Tree of the Week for June 19th-26th, 2011: Robinia pseudoacacia
As you may have noticed, I didn't post a second plant two weeks ago. Or one last week. But instead of continually backdating my posts until the week I finally get the wherewithal to post twice, I'm going to just forge ahead and post a new plant this week. I wrote most of one last week, and I'm going to finish it up and post it this week, even though the feature that made it catch my attention - its spectacular show of flowers - is now essentially over. Still, I'm sure anyone among my few readers who lives in eastern North America has also recently noticed the flowers of this week's tree, the...
Black Locust, Robinia pseudoacacia
Synonyms: Yellow locust, sweet locust, honey locust (although that name is usually reserved for the related species Gleditsia triacanthos.), false-acacia, "acacia", shipmast locust.
Native? Y/N: Y, but its range has expanded well beyond its original one even here in its native east.
Range and Habitat: Originally, black locust grew in the wet and mountainous portions of the east, all along the Appalachian ridges from middle Pennsylvania down to northern Alabama and down into the hilly portions of West Virginia and eastern Tennessee and Kentucky. It also could be found in the Ozark and Ouachita mountains of Missouri and Arkansas, and the rocky regions of extreme southern Illinois. Since the 17th century, however, foresters, farmers, and gardeners on 4 continents have been planting this remarkable tree, hugely increasing its range1. It has naturalized extensively, and can now be found in all 48 states of the continental U.S., as well as in much of southern Canada. In France, eastern Europe, and China it has also been planted extensively, and naturalized itself successfully too. But more on that later.
Plant ID: Look! Do you see those trees bedecked in long garlands of luminously white, pea-like flowers? Those are black locusts. But even when it is not in flower - which is most of the year, for its flowering lasts little more than a week - it easy one of the most instantly recognizable of trees. It has long, fern-like compound leaves2, similar to those of its relatives the honey locust3 and the Kentucky coffee-tree4, the only trees common in the eastern U.S. it could really be confused with. But the black locust has larger leaflets than the honey locust, blunter-tipped leaflets than the coffee-tree, and its leaves are never doubly compound, unlike the other two species whose leaves frequently may be. Furthermore, black locust has bark that is incredibly deeply furrowed5. This slightly ruddy, cross-checked, and slightly fibrous or scaly bark, combined with its crooked, snake-like form allows one to readily ID the tree at any time of year.
Ecology: Black locust is what the Europeans call a "thermophilic" species, thriving in light and warmth6. In spring, weeks after other trees have put out their leaves, black locust is still bare, as if skeptical of the warm weather's permanence and waiting to make sure that no late freeze will scald its delicate foliage. Again in autumn, it is already losing its leaves in late September, the first touch of the chill Arctic air causing it to hastily put away its work and go to sleep for the winter. And light is just as important; black locust does poorly under the shade of other trees, instead colonizing meadows, prairies, barrens, and large forest openings. It may grow best in the cool cove forests of the southern Appalachians, as do all North American trees fortunate enough to be found there, but it is rare in those shady forests primeval, dependent on storms or landslides to open up a large enough area of forest for its light-hungry seedlings.
But while it is intolerant of shade, it is very tolerant of many other things. Black locust puts forth a wide-spreading root system from which it readily produces new suckers. Therefore, although the mature tree is not particularly invulnerable to fire, after fire scours a stand containing black locust it will rise up like a Hydra, with many new heads replacing the one lopped off. The wide-spreading, though shallow, root system of black locust hides within itself a secret, too. On impoverished soils, it will begin to produce small nodules on its roots, within which it cultivates varieties of bacteria who have themselves evolved the trick of "fixing" - that is, rendering biologically available - atmospheric Nitrogen. These bacteria are greedy, requiring a huge percentage of the sugar produced by the tree as it photosynthesizes, but on exhausted, extremely sandy, or extremely rocky soils where otherwise it would be almost impossible to grow it is very worthwhile. This not only allows black locust to grow on otherwise barren ground, the Nitrogen fixed by the tree's bactericultural technology is introduced into the soil upon its death, thus enriching the soil and making it easier for the next generation of plants of any species to grow on that spot.
