Blackgum

Gene Wengert, wooddoc@uwalumni.com

Also known as black tupelo, tupelo gum, or tupelo.

Blackgum

Blackgum, also called black tupelo, tupelo gum, or just tupelo, is a tree that loves to grow in water and water-soaked soils. In fact, the genus Nyssa is the name of a water nymph. The tree grows throughout the eastern states, from Maine to Texas. Tupelo, Miss., (Elvis’ birthplace) was named after this tree. A close relative is water tupelo (Nyssa aquatica) which has nearly the same characteristics. Honey from water tupelo is thought to be the best honey in the world. It is very high in fructose sugar.

The tree in the forest is often 3 feet in diameter and 80 feet tall. Some of the trees are over 400 years old. Unlike most trees, the tupelo trees are either male or female, with flowers being abundant on the female trees. It is not unusual 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 interlocked grain (the grain swirls every which way), which leads to warp when drying, warping in use when the MC changes, and difficulty when machining. In the past, blackgum was used for oxen yokes and chopping bowls due to the toughness resulting from the interlocked grain. Today, in spite of the fact that this is not an outstanding, beautiful appearing species, it still is widely used for furniture, cabinets, caskets, and railroad ties. Carvers of duck decays also appreciate this wood.

Processing suggestions and characteristics

Weight. Blackgum has a dry specific gravity (SG) of 0.52 (which means it is roughly half as dense or heavy as water). The weight, when dry, is 32 pounds per cubic foot or about 2.6 pounds per board foot.

Strength. For dry wood, the ultimate strength (MOR) is 9,600 psi, stiffness (MOE) is 1.20 million psi and hardness is 810 pounds. Comparative oak values are 14,300 psi, 1.82 million psi, and 1290 pounds. Although oak is a little stronger, oak also weighs 30 percent more.
Interlocked grain means that splitting is very difficult. Nailing is also difficult, as the interlocked grain wants to change the nail’s direction. Pre-boring holes for nails and screws can be helpful.

Drying and stability. The wood dries with considerable difficulty due to warp, especially twisting. End coating, even 4/4 stock, is suggested.
Shrinkage in drying is moderate. Overall shrinkage from green to 6 percent MC is 6.2 percent tangentially (the width in flatsawn lumber) and 3.5 percent radially (the thickness of flatsawn lumber). Once dried, the wood does move slightly if there are large RH changes or if the MC is not matched to the environment’s EMC conditions. A typical, desired, final moisture range is 6.0 to 7.5 percent MC.
Once dry, it takes a 4.5 percent MC change to result in 1 percent size change tangentially and 8 percent MC change radially. It is prone to some warping in-use if the MC changes.

Machining and gluing. This wood machines moderately well, with some chipped grain due to the interlocked grain. If over-dried, the wood appears to be quite brittle. Sharp tools are essential.
This wood glues without much difficulty if surfaces are perfectly flat. However, if the MC changes and the pieces warps in a small amount, it is difficult to obtain high strength joints.

Grain and color. The wood, which is mostly heartwood, is grayish brown to a light brown with hints of yellow at times. The finished surface is smooth, but without natural luster. If the lumber is quartersawn, blackgum exhibits a very attractive figure. The wood is odorless. However, when the tree is bacterially infected, the wood sawn from such a tree will have a putrid odor that can persist and is noticeable (and objectionable) in dry wood products especially when the humidity is high.

Gene Wengert, "The Wood Doctor," has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.

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