Michael Van Valkenburgh, FASLA, and his fellow speakers got multiple rounds of spontaneous applause at the 2011 ASLA annual meeting for hosting a session on a topic near and dear to many design professionals and wood experts: how to end the unsustainable harvesting of Ipe wood and scale up the use of sustainable alternatives. The real alternative may be Black Locust (Robinia pseudoacacia), which Van Valkenburgh and other progressive landscape architects, architects, engineers, and wood manufacturers have already been using for some time. In addition, domestically-grown Black Locust may offer new opportunities for local sustainable forestry businesses. The trees grow fast and are hardy (in fact, in many areas, they are treated as invasives) and can even take root in urban areas, so they can provide a new source of employment in cities like Cleveland and Detroit, where populations are collapsing and landscapes aren’t as productive as they could be.
Ipe is a tropical hardwood often used in outdoor decks and furniture because it’s so resilient to rain, insects, and weather changes. It’s special properties also mean that it lasts a long time. However, there is a dark side to this wood, which is all too often still used in park and residential projects. Just a few Ipe trees are found per acre in dense, lush tropical forests, which means foresters must wreck havoc on the forest to extract and process those single old trees. Van Valkenburgh and others argue that there must be alternatives.
Why Black Locust?
Stephen Noone, ASLA, senior associate, Michael Van Valkenburgh Associates, said Black Locust lasts just as long as Ipe. He said it’s a “pioneering, not invasive” plant that “takes root on sites that other plants don’t like.” Unlike Ipe, the tree grows together in densely planted areas. In Europe and Asia, it’s already treated as an acceptable crop. In fact, a number of countries are moving forward with planting large groves for wood production, a business that, oddly, has failed to take root in the U.S.
A research project by Michael Van Valkenburgh Associates’ interns found a plantation in Hungary producing a range of different quality Black Locust woods, including top quality woods. They found that Black Locust can “only be planted in marginal areas where Oak can’t be established.” In a smart move, a local forestry research organization and local wood producers aforested a massive area. Now, 8,500 hectares of dense Black Locust forests are being harvested in just one area there.
Black Locust has “high natural durability, is heavy and hard, but has a tricky kiln drying process,” said Noone. It’s “not going to rot and is insect resistant.” Noone delved into the details of moisture content, and the process needed to achieve the desired content levels. There is a complex multi-step process that involves lettting the freshly cut wood air-dry to reduce moisture and then using a “dehumidifier kiln.” Noone said “the process is very strict,” and “dilligence is required on the part of the drier.” There’s also a long lead time for landscape architects: 40-50 days until the wood can be used. But it’s worth it: Beyond the sustainability benefits, Black Locust is also cost-effective. In bulk, it’s $5.44 per square foot, while Ipe is more than $7 per square foot.
Building a Bridge in Brooklyn
A new pedestrian bridge made entirely of Black Locust and designed to move people from neighborhoods in Brooklyn into Van Valkenburgh’s Brooklyn Bridge Park (see earlier post) is now taking shape. Ted Zolli, HNTB, said in this case, “black locust is better than concrete in terms of its compression, strength, and flexibility” and an “incredibly viable structural material.”
Zolli showed how timber bridges aren’t a new thing. Some 20 percent of current bridges are made up of wood and some are more than 100 years old. This 400-foot long bridge is comprised of pre-fabricated pieces created off-site and the delivered and installed in BK. Some parts of the bridge span 120 feet. All together, there’s about 30 tons of wood. He said for this gangway, Black Locust was the right way to go.
HNTB purposefully tested how vulnerable the wood is to fire and found that it doesn’t lose its strength as it burns. “It’s better than steel and will do better in a fire than the cable wires we are using.” For him and his firm, the real challenge was getting a hold of longer planks and finding the right connector systems for the bridge components.
The Properties of Black Locust
Don Lavender, Landscape Forms, a man Van Valkenburgh called a “national treasure,” discussed the opportunities and challenges in scaling up a domestic Black Locust industry. He said there is great potential for the tree in the U.S. but it’s about “obtaining prime examples and taking selections.” Lavender said the best trees are found in the Appalachian region.