Uses and Horticulture: The black locust is one of those creatures which has come to much greater fame abroad than at home. In Europe and Asia, millions of acres of forests have been planted with black locust. The wood is terrifically durable, refusing to rot for centuries even in wet, mouldy climes like Western Europe's. It can be used for fencing, too, for even when stuck into the soil, where so many wood-decaying fungi spend their time, it will yet resist for many decades. It is hard, too, comparable to more famous tropical hardwoods like mahogany. Black locust wood is also very dense and heavy, which makes it an excellent firewood; for any given volume of firewood, black locust will contain among the highest amount of caloric energy. A cord of black locust burns to produce 23.2*106 BTUs worth of energy, nearly as much as a ton of high-grade coal, and more than 1.5 times as much as a cord of pine logs.7 The tree is also a major source of nectar for bees. In France, the so-called "acacia honey" produced by bees feeding on locust flowers is considered a delicacy, and an old UN document claims that, in Hungary, "[w]ithout [black locust] no commercial apiculture could exist"8. Finally, R. pseudoacacia's leaves have been used as feed for livestock. As a leguminous, nitrogen-fixing plant, black locust's leaves are high in protein - higher even than alfalfa, as it turns out!9 The same UN document I keep referencing mentions that in South Korea, pulverized black locust leaves are mixed into rice bran at a 3:7 ratio and fed to pigs.
So why hasn't such a useful tree gained greater fame here in these United States? There is a single reason, and that reason is the locust borer, Megacyllene robiniæ. This insect burrows into the trunk of the black locust to lay its eggs, and the larvæ then feed on the inner bark of the tree. This typically does but little damage to the tree itself, as the two species have co-evolved over millions of years. But it does create a crook in the tree's trunk, and so most black locusts found in the United States have a bent and snake-like bole which, though picturesque, makes the tree essentially worthless to the lumber industry. So far, the borer has not escaped into any of the other continents, and so trees both planted and feral there grow straight and pretty as you please. Now, for decades it has been known that resistance to locust borer is in part controlled by genetics, and there exists at least one variety of R. pseudoacacia - the famous "shipmast locust" of Long Island - which generally grows quite straight even in these borer-infested lands of ours. Still, forestry involving this variety remains uncommon, possibly for some technical reason I am not privy to, or possibly due to sheer institutional inertia.
Taxonomy: R. pseudoacacia is not the only Robinia in the world, or even in the U.S. In the southern Appalachians, black locust may grow alongside its little sisters, Robinia viscosa, the "Clammy locust", and R. hispida, the bristly locust. Despite their rather unflattering monikers, these shrubs or small trees produce charming rose-colored flowers that look every inch the fancy Southerner. Farther west, in both New and Old México, R. neomexicana grows along mountain streams and valley bottoms. All of these trees are, of course, in the Fabaceæ, the legume family, which is second only to the grasses in terms of economic importance to humanity. From peanuts to navy beans, from the dye of indigo to the 'gum Arabic' of Acacia, and from bitter vetch to mesquite, legumes are terrifically important to our civilization, and for a bewilderingly diverse number of reasons.
Terrifically important for a bewilderingly diverse number of reasons since 1986,
--mark
1 According to the Food and Agricultural Organization of the United Nations here, Black Locust was introduced to Eurasia as early as 1601! Peattie similarly says that it was introduced between 1601 and 1636.
2 Image courtesy of the USDA PLANTS database.
3 Image courtesy of the University of Waterloo.
4 Image courtesy of the city of Troy, Michigan.
5 Image courtesy of the science photo library.
6 It should be noted that thermophilic means something much more extreme when it comes to bacteriology.
7 Information on energy content of various woods from "Chimney Sweep Online", here; that on the energy of coal from the University of Wisconsin-Stevens Point here. Any error in calculations is mine alone.
