The Black Locust (Robinia pseudoacacia) is an enigma, a tree of contradictions with both great beauty and utility and yet condemned by many to the list of “weed trees.” I hope to enlighten our readers to both sides of the Black Locust story.
The black locust is a tree native to the eastern United Sates with a long established ecological and cultural history in the traditions of both the pre-European cultures and those of European settlers. Yet today, this tree is commonly disgraced by ecologists and landscapers alike as an invasive. The tree grows very well on disturbed and degraded soils, leading to its ability to proliferate on these sites and seemingly “invade” them. This tree commonly fills in the untended gaps left by ecological disturbances in the anthropomorphic environment. Walk the alleys of D.C. and you are likely to stumble across this tree. Beyond the ability to seed itself well, this tree commonly sprouts clonal shoots directly from the roots to form new trees in tight groves. In the cultivated home landscape, the extra maintenance required to manage the shoots is a nuisance – but this same trait makes the black locust an excellent tree for quickly stabilizing eroding slopes and re-vegetating cleared lands. This tree is a classic early succession species that can be both of tremendous use and a pain to contain.
Like the rose, the black locust presents both beauty and a sharp warning to stay back. In the spring, a lovely fountain of white flowers tumbles from the branches of the tree, providing interest to the eye, food for the bee, and honey for the beekeeper. Yet be careful when getting tactile with the black locust or you may find your finger punctured by the half-inch thorns that cover the young branches. Goats, however, find their way around the thorns and consider the leaves of this tree a tasty treet (pardon the pun). As it has a very high protein content, research is actually under way to utilize the black locust as a fodder crop for livestock.
The merits of black locust as a functional tree in an agricultural setting are many-fold. We’ve already discussed the ability of this tree to grow quickly on infertile sites, but it also contributes to soil fertility by partnering with specialist bacteria to fix nitrogen. In one of the contradictions that baffles me to this day, the black locust actually has some of the densest and most rot-resistant wood of any tree native to this region. The abilities to grow fast and produce sturdy wood do not often go together in the world of timber production. The wood of mature trees is reported to have the thermal heat potential of anthracite coal and when set in the soil as a fence post, the wood of this tree can resist rot for 100 years. The flowers of this tree also fuel the honey industry of many regions (Hungary in particular, apparently). The wood has had a plethora of uses, from traditional bow making to ship-building. In East Asian and European countries, the merits of black locust as a timber species have spawned a burgeoning agro-forestry industry. Black Locust has the potential to provide a locally available, sustainably harvestable option for rot-resistant, durable wood needed for decking and other outdoor applications. Currently, endangered tropical hardwoods like Ipe wood are used. A growing movement works to promote the use of this wood. The black locust does have one key insect threat in the locust borer (Megacyllene robiniae), which attacks young and unhealthy trees by chomping through the cambium in the heartwood. It is reported that the insect is less of a problem when the tree is grown in more favorable soils.
The French have used the Black Locust as an ornamental landscape tree for centuries now. Many countries have planted thousands of acres for timber. The Hungarians and other eastern Europeans depend on it for honey production. We would do well to appreciate the gift that our native lands have given us in this great tree. When used with care and planted in the right spot, the black locust is a tree that will serve. And in the nooks and crannies of our city and on the derelict lots that no one cares to tend to, the black locust quietly works to stabilize and enrich the soil, soak up storm water, and add to our urban forest canopy.