Black locust trees (Robinia pseudoacacia, USDA zones 4 through 8) are at their best in late spring, when trailing clusters of 5-inch (13 cm.), fragrant flowers bloom at the tips on new branches. The flowers attract honey bees, which use the nectar to make excellent honey. Growing black locust trees is easy, but they can become weedy if you aren’t diligent about removing suckers. Read on for more black locust information.
| Common name | Black Locust, Common Locust, False Acacia, Green Locust, Pea Flower Locust, White Locust, Yellow Locust |
| Botanical name | Robinia pseudoacacia |
| Family | Fabaceae |
| Species | pseudoacacia |
| Origin | Eastern and central United States |
| Life cycle | Woody |
| Plant type | Native Plant |
| Hardiness zone | 3, 4, 5, 6, 7, 8 |
| Sunlight | Full Sun |
| Maintenance | Medium |
| Soil condition | Clay |
| Soil ph | Acid |
| Drainage | Well-Drained |
| Growth rate | Fast |
| Spacing | 24 – 60 ft. |
| Harvest time | Fall |
| Flowering period | Spring |
| Height | 5- 50 ft. |
| Width | 5- 50 ft. |
| Flower color | White |
| Leaf color | Blue |
| Fruit color | Brown, Copper |
| Fruit type | Legume |
| Leaf benefit | Showy |
| Flower benefit | Fragrant |
| Garden style | Butterfly Garden |
| Uses | Naturalized Area |
I. Appearance and Characteristics
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 native 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, such as the temperate east coast of Australia where the cultivar ‘Frisia'(Golden Robinia) was widely planted as a street tree before being classed as a weed.
Another common name is false acacia, a literal translation of the specific name (pseudo [Greek ψευδο-] meaning fake or false and acacia referring to the genus of plants with the same name).
The roots of black locust contain nodules that allow it to fix nitrogen, as is common within the pea family. Trees reach a typical height of 12–30 meters (40–100 feet) with a diameter of 0.61–1.22 m (2–4 ft). It is a very upright tree with a straight trunk and narrow crown that grows scraggly with age. The bark is a reddish black and gray and tinged with red or orange in the grooves; it is deeply furrowed into grooves and ridges which run up and down the trunk and often cross and form diamond shapes. The branches are typically zig-zaggy and may have ridges and grooves or may be round. When young, the branches are at first coated with white silvery down; this soon disappears, and they become pale green and afterward reddish or greenish brown.
The dark blue-green leaves are lighter on the underside, and are compound, meaning that each leaf contains many small leaflike structures called leaflets, which are roughly paired on either side of the stem that runs through the leaf (rachis). There is typically one leaflet at the tip of the leaf (odd pinnate), and the leaves are alternately arranged on the stem. Each leaf is 15–36 cm (6–14 in) long and contains 9–19 leaflets, each being 2.5–5.1 cm (1–2 in) long, and .65–1.9 cm (1⁄4–3⁄4 in) wide.
The leaflets are rounded or slightly indented at the tip and typically rounded at the base. The leaves come out of the bud folded in half, yellow green, covered with silvery down which soon disappears. Each leaflet initially has a minute stipel, which quickly falls, and is connected to the (rachis) by a short stem or petiole. The leaves are attached to the branch with slender hairy petioles which are grooved and swollen at the base. The stipules are linear, downy, membranous at first and occasionally develop into prickles. The leaves appear relatively late in spring, and turn a clear pale yellow in autumn. The leaflets fold together in wet weather and at night (nyctinasty), as some change of position at night is a habit of the entire leguminous family.
Young trees are often spiny, especially on root suckers and branches near the ground; mature trees often lack spines. R. pseudoacacia is quite variable in the number of spines present, as some trees are densely prickly and other trees have no prickles at all. The spines typically remain on the tree until the young thin bark to which they are attached is replaced by the thicker mature bark. They develop from stipules (small leaf-like structures that grow at the base of leaves), and since stipules are paired at the base of leaves, the spines will be paired at the bases of leaves. They range from .65–2 cm (1⁄4–3⁄4 in) in length and are somewhat triangular with a flared base and sharp point. Their color is a dark purple and they adhere only to the bark.
