundant on female trees. It is not uncommon for the larger, older tree to be hollow due to decay fungi; yet the tree can live with this condition for hundreds of years.

The wood itself is characterized by an interlocked grain (the grain swirls in all directions), which leads to warping in drying, warping in use as the MC changes, and difficulty in machining. In the past, black gum was used for ox yokes and chopping bowls due to the toughness resulting from the interlocked grain. Today, although not an exceptional species and a beautiful appearance, it is still widely used for furniture, cabinets, caskets and railway sleepers. Duck decoy carvers also appreciate this wood.

Treatment Suggestions and Features

Weight: Blackgum has a dry specific gravity (SG) of 0.52. The weight, when dry, is 32 pounds per cubic foot or approximately 2.6 pounds per board foot.

Strength: For dry lumber, ultimate strength (MOR) is 9,600 psi, stiffness (MOE) is 1.20 million psi, and hardness is 810 lbs. Interlocked grain means splitting is very difficult. Nailing is also difficult because the interlock wants to change the direction of the nail. Pre-drilled holes for nails and screws can be useful.

Drying and stability: The wood is very difficult to dry due to warping, especially twisting. Final coating is suggested, even with 4/4 paper. Shrinkage on drying is moderate. The overall 6 percent MC green shrinkage is 6.2 percent tangential (the width of the flat-sawn lumber) and 3.5 percent radial (the thickness of the flat-sawn lumber). Once dried, the wood moves slightly if there are large changes in relative humidity or if the MC is not suitable for the EMC conditions of the environment. A typical and desired final moisture range is 6.0-7.5% MC. When dry, it takes a 4.5% change in MC to cause a 1% size change tangentially and an 8% radial change in MC.

Machining and gluing: This wood works moderately well, with some chipped grains due to the interlocked grain. If it is too dry, the wood seems quite brittle. Sharp tools are essential. This wood glues without too much difficulty if the surfaces are perfectly flat. However, if the MC changes and the parts deform a little, it is difficult to achieve high strength joints.

Grain and color: The wood, which is mostly heartwood, is greyish brown to light brown with occasional yellow tinges. The finished surface is smooth, but without a natural sheen. If the wood is quarter-sawn, the black gum presents a very attractive figure. The wood is odorless. However, when the tree is infected with bacteria, the sawn wood will have a putrid odor and is noticeable (and unpleasant) in dry wood products, especially when the humidity is high.