This article, which appeared in the May/June 2008 issue of Sawmill & Woodlot Magazine, is part of S&W Publishing LLC’s “Tree Species” series and is not intended for reprint or republication. It is posted here with permission from Sawmill & Woodlot Magazine. To access copies of Sawmill & Woodlot’s “Tree Species” articles, visit http://sawmillmag.com/treespecies.php
Includes: Rock Elm (Ulmus thomasii), Winged Elm (Ulmus alata), Cedar Elm (Ulmus crassiflia and September Elm (Ulmus serotina)
NORTH AMERICAN ELMS are divided into two groups, hard (the subject here) and soft (including American elm and slippery, or red, elm). Hard elms are considerably harder (50% or more) than the soft elms. In this case, the group name actually makes sense! Unfortunately, all of the hard elms are subject to Dutch elm disease, so supplies are not as abundant as they once were. Rock elm (also known as cork elm) is a hardwood tree that is a native to Canada and the northeastern United States. The highest quality lumber is found at mills in north central Wisconsin, lower Michigan, and southeastern Ontario. Winged and September elms are southern elm; cedar elm is most common in Texas; and September elm is found mainly in Arkansas and Tennessee. However, because the four hard elms are so similar, the lumber from all four is commonly lumped together and sold as rock elm. (In this article, rock elm data is provided; the other three are quite similar.) Many sailing ships were made with rock elm timbers because of its high strength. Caskets have been made from elm for many centuries, both in North America and Europe. More recently, rock elm has been used for cheese boxes, vegetable boxes, furniture (especially Danish styles), and upholstery frames. Bending qualities are superb; more severe bending operations should consider using elm because rejects are few, even though the lumber can be expensive and hard to obtain at times. The beautiful grain makes the hard elms particularly attractive for interior paneling.
PROCESSING Suggestions and Characteristics
Rock elm is a heavy wood, comparable to red oak, with a density at 6% MC of 42 lbs. per cubic foot or a specific gravity of 0.65. A board foot, when freshly sawn, weighs about 5.7 lbs.; when dried to 6% MC and planed, a board foot weighs about 2.5 lbs.
The grain pattern of elm is an important feature of the wood, so flatsawn lumber is very important. This means that the log must be turned often to avoid quartersawn grain and an uninteresting grain. Due to stresses in the log, it is advisable to make the second face opposite or 180 degrees from the first or opening face. Most markets want 4/4 lumber, but for paneling, 5/4 (that is then resawn into two thinner pieces after drying) is a good choice. The market for thicker stock is poor. Likewise, lower grade lumber has poor markets (meaning it is hard to sell even when prices are low). For this reason, it is a good idea to consider a second saw, rather than using the head saw, for sawing the low quality cants into lumber.
The wood dries easily, but warping due to interlocked grain is a serious risk. Stickers are usually spaced 12 in. apart, twice as close as with other hardwoods. Extra weight on the tops of the piles helps to control warp. End coating is required to prevent end cracking and splitting.
GLUING & MACHINING
As with all heavier weight woods, hard elm requires moderate care to develop good glue joints. That is, it is not too forgiving if everything is not perfect. All adhesives work well. Hard elm machines fairly well for a dense wood. Tools and sandpaper must be sharp. Chip-out, tear-out, or grain tearing can result due to interlocked grain, especially if the wood is overdried or tool setup is not optimum. Turning quality is quite good.
Overall, shrinkage from green to 6% MC is 7.1% tangentially (the width in flatsawn lumber) and 3.9% radially (the thickness of flatsawn lumber). Once dried, the wood is fairly stable when the RH (relative humidity) changes; it takes a 7% MC change to result in 1% size change radially and 4% MC change tangentially.
Rock elm is quite strong. For dry wood, the ultimate strength (MOR) is 14,800 psi, stiffness (MOE) is 1.54 million psi, and hardness is 1,320 lbs. Comparative oak values are 14,300 psi, 1.82 million psi, and 1,290 lbs.
COLOR & GRAIN
It is difficult to split the hard elm logs because the grain is interlocked (which means it goes every which way). This interlocked grain also means that impact resistance is high, a good feature for hockey sticks. On the other hand, it also means that the planer knives and other tools, when machining, are always going partly against the grain, which leads to chip-out. Interlocked grain pattern means that bending failures will be rare. The wood itself is light brown to medium brown, with a reddish hue at times.
PESTS & DISEASES
The hard elms, as well as American and red elm, are plagued by two diseases that overshadow all other elm pest and pathogen problems. The first is the infamous Dutch elm disease (DED), caused by either of two closely related vascular wilt fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. The second, known as elm yellows, is just as devastating but doesn’t receive nearly as much publicity, perhaps in part because of its obscure nature. For example, both diseases appear to have been introduced to North America early in the 20th century, but the origin of elm yellows remains uncertain. Indeed, the organism responsible for elm yellows, a phytoplasma, eluded detection and identification for many decades.
Phytoplasmas are small bacteria-like creatures that make a living exclusively in the phloem (food-conducting inner bark) of specific hosts. Like DED, elm yellows is spread from tree to tree by insects or through root grafting. However, DED is vectored by a couple of beetles (elm bark beetle and European elm bark beetle), whereas elm yellows is spread by phloem- sucking leafhoppers. Once in the host, the elm yellows phytoplasma travels through the phloem to the root system, where it proliferates, cuts off the food supply, and eventually kills the fine roots. Subsequent decline and death of the tree crown is merely a symptom of severe root system damage. While DED and elm yellows cause broadly similar symptoms and ultimately tree death, they differ in important ways. Knowing how to distinguish between the two may save a landowner or manager considerable time and money, because DED is treatable (e.g., with expensive fungicide injections) but elm yellows is not, and misdiagnosing the latter as the former could prove costly.
Classic DED symptoms include browning and wilting of foliage on individual branches, whereas elm yellows symptoms typically include simultaneous yellowing of the entire crown, often followed by premature leaf drop. Additionally, DED causes a brown streaking in the sapwood, while trees with elm yellows have a butterscotch discoloration and characteristic wintergreen odor in the inner bark.
The strongest market overall is for decorative interior paneling. This means that the wood must be dried and is often planed and then has a tongue-and-groove pattern on the edges. As mentioned, 5/4 is often sawn, dried, and then resawn, which allows 1.25 bf of lumber to produce 2 square feet of paneling. The purchasers will often be builders and remodelers; personal visits may be required to cultivate their business.
One idea that can help is to provide a laser engraving indicating the type of wood, so that the purchaser can “show it off” to the neighbors. Certainly, rough or planed, dried 4/4 lumber can be sold, but the profits and most active markets are for products that have been manufactured after drying. Rough lumber buyers will most likely be persons who accumulate lumber and then remanufacture it into various products, furniture and cabinet parts, paneling, or flooring, often on a customized basis. Producing cants for railroad ties, if the core is solid, can be worthwhile, especially if the purchaser is near the sawmill. Otherwise, hauling costs can quickly eat up profits. Oftentimes, the purchaser has special size requirements.
Gene Wenger is a Professor Emeritus at the University of Wisconsin-Madison and president of The Wood Doctor’s Rx. He can be reached at [email protected].
Eric Kruger is a professor in the Department of Forest Ecology & Management at the University of Wisconsin-Madison. He can be reached at [email protected].