Robinia pseudoacacia

Robinia pseudoacacia, commonly known in its native territory as black locust, is a medium-sized hardwood deciduous tree, belonging to the tribe Robinieae of the legume family Fabaceae. It is endemic to a few small areas of the United States, but it has been widely planted and naturalized elsewhere in temperate North America, Europe, Southern Africa and Asia and is considered an invasive species in some areas. Another common name is false acacia, a literal translation of the specific name (pseudo [Greek pseudo-] meaning fake or false and acacia referring to the genus of plants with the same name).

Description

Black locust reaches a typical height of 40-100 feet (12-30 m) with a diameter of 2-4 feet (0.61-1.22 m). Exceptionally, it may grow up to 52 metres (171 ft) tall and 1.6 metres (5.2 ft) diameter in very old trees. The tree is upright with a narrow crown and straight trunk that becomes more scraggly as it ages. The dark blue-green compound leaves with a contrasting lighter underside give this tree a beautiful appearance in the wind and contribute to its grace.

Black locust is a shade-intolerant species and therefore is typical of young woodlands and disturbed areas where sunlight is plentiful and the soil is dry. In this sense, black locust can often grow as a weed tree. It also often spreads by underground shoots or suckers, which contributes to the weedy character of this species. Young trees are often spiny, but mature trees often lack spines. In the early summer black locust flowers; the flowers are large and appear in large, intensely fragrant clusters reminiscent of orange blossoms. The leaflets are prone to bending in wet weather or at night (nyctinasty), since the whole leguminous family has a nightly habit of changing their position.

Although similar in general appearance to the honey locust, the black locust lacks that tree’s characteristic long branched thorns on the trunk, having instead the pairs of short prickles at the base of each leaf; the leaflets are also much broader than honey locust. It may also resemble Styphnolobium japonicum, which has smaller flower spikes and lacks spines.

Detailed description

Uses

Cultivation

Black locust is a major honey plant in the eastern US, and has been planted in European countries. In many European countries, it is the source of the renowned acacia honey. Flowering starts after 140 growing degree days. However, its blooming period is short (about 10 days) and it does not consistently produce a honey crop year after year. Weather conditions can also have an impact on how much nectar is collected. Ohio, for instance, has good locust honey flow in only one of five years.

It can be easily propagated from roots, softwood, or hardwood and this allows for easy reproduction of the plant. Cultivars may also be grafted as this ensures the parent and daughter plant will be genetically identical.
R. pseudoacacia can be grown in areas that are highly disturbed as an erosion control or plant. The shallow, aggressive roots help hold onto the soil; the tree grows quickly and on poor soils due to its ability to fix nitrogen.

Black locust has nitrogen-fixing bacteria on its root system, so it can grow on poor soils and is an early colonizer of disturbed areas. With fertilizer prices rising, the importance of black locust as a nitrogen-fixing species is also important. Mass fertilizer application in agriculture and forestry has become more expensive. Therefore, nitrogen-fixing tree species and shrubs are becoming increasingly important in managed forest.

Black locust is also planted for firewood because it grows rapidly, is highly resilient in a variety of soils, and it grows back rapidly from its stump after harvest by using the existing root system. Coppicing is also available.

It tolerates polluting well and is commonly planted in Europe along streets and parks.

Cultivars

Several cultivars exist but ‘Frisia’ seems to be one of the most planted ones.

Wood

The wood is extremely hard, being one of the hardest woods in Northern America with a Janka hardness test of 1,700 lbf (7,560 N). It is extremely resistant to rot and durable. This wood is prized for furniture and flooring as well as paneling and fence posts. Black Locust is a highly durable, organic wood product, that does not require chemical treatment to preserve its beauty for 50+ years. Wet, newly cut planks have an offensive odor which disappears with seasoning. Black locust is still in use in some rustic handrail systems. In the Netherlands and some other parts of Europe, black locust is one of the most rot-resistant local trees, and projects have started to limit the use of tropical wood by promoting this tree and creating plantations. The wood can last more than 100 years in soil thanks to the flavonoids found in its heartwood.

Black locust is highly valued as firewood for wood-burning stoves; it burns slowly, with little visible flame or smoke, and has a higher heat content than any other species that grows widely in the Eastern United States, comparable to the heat content of anthracite. For best results, it should be seasoned like any other hardwood, but black locust is also popular because of its ability to burn even when wet. In fireplaces, it can be less satisfactory because knots and beetle damage make the wood prone to “spitting” coals for distances of up to several feet.^{[citation needed]} If the black locust is cut, split, and cured while relatively young (within 10 years), thus minimizing beetle damage, “spitting” problems are minimal^{[citation needed]}.
In 1900, Robinia pseudoacacia‘s value was said to have been almost destroyed in all of the United States. This is because it lives in mountain forests and is inhabited by locust borers that gnaw on the trunk and branches. Were it not for these insects, it would be one of the most valuable timber trees that could be planted in the northern and middle states. Young trees grow quickly and vigorously for a number of years, but soon become stunted and diseased, and rarely live long enough to attain any commercial value.

Food and medicine

In traditional medicine of India, different parts of R. pseudoacacia are used as laxative, antispasmodic, and diuretic.

Sometimes, the flowers of Romania are used to make a sweet and fragrant jam. This means manual harvesting of flowers, eliminating the seeds and boiling the petals with sugar, in certain proportions, to obtain a light sweet and delicate perfume jam.

Although the bark and leaves are toxic, various reports suggest that the seeds and the young pods of the black locust are edible. The shelled seeds can be harvested from summer to fall and are both safe and delicious. Due to the small size of the seeds, shelling them efficiently can prove tedious and difficult. In France, Italy and Romania, R. pseudoacacia flowers are eaten as beignets after being coated in batter and fried in oil; they are also eaten in Japan, largely as tempura.

Ecology

When growing in sandy areas this plant can enrich the soil by means of its nitrogen-fixing nodules, allowing other species to move in. On sandy soils black locust may also often replace other vegetation which cannot fix nitrogen.

Black locust is a typical early successional plant, a pioneer species, and it grows best in bright sunlight and does not handle shade well. It specializes in colonizing disturbed areas and edges of woodlots before it is eventually replaced with taller or more shade-tolerant species. It prefers dry to moist limestone soils but will grow on most soils as long as they are not wet or poorly drained. This tree tolerates a soil pH range of 4.6 to 8.2. Within its native range it will often grow on soils of Inceptisols, Ultisols, and Alfisols groups. Black locust is not able to grow on clayey, compacted or eroded soils. Black locust is a part of the Appalachian mixed mesophytic forests.

Black locust is not a particularly valuable plant for wildlife^{[citation needed]}, but does provide valuable cover when planted on previously open areas. Its seeds are also eaten by bobwhite quail and other game birds and squirrels. Woodpeckers may also nest in the trunk since older trees are often infected by heart rot.

Taxonomy

The black locust is a plant from the subfamily of Faboideae in the family of legumes (Fabaceae) and is a relative of the pea and bean.

After its species name, the black locust is often called “false Acacia” or “false Acacia”. Although it is not particularly closely related to the acacia which belongs to a subfamily of the mimosa family (Mimosoideae), both species are similar in the form of their feathered leaves and thorns but the flower shapes are very different. It is nearly impossible to confuse the species of the two genera at higher latitudes. Acacias are only found in tropical and subtropical areas, and they do not thrive in cooler climates.