8 Accessed here, the same document referenced in note 1.
9 Black locust has 22.3% crude protein content by mass, according to this paper accessed on the website of Purdue University; whereas alfalfa hay has 15-20% crude protein by mass, according to this paper found on the California Alfalfa Workgroup page.
Black Locust, Robinia pseudoacacia
Synonyms: Yellow locust, sweet locust, honey locust (although that name is usually reserved for the related species Gleditsia triacanthos.), false-acacia, "acacia", shipmast locust.
Native? Y/N: Y, but its range has expanded well beyond its original one even here in its native east.
Range and Habitat: Originally, black locust grew in the wet and mountainous portions of the east, all along the Appalachian ridges from middle Pennsylvania down to northern Alabama and down into the hilly portions of West Virginia and eastern Tennessee and Kentucky. It also could be found in the Ozark and Ouachita mountains of Missouri and Arkansas, and the rocky regions of extreme southern Illinois. Since the 17th century, however, foresters, farmers, and gardeners on 4 continents have been planting this remarkable tree, hugely increasing its range1. It has naturalized extensively, and can now be found in all 48 states of the continental U.S., as well as in much of southern Canada. In France, eastern Europe, and China it has also been planted extensively, and naturalized itself successfully too. But more on that later.
Plant ID: Look! Do you see those trees bedecked in long garlands of luminously white, pea-like flowers? Those are black locusts. But even when it is not in flower - which is most of the year, for its flowering lasts little more than a week - it easy one of the most instantly recognizable of trees. It has long, fern-like compound leaves2, similar to those of its relatives the honey locust3 and the Kentucky coffee-tree4, the only trees common in the eastern U.S. it could really be confused with. But the black locust has larger leaflets than the honey locust, blunter-tipped leaflets than the coffee-tree, and its leaves are never doubly compound, unlike the other two species whose leaves frequently may be. Furthermore, black locust has bark that is incredibly deeply furrowed5. This slightly ruddy, cross-checked, and slightly fibrous or scaly bark, combined with its crooked, snake-like form allows one to readily ID the tree at any time of year.
Ecology: Black locust is what the Europeans call a "thermophilic" species, thriving in light and warmth6. In spring, weeks after other trees have put out their leaves, black locust is still bare, as if skeptical of the warm weather's permanence and waiting to make sure that no late freeze will scald its delicate foliage. Again in autumn, it is already losing its leaves in late September, the first touch of the chill Arctic air causing it to hastily put away its work and go to sleep for the winter. And light is just as important; black locust does poorly under the shade of other trees, instead colonizing meadows, prairies, barrens, and large forest openings. It may grow best in the cool cove forests of the southern Appalachians, as do all North American trees fortunate enough to be found there, but it is rare in those shady forests primeval, dependent on storms or landslides to open up a large enough area of forest for its light-hungry seedlings.
But while it is intolerant of shade, it is very tolerant of many other things. Black locust puts forth a wide-spreading root system from which it readily produces new suckers. Therefore, although the mature tree is not particularly invulnerable to fire, after fire scours a stand containing black locust it will rise up like a Hydra, with many new heads replacing the one lopped off. The wide-spreading, though shallow, root system of black locust hides within itself a secret, too. On impoverished soils, it will begin to produce small nodules on its roots, within which it cultivates varieties of bacteria who have themselves evolved the trick of "fixing" - that is, rendering biologically available - atmospheric Nitrogen. These bacteria are greedy, requiring a huge percentage of the sugar produced by the tree as it photosynthesizes, but on exhausted, extremely sandy, or extremely rocky soils where otherwise it would be almost impossible to grow it is very worthwhile. This not only allows black locust to grow on otherwise barren ground, the Nitrogen fixed by the tree's bactericultural technology is introduced into the soil upon its death, thus enriching the soil and making it easier for the next generation of plants of any species to grow on that spot.
