Tree of the Month, May 2024

Black Locust, False Acacia, Robinia pseudoacacia

by Martin (Mort) Schmidt, for Simply Living

Photo by Mort Schmidt.

Black Locust is an interesting and attractive part of the Ohio forests, and for a few weeks in May, an incredibly sweet-smelling addition. But its hardiness and ability to spread makes it unpopular in much of the country. 

Black Locust (Robinia pseudoacacia) has alternately-arranged pinnately-compound leaves. This describes a number of other Ohio trees, including Black Walnut, Sumacs, and Hickories, but Black Locust leaflets are distinctly smaller and more oval-shaped. The petioles (leaf stems) are approximately 6 to 12 inches long and hold 7 to 19 leaflets. The oval leaflets are 1 to 1.75 inches long and half as wide, and they have smooth edges (as described by the Audubon Field Guide to North American Trees, Eastern Region). Notice that the number of leaflets is given in odd numbers, 7 to 19, rather than even numbers, such as “8 to 20 leaflets”. This indicates that there is normally a single leaflet at the end of the stem, unlike Black Walnut, which usually has an even number of leaflets and a pair of leaflets at the end of the stem. 

Left – Alternate versus opposite arrangement. Right – Pinnately- versus palmately-compound leaves. Illustrations by Mort Schmidt.

Not surprisingly, Black Locust leaves are similar to those of Honeylocust (Gleditsia triacanthos). It’s more obvious in real life than in photos, but Honeylocust leaflets are smaller and more elongated than Black Locust leaflets. Additionally, Honeylocust leaves are often bipinnately-compound (doubly compound), with small petioles attached to larger petioles. (The difference between petioles, which are used only one year, and twigs, which remain on the tree for years, is obvious much of the year, when the new petioles are green and the older twigs they’re attached to are brown, as seen in the picture of Black Locust, below). 

Clockwise from left – Black Locust leaves, Honeylocust leaves, Kentucky Coffeetree Leaves. Honeylocust leaflets are considerably smaller and more elongated than Black Locust and Coffeetree, and Coffeetree leaflets are more pointed. Photos by Mort Schmidt.

Black Locust flowers are highly recognizable, and can even be distinguished along the roadside when you’re traveling at highway speed. The flowers are papilionaceous (resembling a butterfly), rather than radially-symmetrical like a daisy, and they are mostly white with yellow highlights. R. pseudoacacia flowers are perfect – they have both male and female parts in the same flower, but the flower is divided such that it rarely self-pollinates. The April 2024 Tree of the Month, Pawpaw, also has perfect flowers and doesn’t self-pollinate. Both trees seem to compensate for their lack of self-fertilization by spreading from root suckers. Black Locust flowers are especially distinctive because they hang in clusters like grapes, and they’re extremely fragrant and appealing to honey bees. Purple Robe Locust is a variety of locust with pink flowers that’s used in landscaping. Various sources indicate that it’s widely marketed as “Robinia pseudoacacia”, but is probably a hybrid of R. pseudoacacia and the pink-flowered shrub R. hispida

Like other legumes, Black Locust seeds are lined up in a pod. The 2- to 4-inch pods contain 5 to 15 small seeds. And like most legumes, Black Locust fixes nitrogen in the soil. 

Clockwise from upper left – Black Locust flowers, Purple Robe Locust flowers, Purple Robe flower close-up, and Black Locust seed pods.

I’ve long focused on recognizing trees by their bark. Bark has the advantages of being present throughout the year, and being recognizable on trees whose leaves are too high to see. Historically, many trees were felled in winter by farmers, because they had no crops to tend, they didn’t have to deal with bugs and weeds, and much of the trees’ sap was in the roots, giving a head start on drying the lumber. Consequently, farmers, and anyone else cutting trees in winter, needed to recognize bark. Black Locust bark is one of several Ohio trees with a criss-cross, diamond pattern, but the diamonds on R. pseudoacacia are far larger than those of White Ash (Fraxinus americana), Black Walnut (Juglans nigra), or Bitternut Hickory (Carya cordiformis). Black Locust bark and roots may also have a distinct orange color, similar to that of Mulberry (Morus) and Osage Orange (Maclura pomifera).

Left to right – Black Locust, Ash, Black Walnut, Bitternut Hickory bark. Photos by Mort Schmidt.

Black Locust is easily recognized in small trees by its 0.5 to 1.0 inch long paired thorns, separated by an angle of approximately 90 to 120 degrees. Unfortunately, the thorns are usually not visible on mature trees whose twigs are far above eye level. Honeylocust also has highly recognizable thorns, often exceeding 6 inches in length with multiple branches. Black Locust is commonly attacked by shelf fungi, and can often be recognized by its presence.

