Black Locust is a fast growing introduced (to Minnesota), deciduous, medium size tree, growing 40 to 70 feet in height and up to 2 feet in diameter, with an irregular crown of crooked somewhat brittle branches. The trunk is usually straight. It is considered invasive outside of its native range in the eastern states.
The bark is gray to light brown, thick and fibrous, and with age becoming heavily furrowed into long forking ridges.
Twigs are zigzag shape, green initially, then brown to red-brown with numerous light colored lenticels and with a pair of 1/4 to 1/2 inch stout spines at the leaf scars. The buds are buried beneath the leaf scars. Older twigs may have dropped the spines.
Leaves are alternate, pinnately compound, 6 to 12 inches long, with 7 to 19 leaflets that are oval, about one to two inches long, with a very short stalk, thin, with smooth margins and a tiny bristle tip. The terminal leaflet is more ovate than the laterals. The upper side is dark blue-green and the under is paler, somewhat hairy when young. Fall color is yellow.
Flowers: The tree usually has perfect flowers on the same plant. These hang in showy hanging clusters (racemes), 4 to 8 inches long from the leaf axils near the tip of new shoots. They are composed of 1 inch long pea-like flowers that have 5 unequal white petals; the large upper petal, the 'standard', has some yellow near the base. There are two lateral petals and two forming a keel. The yellowish-green to reddish-green calyx has 5 teeth. The stamens (several) and single style are hidden within the keel petals. Pollination is by insects that can penetrate the keel petals. The flowers are fragrant and form after the leaves.
Seed: Fertile flowers mature in the autumn forming a brown, flattened legume pod, 3 to 4 inches long, containing 4 to 8 kidney shaped smooth 5 mm long reddish-brown seeds. Pods normally remain attached to the tree into the winter. Seeds are dispersed by falling from the tree and to a lesser extent by birds. They have a long viability period. Trees must be around 6 years old to produce seed (which is relatively fast for a tree).
Habitat: Black Locust has a shallow but spreading root system that is nitrogen fixing and that can send out runners to produce clonal plants. It prefers a well drained, dry to mesic site with looser soils and full sun. Due to its vigorous root system it invades disturbed areas via suckering and runners. It grows quickly compared to Oak, Elm or Beech.
Names: The genus name, Robinia is named for French Royal Gardener Jean Robin (1550-1629) who cultivated specimens received from Canada in the New World. The species name, pseudoacacia, is literally the alternate common name for False Acacia. The author name for the plant classification, 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: There are not any plants that would make for a difficult comparison. One of the few that has spines on the twigs is Prickly Ash, but there the buds are not buried but clearly visible and the flowers are much different.
Above: Flower clusters form after the leaves have formed. Drawing from North American Sylva by Franccois Michaux & Thomas Nuttall
Below: 1st photo - A young tree. 2nd photo - An older tree in flower. 3rd photo - The pinnately compound leaves have 7 to 19 oval leaflets.
Above: The Full leaf. Below: 1st photo - The terminal leaflet of the compound leaf is more ovate than the lateral leaflets. 2nd photo - Black Locust has perfect flowers with 5 unequal white petals (pea-like) and a reddish-green 5-pointed calyx, in showy drooping clusters from the tips of new growth.
Below: 1st photo - Detail of the petal arrangement. Note the yellow on the erect "standard". 2nd photo - The stamens, pistil and style are hidden with the two keel petals.
Below: Fertile flowers mature in the autumn forming a brown, flattened legume pod, 3 to 4 inches long, containing 4 to 8 kidney shaped smooth reddish-brown seeds. Pods normally remain attached to the tree into the winter.
Below: 1st photo - A pair of stout spines grow on newer twigs at the leaf scars. 2nd photo - Older twigs are reddish-brown with light colored lenticels. Buds are buried in the leaf scars. Twigs form a zigzag shape.
Below 1st photo - the bark of the young tree is a lighter color and shows the elongated lenticels 2nd photo - the bark of the older trees is darker and becomes heavily furrowed into long forking ridges. 3rd photo - Flower clusters form after the leaves have formed.
Notes: Black Locust has been in the Garden on and off for some time. Martha Crone noted it in bloom in her 1938 Garden Log. It was not listed on her 1951 census, nor on the 1986 census, but was present on the 2009 census. It was originally growing on the east coast of the U.S., discovered by the colonists at Jamestown in 1607 although Michaux - see below- states that it had already been shipped back to England and was being cultivated about 1601. Black locust’s native range follows the Appalachian Mountains from Pennsylvania to Alabama, and a secondary population exists primarily in the Ozark Mountains. It has since spread extensively throughout the U.S. and parts of southern Canada, most likely by plantings as the tree has numerous good points. Within Minnesota it has been found in about 28 counties in widely scattered parts of the state, including Hennepin. The Minnesota DNR, however lists it as a non-native invasive terrestrial plant.
Uses: Wood of Black Locust is hard, strong and durable and can take a fine polish. It is used extensively for wood that is contact with the ground such as posts, landscape timbers and in mining, exceeding the life of Cedar. It has been widely planted both in North America and in Europe for landscape purposes, erosion control and for timber. Heartwood that is reddish is considered the must durable. In earlier times the wood was used for 'tree-nails', that is the wooden pins that attached side planks of ships to the frames, as the wood hardened with time instead of decaying.
Over the years, the tree has had great acceptance and then opposition, although no one disputed the beauty of the foliage and fragrance of the flowers. After giving a lengthly discourse on the tree, botanist Francois Michaux, in his North American Sylva of 1841 wrote the following:
"If I may be allowed to give an opinion, I should say that its principal advantages consist in the rapidity of its growth, and in the excellent qualities by which it wood is fitted for the most important uses. To these must be added another property by which it is distinguished from other trees of rapid growth, and which has not been placed in a sufficiently striking light by the authors who have treated of the Locust: It is that of beginning from the third year to convert sap into perfect wood; which is not done by the Oak, the Chestnut, the Beech, and the Elm, till after the tenth or the fifteenth year. Hence, if all these species were planted at the same time upon good land, in twenty-five or thirty years the Locusts, already one-third larger in general than the others, and often twice as large, would be found almost wholly composed of heart, and would be of sufficient dimensions for the various uses to which their wood is adapted; while the others, besides being too small at this age to be employed with advantage, would have only half the diameter of the trunk converted into perfect wood. This is a most important consideration; for it is well known that every species of wood must be deprived of the sap before it is used, as this part is subject to become worm-eaten if it is sheltered, and to decay if it is exposed to the air."
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"