The winter buds are minute, naked (having no scales covering them), three or four together, protected in a depression by a scale-like covering lined on the inner surface with a thick coat of tomentum and opening in early spring. When the buds are forming they are covered by the swollen base of the petiole. The large flowers open in May or June for 7–10 days, after the leaves have developed.
They are arranged in loose drooping clumps (racemes) which are typically 10–20 cm (4–8 in) long. The flowers themselves are cream-white (rarely pink or purple) with a pale yellow blotch in the center and imperfectly papilionaceous in shape. They are about 2.5 cm (1 in) wide, very fragrant, and produce large amounts of nectar. Each flower is perfect, having both stamens and a pistil (male and female parts). There are 10 stamens enclosed within the petals; these are fused together in a diadelphous configuration, where the filaments of 9 are all joined to form a tube and one stamen is separate and above the joined stamens.
The single ovary is superior and contains several ovules. Below each flower is a calyx which looks like a leafy tube between the flower and the stem. It is made from fused sepals and is dark green and may be blotched with red. The pedicels (stems which connect the flower to the branch) are slender, 1.3 cm (1⁄2 in), dark red or reddish green.
The fruit is a typical legume fruit, being a flat and smooth pea-like pod 5.1–10.2 cm (2–4 in) long and 1.3 cm (1⁄2 in) broad. The fruit usually contains 4–8 seeds. The seeds are dark orange brown with irregular markings. They ripen late in autumn and hang on the branches until early spring. There are typically 25,500 seeds per pound. Cotyledons are oval in shape and fleshy.
The bark, leaves, and wood are toxic to both humans and livestock. Important constituents of the plant are the toxalbumin robin, which loses its toxicity when heated, and robinin, a nontoxic glucoside.
II. How to Grow and Care
Sunlight
Black locust likes plenty of sunlight, so strive to provide the tree with no less than 6 hours of light every day. plant it on the sunny side of the courtyards to ensure the best results. The shade will inhibit its growth, but a short-term stay in a partially-shaded environment will have little impact. It should be noted that the seedling grows well under proper shading because seedling photosynthesis is inherently imperfect. The tree is easily burned by blazing light, so do not expose it to direct sunlight in summer; rather, shade it by placing it in or near a structure like a shed.
Watering
Black locust is somewhat drought-resistant. However, excessive water can make the tree prone to pests and diseases like root rot. Therefore, it should not be watered too often and the soil should be kept slightly dry. Water the tree once a day after transplanting, and switch to once a month after the seedling is established.
As for adult trees, remember to water them during their flowering period in late spring and early summer. After one-time irrigation in early autumn, suspend watering for a short period to promote the growth of strong trees’ trunks that can safely overwinter. Water again just before winter begins, and then no water is needed until spring comes.
Additionally, watering frequency should be adjusted according to weather conditions. For example, there is little rain in the early spring; the weather is dry, and transpiration is strong. As a result, it is necessary to replenish water appropriately. By contrast, during the rainy season black locust is sensitive to waterlogging and must be kept drained to prevent root rot.
Soil
Black locust doesn’t have strict requirements for soil and has very strong adaptability thanks to a deep root system. The tree prefers thick, sandy, fertile, and well-drained soil. If planted in barren soil or clay with accumulated water and poor ventilation, it will grow poorly. It is not sensitive to soil pH, so it can cope with a pH range of 5.1-7.8.
Fertilizing
Black locust grows fast, so sufficient nutrients are required. Apply tree plant food once every 15-20 days for small trees. Use nitrogen fertilizer in the early stages of growth, and use nitrogen and phosphorus compound fertilizer in the later stages. The peak growth period of adult trees is in summer, so be sure to fertilize in early summer.