Left to right – Black Locust thorns, Honeylocust thorns, Black Locust with shelf fungus. Notice the distinct orange color in Black Locust bark. Photos by Mort Schmidt.

Black Locust typically grows to a height of 60 to 80 feet, with a trunk diameter of 1 to 2 feet. Ohio’s Champion Tree has a height of 106 feet, a trunk diameter of 5.5 feet, and a crown width of 112 feet. Black Locust needs full sun and good drainage. This, its ability to spread rapidly, its tolerance for poor soil, and the fact that it adds nitrogen to soil, makes it a common choice for land restoration after surface mining. Black Locust is native to the Eastern United States, but it grows in all of the conterminous 48 states, often as an invasive species. In fact, the Missouri Botanical Garden says, “This plant is listed as a noxious weed in one or more Midwestern states outside Missouri and should not be moved or grown under conditions that would involve danger of dissemination.” Black Locust is a short-lived tree, with a lifespan of only about 90 years, according to Illinois Wildflowers.

Robinia pseudoacacia range map. Image from U.S. Geological Survey

ID Summary: Deeply criss-crossed bark and paired thorns on twigs. Summer: Pinnately-compound leaves with 7 to 19 oval leaflets 1.5 inches long, and white flowers in bunches like grapes. Fall: seed pods similar to snow peas. 

Black Locust lumber has a Janka hardness of 1,700 lbf and a density of 0.77 g/cc, according to the Wood Database, compared to the median of my list of 73 Ohio trees of 1,075 lbf and 0.64 g/cc. This hardness makes Black Locust lumber harder than Ohio’s oaks, and only a little less hard than hickory. The wood is ring-porous, i.e., the large pores are lined up parallel to annual rings, like Ash, Oak, and Hickory, rather than spread throughout the wood (diffuse-porous), like Maple or Beech. Like White Oak, Black Locust pores are filled with a foam-like substance called “tyloses”, which impede the flow of water and probably account for their great rot resistance. R. pseudoacacia’s durability makes it a traditional choice for fence posts, railroad ties, and mine-support posts. Black Locust is also used for flooring, veneer, and turned objects. Two of its more obscure uses were the pins that held insulators for telephone and telegraph lines, and dowels for holding together wooden ships. Donald Culross Peattie’s A Natural History of Trees of Eastern and Central North America points out that Locust is in fact the stiffest, most rot-resistant, hottest burning American wood, and it’s almost entirely heartwood (the outer sapwood in most trees is typically lighter in color, softer, and far less valuable). Unfortunately, while Black Locust has high rot resistance, it’s subject to insect and fungal attacks during life, reducing its utilization as lumber. The Locust Borer (Megacyllene robiniae) is a particularly serious pest. Left alone, Black Locust would be one of our most valuable trees. (Notice that while Robinia pseudoacacia is a hot-burning firewood, the smoke occasionally causes an allergic reaction). 

Black Locust has been planted in Europe for its lovely flowers and excellent lumber since the early 1600s. Romeyn Beck Hough’s Handbook of the Trees of the Northern States and Canada (1907) says that, “…on account of its valuable wood, the delicacy and beauty of its graceful foliage and fragrant flowers it has been probably more extensively planted both in this country and Europe for ornament and use than any other North American tree…” Today, Honeylocust is used for landscaping far more than Black Locust, at least in Central Ohio, but the purple robed variety of R. pseudoacacia is seen occasionally. The City of Columbus Street Tree Inventory, 2023, lists Honeylocust as the third most common tree, while Black Locust is outside of the top 80.

One final use for Black Locust today is that its flowers are an excellent source of nectar and provide highly-regarded honey. 

Historically, according to the Native American Ethnobotany Database, the Cherokee used Black Locust in the following ways:

  • Root bark was used as an emetic,
  • Beaten root was held on the teeth for toothaches,
  • Infusions were given to cows as a tonic,
  • Wood was used to make fence posts, sills for houses, and pegs to hold together log cabins,
  • Wood was used to make bows and blowgun darts.

The Mendocino fed Robinia pseudoacacia leaves to horses, while the Wailaki fed the seeds to chickens. The Menominee used trunk bark to flavor medicines. William Harlow’s Trees of the Eastern and Central United States and Canada reports that the Natives extracted a blue dye from the leaves. 

According to Illinois Wildflowers, Black Locusts are pollinated primarily by bumblebees, but also by hummingbirds, butterflies, and moths. The foliage is food for caterpillars of the Funereal Duskywing (Erynnis funeralis), Locust Underwing (Euparthenos nubilis), Locust Leafroller (Sciota subcaesiella), and various leaf hoppers. 

Image by Mort Schmidt.

In the coming weeks, be on the lookout for Honey Locust’s clusters of white, fragrant flowers.

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You can also listen to my talks on Ohio Broadleaf Tree Identification on YouTube, Part 1 and Part 2

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