Black Locust was originally given to settlers by the government during the early expansion of the U.S. because it’s very fast growing. Within 15-20 years, the material can be cut down and burned. At 30 years old, it can be used for materials in homes. Lavender said the best of these trees “competed for sunlight with other trees.”
The tree can be used for many products, and even lesser-grade woods aren’t wasted. “100 percent of the tree can be purposefully used.” Lavender said lower grades can be used for mulch, biomass fuel, parquet, and greenhouse poles. The higher grades, #1 grade, premium and premium plus (the top 5 percent), are the result of a more challenging “kiln drying process” that requires “patience.” The wood is tough and resistant to drying for the “same reason it’s so resistant to decay.”
Once dried properly, it can easily be “cut, sawed, drilled, sanded, and shaped.” No outdoor finishes are needed and its screw retention is good. Its Janka hardness also compares favorably with other woods. At 1700, it’s better than Red Oak (1290), but a bit less than tropical hardwoods like Jarrah (1910) and Ipe (3684). It’s also difficult to glue. But biologically, Black Locust is “remarkably decay resistant.”
Scaling up Cultivation and Production in the U.S.
Van Valkenburgh said the U.S. is falling further and further behind globally. “The country is losing its edge.” Currently, there are 5 million acres of Black Locust under cultivation worldwide, but “virtually zero in the U.S.” Korea has 1.2 million acres, China has another 1 million, while Hungary has 270,000 acres. “This is something that has the potential to be an economic engine in many parts of this country.”
Instead of being viewed as an invasive, as it is in many parts of the U.S., Van Valkenburgh said it should be grown in set-aside areas. Lavender added that the Amish, who “don’t waste anything,” has been using Black Locust for ages. The Amish, who have perfected techniques over generations, are in fact a perfect model for Black Locust production: “proper kiln drying is not something you just get into one day. It takes generations to learn this.” He added that a number of firms have “blundered into kiln drying” and ended up with kindling. “It needs to be done methodically” if an industry is going to bloom here.
Van Valkenburgh is currently using Black Locust imported from Hungary. A firm in Massachusetts is importing containers from Hungary and “taking it upon themselves” to expand the domestic market. While using this wood puts him out of the 500 mile range the Sustainable Sites Initiative (SITES) calls for in sourcing sustainable materials, he said “we have to think bigger about sustainability. The lifespan of these woods is several multiples higher than others.” Still, he wants to see the woods grown domestically.
The Black Locust market in the U.S. is “still in its infancy” despite the advocacy efforts of Van Valkenburgh and others. Hopefully, some smart city officials will see an opportunity. As one audience member said, Detroit and other cities could not only turn their abandoned lots into forests, but Black Locust forests. Van Valkenburgh went even further: “Planting on reclaimed sites is a great idea.”
Van Valkenburgh, Noone, Zolli, and Lavender kindly shared their 84-page presentation (8MB) full of rich content, photos, and data. Download and help spread the word.
Image credit: (1) ASLA 2006 General Design Honor Award. Small is Beautiful. Michael Van Valkenburgh Associates / Elizabeth Felicella, (2) Walkway into Brooklyn Bridge Park / HNTB
Very interesting post, thank you!
Loved this article – thanks for the great info ! Our neighborhood group helps manage a small Jens Jensen park in Madison, WI which has become highly dominated by black locust. Our city forestry & parks officials have approved removal of all black locust over time via an urban forestry plan; now we are working to convince same officials to allow us to sawmill the removed trees into useful wood for community gardens and other park amenities.
Using black locust as a more sustainable option to ipe wood is highly commendable in order to preserve our fragile rainforests. However, the assertion that black locust is a pioneering species instead of an invasive species is woefully incorrect. Perhaps in the east coast it is a pioneering species, but here in California it is in fact an invasive species. I do extensive riparian restoration work along the American, Sacramento, and San Joaquin Rivers in the Central Valley. Black locust grows right alongside oaks, cottonwoods, and other riparian species, and crowds them out. We have to rip them out whenever we restore an area to make room for our natives. Encouraging groves of black locust for commercial use in an area like ours would add a horrific environmental burden. They don’t live in a vacuum, as birds carry their seeds quite far. Extreme caution should be used in recommending an invasive species such as black locust as the miracle ipe substitute. This won’t be the first time that well-meaning professionals have brought in a destructive non-native species that wreaks havoc in its new home.
