Black Locust

Black locust tree at Marsh Botanical Garden
Basic Information
Tree ID: 
Genus and species: 
The black locust is native to United States, although it has been planted all over North America, Europe, Asia, and South Africa. The species, Robinia pseudoacacia, is a deciduous angiosperm that is considered by many to be an invasive species. The wood of the black locust is very durable and used for many human purposes. Historically, Native Americans used the wood to make bows and tools. The flowers of the tree are strongly fragrant and pleasing to look at, although they often only appear for 7-10 days between May and early June.
Collected Data
Tree shape: 
Irregularly Open
Date of tree entry: 
15.90 m
Diameter at breast height: 
0.74 m

The bark of the mature black locust is colored dark reddish-brown to black to grey, depending on the tree, and is heavily grooved and scaly. The bark of younger black locust trees is colored lighter brown. Divots and grooves in the bark do not run linearly, but rather intersect and criss-cross forming diamond-like shapes. Bark near the base of the trunk and on low branches is often adorned with spines in a young tree until it has grown thicker, more durable bark to protect itself. spines, which are 1-2 cm in length, appear right next to the base of leaves and develop from stipules, or rather small leaf-like structures. The prevalence of prickles varies among trees of the species.
Twigs & branches
Black locust branches appear in an alternating pattern, which creates a zig-zag effect when looking up the tree trunk. Young branches appear coated with white silvery down, but later transform into green and then reddish-brown, just like the bark on the trunk. Twigs are stout and angular. The places on the twigs or branches where leaves jut out are often paired with spines.
The leaves of the black locust take the pinnate compound shape and exhibit alternating leaflets. A central stalk of 20-30 cm in length produces 7-9 leaflets, as well as a terminal leaflet. The leaflets themselves are 1-5 cm long and 1-3 cm wide. The black locust is deciduous so that it leaves are only visible for part of a year. In the northern hemisphere, black locust leaves turn yellow and then fall in autumn and return in spring. The leaflets are colored green and ovoid or oval shaped. The bottom-facing side of the leaves are smooth, while the top-facing side is rough.
Reproductive Structures
The black locust reproduces both sexually and asexually. Sexual reproduction relies on flowers and pollination vectors. The flowers of the black locust appear for a very limited time frame, between 7-10 days in late May or June. The flowers are colored cream white with an yellow region near the center and loosely hang in clumps. These clumps are called racemes and are 10-12 cm in length. Each black locust flower is bilaterally symmetrical. Within each flower, there are 10 stamens, 9 of which are fused, and a superior ovary containing many ovules. The flowers attract pollinators with a strongly scented nectar. Insects are the primary pollination vector for black locust trees, although hummingbirds may also pollinate. Self-pollination rarely occurs. Black locusts reproduce asexually using root suckers. Root suckers can develop spontaneously, or following damage to the tree. The suckers themselves are stems that grow above ground from existing roots and can develop into new trees. This enables black locust trees to regrow from a trunk after being cut down by humans. This also helps explain why black locusts are thought to be an invasive species.
Black locusts have a typical legume fruit which is usually 2-4 inches and a flat and smooth pea-like pod. The legumes have hard and impermeable coats and are dry/paper-like, breaking open when ripe. These fruit typically have 3-8 kidney-shaped seeds which are dark orange and brown with irregular markings. The pods typically has straight edges, is not wavy and tapers abruptly. Most fruit tend to ripen in late October and can last on the branches until early spring. There is an average of 25,500 seeds per pound.
Natural range of distribution: 
Black locusts are native to the United States but the extent of its original range is not completely known. It is however, though to be originally found in two regions-- the eastern and western region. The eastern region centered in the Appalachian Mountains and this range spanned from central Pennsylvania, southern Ohio to northwestern South Carolina and northern Georgia. The western region centered around the Ozark Plateau in southern Missouri and ranged towards the the Ouachita Mountains of central Arkansas and southeastern Oklahoma. Black locusts have been heavily distributed across North America since their origin because they have been widely planted and is able to avoid cultivation. They are native to a broad range of forest types and conditions, however, they thrive within the cove or mixed-mesophytic forests of the central and southern Appalachian region. They are well adapted to many soils but cannot survive on permanently wet; they most prefer moist, loamy soils of limestone origin. They were able to exist in all states within the native range with the exceptions of Illinois, Pennsylvania, South Carolina, and Georgia.
Origin, history, and uses: 

The wood of the black locust has been used by humans throughout history and typically was used for mine props, railroad ties and fence posts. Native Americans used the locust for their bows and other tools. It was favored heavily in colonial times and is considered one of the strongest North American timbers, helping to build Jamestown and ships during the War of 1812. It was first mentioned in an account by William Strachey in “The Historie of Travell into Virginia Britannia” as “a kind of low tree which bears a cod like to the peas, but nothing so big: we take it to be locust”, which is how the name stuck. Because they are transplanted easily, they also serves a large role in preventing soil erosion as well as for ornamental purposes (street-trees).

The black locust flowers at a relatively early age, usually around 3 years of age. Between September and October, fruits ripen and open on the tree and between September and April seeds are dispersed; the seeds can then persist in the soil for a long amount of time, even years. Full seed production begins at around age 6 and ends around age 60 with seed crops every 1-2 years. Seed production is highest between 15-40 years of age although it can surpass this. Flowering occurs usually after leaf emergence in May or June and rely on pollination by insects, usually bees. Flowering can be advanced in cities by urban heat effects. Typically, black locusts have shallow and wide-spreading roots that can be about 1 to 1.5 times the tree height.
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