Uses and Horticulture: The black locust is one of those creatures which has come to much greater fame abroad than at home. In Europe and Asia, millions of acres of forests have been planted with black locust. The wood is terrifically durable, refusing to rot for centuries even in wet, mouldy climes like Western Europe's. It can be used for fencing, too, for even when stuck into the soil, where so many wood-decaying fungi spend their time, it will yet resist for many decades. It is hard, too, comparable to more famous tropical hardwoods like mahogany. Black locust wood is also very dense and heavy, which makes it an excellent firewood; for any given volume of firewood, black locust will contain among the highest amount of caloric energy. A cord of black locust burns to produce 23.2*106 BTUs worth of energy, nearly as much as a ton of high-grade coal, and more than 1.5 times as much as a cord of pine logs.7 The tree is also a major source of nectar for bees. In France, the so-called "acacia honey" produced by bees feeding on locust flowers is considered a delicacy, and an old UN document claims that, in Hungary, "[w]ithout [black locust] no commercial apiculture could exist"8. Finally, R. pseudoacacia's leaves have been used as feed for livestock. As a leguminous, nitrogen-fixing plant, black locust's leaves are high in protein - higher even than alfalfa, as it turns out!9 The same UN document I keep referencing mentions that in South Korea, pulverized black locust leaves are mixed into rice bran at a 3:7 ratio and fed to pigs.
So why hasn't such a useful tree gained greater fame here in these United States? There is a single reason, and that reason is the locust borer, Megacyllene robiniæ. This insect burrows into the trunk of the black locust to lay its eggs, and the larvæ then feed on the inner bark of the tree. This typically does but little damage to the tree itself, as the two species have co-evolved over millions of years. But it does create a crook in the tree's trunk, and so most black locusts found in the United States have a bent and snake-like bole which, though picturesque, makes the tree essentially worthless to the lumber industry. So far, the borer has not escaped into any of the other continents, and so trees both planted and feral there grow straight and pretty as you please. Now, for decades it has been known that resistance to locust borer is in part controlled by genetics, and there exists at least one variety of R. pseudoacacia - the famous "shipmast locust" of Long Island - which generally grows quite straight even in these borer-infested lands of ours. Still, forestry involving this variety remains uncommon, possibly for some technical reason I am not privy to, or possibly due to sheer institutional inertia.
Taxonomy: R. pseudoacacia is not the only Robinia in the world, or even in the U.S. In the southern Appalachians, black locust may grow alongside its little sisters, Robinia viscosa, the "Clammy locust", and R. hispida, the bristly locust. Despite their rather unflattering monikers, these shrubs or small trees produce charming rose-colored flowers that look every inch the fancy Southerner. Farther west, in both New and Old México, R. neomexicana grows along mountain streams and valley bottoms. All of these trees are, of course, in the Fabaceæ, the legume family, which is second only to the grasses in terms of economic importance to humanity. From peanuts to navy beans, from the dye of indigo to the 'gum Arabic' of Acacia, and from bitter vetch to mesquite, legumes are terrifically important to our civilization, and for a bewilderingly diverse number of reasons.
Terrifically important for a bewilderingly diverse number of reasons since 1986,
--mark
1 According to the Food and Agricultural Organization of the United Nations here, Black Locust was introduced to Eurasia as early as 1601! Peattie similarly says that it was introduced between 1601 and 1636.
2 Image courtesy of the USDA PLANTS database.
3 Image courtesy of the University of Waterloo.
4 Image courtesy of the city of Troy, Michigan.
5 Image courtesy of the science photo library.
6 It should be noted that thermophilic means something much more extreme when it comes to bacteriology.
7 Information on energy content of various woods from "Chimney Sweep Online", here; that on the energy of coal from the University of Wisconsin-Stevens Point here. Any error in calculations is mine alone.
8 Accessed here, the same document referenced in note 1.
9 Black locust has 22.3% crude protein content by mass, according to this paper accessed on the website of Purdue University; whereas alfalfa hay has 15-20% crude protein by mass, according to this paper found on the California Alfalfa Workgroup page.