The curious case of Robinia pseudoacacia

For those who wish to convert their land, whether it's an empty lot, grassy suburban lawn, or abandoned pasture, into something more productive and environmentally friendly, the sheer number of possible courses of action can be overwhelming. Decisions on what edibles to plant can be made by considering what plants grow well in your climate that you like to eat. Unless you're an expert on local ecology, gardening for wildlife and land restoration is more difficult. Observation of your site will certainly help, but you'll no doubt want to tap into the collective knowledge available on the internet. There are innumerable sites and YouTube channels dedicated to native plants, regenerative farming, wildlife friendly gardening, etc. Information is also available from government agencies like the Department of Natural Resources here in Ohio, as well as state extension services. These sites provide a wealth of resources, but can also create confusion, as contradictory advice is often encountered. Consider the case of black locust, Robinia pseudoacacia.

This tree is native to the Appalachian region including where I live in southeast Ohio, plus areas of Arkansas, Oklahoma, and Missouri. It has been planted extensively outside this range ornamentally, and for erosion control and land reclamation. In my area it was traditionally used for fence posts since the wood is extremely rot resistant and posts made from it can last more than 100 years. When I first moved to the farm I was thrilled to find a pile of logs that turned out to be salvaged locust fence posts. I used them to fence in garden, chicken, and horse areas, and saved a lot of money that would otherwise have been spent at the farm supply store. The ones that were too short or weak made picturesque edges for garden beds.

I've always considered black locust to be a highly desirable tree. Its clusters of seed pods fascinated my sisters and me as children (beans growing on trees!) Young specimens often have serious thorns, though the bark of mature trees rarely sports them, and while still alive the tree is prone to being brittle, and regularly drops sizable branches. For these reasons it isn't a popular yard tree, but it more than made up for these drawbacks in the past by creating free fence posts for my farmer forebears and their cohorts. In addition, the dense wood has a high btu rating, and burns with minimal smoke, making it a great choice for woodstoves and fireplaces. Unfortunately there are only a few mature specimens on the farm, with apparently no saplings to take their place when they're gone. In a previous post I described the transition of an abandoned cornfield that is being colonized by native pioneer species. I keep a close eye on the black cherry, crabapple, pawpaw, sycamore, sweet gum, pine, tulip poplar, American holly, and various maples that are thriving here. Despite the presence of a mature black locust mere feet from this spot, not a single locust seedling has been observed. Here in its native range where it must complete with other locally adapted species, it is not aggressive and doesn't function as a pioneer species as it reportedly does elsewhere.

Black locust is such a useful tree that I was surprised when I saw mention of the fact that in some parts it is considered a nuisance, and land managers are actively removing it. Because plants often share common names, I wondered if the haters were referring to a different tree altogether, and not the well-behaved and useful one I know as black locust. Further investigation revealed that it was indeed the same tree. Turns out black locust is considered an invasive species in much of New England and the Midwest. Woody Invasives of the Great Lakes Collaborative states that black locust is unique in that it is the only North American native tree that the organization addresses as an invasive threat. There is already enormous confusion and controversy over what constitutes invasive species, and what should be done about them. The case of black locust muddies the waters even more. In southeast Ohio it has been traditionally prized for the reasons listed above, and ODNR describes it as a valuable tree for controlling erosion on road cuts and strip mines. Across the border in Michigan however, residents visiting the state's official website will find an invasive species alert for locust and detailed instructions for control and removal.

Even the most greenhorn gardeners are often aware that invasive plants usually come from far off parts such as Europe or Asia. They dominate away from their home turf because they've left behind the living (eating and infecting) creatures that would ordinarily keep them in check. How could such a great native plant such as Robinia pseudoacacia be considered invasive? Curious folk who choose to go down the rabbit hole of black locust research should be prepared for many contradictions and confusions. The tree is recommended by some, but reviled by others, for reasons related to a particular ability shared by a small group of plants. As a nitrogen fixing plant, locust grows well in poor soils, and with the help of beneficial bacteria, converts nitrogen from the air into a form usable by plants. When the trees die or are cut back, the nitrogen becomes available for other nearby plants. For this reason, use of locusts as “nurse” trees is often advised in permaculture. A nurse tree is planted very close to a fruit or other desirable tree to protect it as it grows, then coppiced or cut down completely once the more valuable tree has become established. The nurse tree can thus theoretically provide shelter, mulch, fertilizer, firewood, and building materials, effectively stacking functions.

It is this ability to fix nitrogen, and therefore move into areas of low fertility, that makes black locust such a nuisance outside its native range. Savannas, prairies, pine barrens, and other locally prized ecosystems where native plants evolved to thrive under conditions of limited nutrients can be quickly overrun when locust arrives with its capacity to make its own fertilizer. It suckers freely and can quickly form thickets, as well as spreading by seed. Because it replaces local natives, reduces overall diversity, and creates pockets of monoculture, it has earned the classification of invasive, despite being native to neighboring states. Land managers engaged in protecting and expanding what little native prairie remains, or preserving pine barrens, for example, view the locust as a threat and act accordingly.

Readers like me who are unconcerned about the invasive nature of locust for reasons of geography might hesitate to plant it due to another characteristic: it's considered toxic to humans and livestock. Actual cases of poisoning in humans are very rare, and among livestock it is purportedly most dangerous to horses. To make things even more confusing, the tree is sometimes used for forage for sheep and other ruminants, who are either less susceptible to the toxic effects, or naturally limit their intake. I have a volunteer clump of black locust in my sacrifice paddock, which my two horses have constant access to. It appeared there before I knew about the toxic aspect of the tree, so I ignored it. Apparently the beasts are similarly indifferent as they have never touched it. I actually consider it to be a highly valuable tree for pasture shade and land stabilization because while the horses regularly strip bark and leaves from sycamores and mulberries, making them difficult to establish, the locust remains unmolested and thriving. Interestingly, it is defying the experts who claim locust doesn't tolerate a high water table and prefers dry sandy sites, by growing in regularly-flooded clay bottomland. I guess that tree didn't get the memo.

The case of black locust supports my conviction that decision making for land stewardship must always be made based on local conditions. While principles are universal, details concerning what plants and techniques to use must always be site specific. I have the benefit of being able to observe the tree within its native range, as well as conducting inadvertent experiments on unsuspecting pets. Many would-be land stewards have only second-hand information gleaned from the internet to go on. The case of Robinia pseudoacacia perfectly illustrates the pitfalls of relying on such sources. Of course, much useful knowledge can be accessed online, and in some cases the interweb is the best or only source available. But for those with the opportunity, I highly recommend liaising with locals such as old farmers or gardening clubs members. You risk getting your ears talked off, but will probably gain access to lots of free plants and a wealth of local know-how. Of course, experimentation and thinking (and growing) outside the box are always advisable. Observe your site and exercise caution in planting anything that might pose an invasive threat until you can be certain its safe, but otherwise, it's best to jump right in.