A balanced NPK fertilizer can be used, and diluted liquid fertilizer can also be poured for irrigation and fertilization. At the end of fall, a decomposed organic fertilizer can be applied once before winter to provide nutrients for the second year of tree growth. If the soil is rich in nutrients, there is no need to apply fertilizer.
Planting Instructions
Seedlings of black locust can be purchased from garden centers and transplanted to your garden. 5 to 10 cm seedlings should be selected for transplanting in spring, and it is better to be transplanted on a cloudy day. On a sunny planting day, avoid noontime high temperatures to prevent water loss, and this can impact seedlings survival. plant them in a sunny site, but use a shading net to protect small trees from too-strong light in summer.
The planting pit needs to be 20 to 30 cm deep, and organic fertilizer should be applied and mixed with the original, loosened soil. If black locust will be transplanted again after this planting, the distance between the two small trees should be about 50 cm. If this is a tree’s final planting site, give it a distance of at least 4 m from other trees. plant the seedling, backfill with soil, compact the soil, and water it. Keep the soil moist or pile up some soil at the base of the trunk to maintain moisture around the seedling.
Pruning
To straighten the trunk of black locust, prune the side branches that grow to the left or right during the first winter or the second spring after planting. Remove 1/3 of the branches that grow upward until the trunk height reaches 5 to 6 m. Quickly prune off any new buds and branches growing out at the trunk base as early as possible.
To shape the canopy, you can also prune in the summer. Over long branches should be shortened, over dense branches should be thinned, and diseased branches and dry branches should be trimmed off. It is best not to cut off the branch from its base, but instead to leave a part of it. This prevents other branches from being easily broken by the wind, causing weak growth.
Propagation
Black locust can be propagated by cutting, a quick method that retains the advantageous characteristics of this tree. Cutting propagation is best carried out in spring. Select one-year-old branches, cut from their bases, and cut again about 20 cm from the base to create good cuttings. Leave at least 2 buds on each branch, retain 3-5 leaves, as they are beneficial to photosynthesis and synthesizing nutrients.
Cut the ends of the branches obliquely, and soak the incisions in a rooting powder solution for 30 mins. Then, cover them with plastic film to keep them warm and spray water 1-2 times a day to keep the substrate moist. After a few days, the cuttings will take root and grow into new plants.
For black locust, seed propagation is more difficult than other methods. The seeds have thick and hard coats, so they must be treated for accelerated germination before sowing.
III. Uses and Benefits
Garden Use
The black locust is prized for its fragrant flowerheads and burgundy color. It is often chosen for planting in difficult areas where other trees may not grow as well, due to its suckering habit and resistance to pollution. Companion plants for the black locust may include the Blanket flower, Dwarf arborvitae, and Mock orange.
Food and medicine
- Honey
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 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 have quite an effect on the amount of nectar collected, as well; in Ohio for example, good locust honey flow happens in one of five years.
- Other produce
In traditional medicine of India, different parts of R. pseudoacacia are used as laxative, antispasmodic, and diuretic.
In Liguria, Italy and Romania the flowers are sometimes used to produce a sweet and perfumed 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. Shelled seeds are safe to harvest from summer through fall, and are edible both raw and boiled. 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.
Erosion control
R. pseudoacacia is considered an excellent plant for growing in highly disturbed areas as an erosion control plant. Black locust’s shallow, aggressive roots help hold onto the soil, and nitrogen-fixing bacteria on its root system allow it to grow on poor soils, making it an early colonizer of disturbed areas. Obviating the mass application of fertilizers, black locust and other nitrogen-fixing tree and shrub species have gained importance in managed forestry.
Firewood
Black locust is planted for firewood, as it grows rapidly, is highly resilient in a variety of soils, and it grows back rapidly after harvest from the existing root system.
Construction wood
This native hardwood is also prized by North American shipwrights for making rot-resistant trunnels in traditional wooden ship construction.
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