top of page

BLACK LOCUST, Magnificent Monster

Black locust (Robinia pseudoacacia) is a plant that will challenge how objective you can be -- a conundrum since it is both wonderful (useful and attractive) and horrible (weedy and dangerous). Truly a Magnificent Monster. Permit me to elaborate, but first some background info. The species is a North American endemic, native to the Appalachians and to the NW as far as extreme SE Indiana. It is also native in the Ozark and Ouachita Mountains (Arkansas) and has naturalized elsewhere in North America as well as on parts of every continent except Antarctica. It is now ubiquitous in the Midwest largely due to anthropogenic activity.

Black locust is a member of the large bean family (Fabaceae aka Leguminosiae). Some people confuse honey locust (Gleditsia triacanthos) with black locust. The confusion must be the shared name (locust) because although the plants are related, they are quite different. Further, the species name (pseudoacacia) translates to false acacia. Like acacia and other bean family members, black locust has compound leaves as well as a legume fruit. Many people call these fruits pods but this causes unnecessary confusion since "pod" is commonly used by non-botanists to describe lots of other fruits. Moreover, like many other members of the family, black locust leaves are nyctinastic, i.e., they have a plant sleeping response, closing in the dark to reduce surface area, thus reducing potential water loss when photosynthesis is not possible. Nyctinastic literally means night movement. Some fabaceous species (like sensitive plant) also have a thigmotropic response, i.e., they close when touched. The prefix (thigmo) denotes the stimulus (touch) and the suffix (trophic) refers to the plant response (i.e., growth or movement). Unlike the nastic movement, trophic movements are directional. Black locust is a medium sized tree. The tallest specimens you will find are perhaps 75 feet, although I have seen boles well over three feet in diameter. The species is fast-growing and clonal but not particularly long-lived, perhaps 100 years, and specimens are 2-3x as tall as wide.

The GOOD -- I find the color and form of this fast-growing tree appealing year-round. Don't believe my assessment? Charles Sprague Sargent, preeminent dendrologist and founder of the Arnold Arboretum said, in referring to black locust, "surpassed in beauty by few American trees." Older specimens usually are gnarly and have lots of nooks and crannies used by wildlife. The branches of black locust trees are clustered near the top this allows substantial trunk exposure which givies the trees a unique profile. Further, the bark on new growth is reddish-brown, whereas the bark on older specimens is deeply furrowed and dark grey-brown. The nearly charcoal thickly ridged bark helps to accentuate the vegetation. The darkness of the bark is the basis for the common name. Black locust wood is very hard (durable) and rot resist which makes it an especially good fencing material. Robinia is also a wonderful firewood. Its BTU rating is excellent and the wood burns green. Additionally, I have seen cross sections of the stems and trunk with intact bark used to make visually stunning trivets -- there is a distinctive heartwood / sapwood color difference. The wood itself is prized by woodcarvers. Some people use and promote black locust for control erosion. It is especially useful in sunny locations with poor sandy soil due to its nitrogen fixation capacity. Black locust leaves (leaflets) start out light green and age to a rich green with a hint of blue. The blue seems more pronounced when viewing the trees from a short distance away. The blueness also varies with light conditions. And black locust has fabulous flowers which are produced in large pendant racemes, to nearly a foot in length. The flowers are zygomorphic (asymmetric), creamy white with a pale yellow center, and wonderfully fragrant -- pervasively sweet in late May to early June. The flowering clusters are reminiscent of wisteria, to which black locust is related. The calyx (sepal portion of the flower) can be rufous (see pic).

Now for the BAD -- I am very familiar with this plant and, as noted above, it has many virtues. However, I do not intentionally grow it and would NEVER recommend planting it in a residential setting. It is a MAGNIFICENT MONSTER. A few cultivars are available, notably a couple that are spineless, especially 'Inermis', but I would worry about root sprouting and reversion to the wild form. The cultivar many people seem unable to resist is contorted/twisted and somewhat dwarfed. This novelty is called 'Tortuosa.' Again, I would be concerned with reversion. Yes, 'Tortuosa' is an appealing visual curiosity, and when in flower very hard to resist, but any garden center offering black locust is acting irresponsibly and should know better. I just returned from a local garden center where I saw another form, a dwarf called Twisty Baby, also known as 'Lace Lady' but beware! Know how you should not trust some people? Well, from its pedigree and my experience I do not trust black locust, regardless of form. The bean family (again, Fabaceae) is large and fascinating, but the family is also notorious for species considered weedy and aggressive. Black locust is justifiably considered a horrible invasive species worldwide and has been banned and made illegal in many communities and some states. Part of the problem is the fact it is VERY difficult to eradicate (all living parts must be removed or killed). The eradication usually takes years and super diligence. In addition to seeds, black locust reproduces asexually by prolific root sprouting. I give the attention-getting details of one such personal experience in my Rantings book. Another characteristic of the family is the production of paired stipules at the base of the petiole. The stipules on black locust are woody spines (see middle pic above) which can be painful, and dangerous. This intimidating mechanical defense is more common and bigger on new sprout growth but are not always present. The paired spines are reddish-purple, broadbased, can exceed one-inch, and are able to pierce leather. Expect to bleed! Finally, if the weediness and spines were not enough to dissuade you, when ingested most parts of the plant are poisonous to humans and livestock, especially horses. Ironically, black locust honey is excellent -- commonly referred to as Acacia Honey -- yet when black locust leaves or fruit are immersed in ponded water they will in time render the water toxic.

As I said, the plant is both good and bad. So, are you able to say no? You can still enjoy black locust's virtues without having this formidable species growing in the soil where you live. It is common and widespread. Unfortunately, it is my experience that most people DO NOT take stock of the complete plant profile BEFORE acquiring. Focusing instead on one attribute, either favorable or not, thus the landscape aftermath from many foolish decisions as well as often missing out on some great plants (e.g., if it isn't native, I don't want it). Well, black locust is but one of many natives you should avoid. -- My August & September rants will deal with nativity, purity and plant policing.
Phellinus robiniae (see bottom pic above) is an impressive saprophytic or parasitic fungus primarily found, as the specific epithet implies, on black locust. Back in graduate school (50 years ago) I knew this fungus as Fomes / Polyporus rimosus -- that is, back when natural history and environmental assessment were considered important, worthy of university. The time will come when we will rue that deemphasis! Some of us already do. One of the common names for this woody shelf-forming basidiomycete is cracked cap polypore. The older portion, the part nearer the tree, is darker. The rim of the fungal "fruiting body" is lighter, concentrically pigmented, and much thinner. This outward part of the rim is mitotic, increasing both the diameter and volume. The fungus keeps growing both downward and away from the tree and the annual growth produces discerable rings. The pore surface (underneath), from which spores are released, is tan-brown, flat, and horizontal. The top surface is tapered upward, towards the host, and features deep cracks, hence the common name. I have observed specimens in the Midwest well over a foot in diameter! Older specimens frequently have lichens growing on the top surface. I also occasionally see weedy mosses there. And, while this fungus can be impressive, it rarely kills the tree. The mark of a wise parasite, i.e., does not decimate the host(s).
bottom of page