I couldn’t agree more!
Black Locust is an invasive tree species and is listed as such by the Massachusetts Invasive Plants Advisory Group an advisor to the Mass Dept. of Agriculture.
Why use black locust when we have Osage-orange (Maclura pomifera)? Osage is denser, stronger, and more rot resistant than almost any North American or temperate hardwood. I don’t see it on the Janka scale but would guess it is near or above 2000. Thousands of miles of Midwest and Great Plains Osage-orange hedgerows have been bulldozed, but thousand more await selective logging. it is extremely heavy and like black locust it is not easily glued. But its natural chemical composition resists insects and rot. I have seen 100-year-old Osage-orange fence posts still solidly in the ground supporting rusted-through barbed wire. Its heartwood use in benches and even wood retaining walls should last at least 50 years.
Black Locust is also a legume which fixes nitrogen and is therefore good for the soil.
I am THRILLED to see that attention is being made to black locust. In Germany, where it is certainly not native, black locust is highly regarded. They use it extensively in playground settings where pressure-treated wood is particularly disdained for the contact children have with chemicals. Whereas, here in its native territory, it is seen as a weed tree. Places where it is considered invasive aren’t places for black locust nurseries, but let’s make use of our strengths. If kudzu only had the same prospects…
Plus its flowers are beautiful and fragrant, and start appearing while it’s still much too young to harvest.
Black Locust and Osage orange (as mentioned in a post above) exemplify the possibilities of looking local and learning from the past. (It must be noted that I am writing this through the eys of someone who lives in the native habitat range of this tree.) As the article on Black Locust mentioned, the Amish have used the tree for years and Amish sawmills generally mill the lumber. Farmers used the tree for a variety of uses and many, century old, locust fence posts can still be seen buried in Pennsylvania hedgerows.
On another note, not only does a Black Locust produce a beautiful flower, the tree can also take on a stately shape. Much as a White Oak growing in a field will obtain a stately growth pattern, so too will a Black Locust. The Black Locust that grows in an open environment generally has a fluid branching pattern. When in full leaf, the tree takes on a pagoda-like character and has an almost lacy quality about it. In my opinion, it’s long history as a weed tree in the eastern US has given it a bad rap and should be considered for its ornamental qualities as well.
Try Red Cedar. Good native species for wildlife food, erosion control and grows well in poor soil. We have 200 year old split rail fencing made from this tree in northern N.Y. I’ve had some under water in a boat house for fifty years. Just pulled it out and cut it. Still like it was the day we harvested it- even smelled good. Unbelieveable structural strength. Not a bad firewood for a conifer as well- no pitch. Only problem is it grows slow.
Importing black locust from Germany may be preferable to brining Ipe from S America. But having the option of a domestic hardwood suitable for outdoor products would be ideal. For two years we have been evaluating black locust for use as bench slats. As of right now, having a reliable domestic source of quality black locust is the challenge. We will all need to be a little patient while black locust moves from planting, to forests, to mills, to manufactuers, to the market. It will take awhile, but we’re working very hard on this . . .
I appreciate this article and will send it to some people who grow wood! Thanks. It’s great to use natives!
TRY THE VARIETY PURPLE ROBE IT IS OUTSTANDING. AND TOUGH TOUGH
I love the idea of domestic production and utilization of local resources, especially on sites that are now just a waste of space but the article mentions nothing about the locust borer that is so devastating to black locust. I agree with the above mention about how strong hedge trees are but they are also extremely squirrely, growing with multiple stems and twisting and turning, and the yellow/red color of their wood may not be as desirable for commercial products. Better utilized for biofuel in my opinion. Thank you for the article though, it is interesting.
Can someone supply info on readily available sources for dimensioned timbers, other than in Europe?
Another great alternative is Western Juniper; a native to the American west. Decades of fire suppression and overgrazing have resulted in it encroaching on natural grasslands. Its removal can restore these ecosystems and this beautiful wood is more rot resistant that Redwood or Red Cedar