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Publication #ENH-728

Robinia pseudoacacia: Black Locust1

Edward F. Gilman and Dennis G. Watson2

Introduction

A fast-growing native deciduous tree, black locust is capable of reaching 70 to 80 feet in height but is more often seen 30 to 50 feet tall with a spread of 20 to 35 feet. The upright growth and short, irregular branches form an open canopy and cast light shade below the tree, allowing a lawn to thrive. The 6- to 14-inch-long, dull, blue/green leaves, made up of multiple leaflets, are some of the last to appear in spring and often drop early in the autumn, just barely fading to a sickly yellow/green before dropping. For approximately a 10-day period in late spring, the trees are festooned with 4- to 8-inch-long, dense clusters of extremely fragrant, one-inch white blossoms (similar to sweet-peas) which are literally "alive" with the bustling activity of visiting bees. The honey which is produced is quite delicious and sought-after. The 2- to 4-inch-long, dark red to black, leathery seeds pods which follow will persist on the trees throughout the winter.

Figure 1. 

Middle-aged Robinia pseudoacacia: black locust


Credit:

Ed Gilman, UF/IFAS


[Click thumbnail to enlarge.]

General Information

Scientific name: Robinia pseudoacacia
Pronunciation: roe-BIN-nee-uh soo-doe-uh-KAY-shuh
Common name(s): Black locust, common locust
Family: Leguminosae
USDA hardiness zones: 4A through 8B (Fig. 2)
Origin: native to North America
Invasive potential: weedy native
Uses: reclamation; shade
Availability: somewhat available, may have to go out of the region to find the tree

Figure 2. 

Range


[Click thumbnail to enlarge.]

Description

Height: 40 to 70 feet
Spread: 25 to 35 feet
Crown uniformity: irregular
Crown shape: upright/erect, oval
Crown density: open
Growth rate: fast
Texture: fine

Foliage

Leaf arrangement: alternate (Fig. 3)
Leaf type: odd-pinnately compound
Leaf margin: entire
Leaf shape: elliptic (oval), oblong, obovate
Leaf venation: pinnate
Leaf type and persistence: deciduous
Leaf blade length: less than 2 inches
Leaf color: blue or blue-green, green
Fall color: yellow
Fall characteristic: not showy

Figure 3. 

Foliage


[Click thumbnail to enlarge.]

Flower

Flower color: white/cream/gray
Flower characteristics: very showy

Fruit

Fruit shape: pod or pod-like
Fruit length: 1 to 3 inches, 3 to 6 inches
Fruit covering: dry or hard
Fruit color: black, red
Fruit characteristics: attracts birds; not showy; fruit/leaves a litter problem

Trunk and Branches

Trunk/bark/branches: branches don't droop; showy; typically one trunk; no thorns
Pruning requirement: needed for strong structure
Breakage: susceptible to breakage
Current year twig color: brown
Current year twig thickness: thin
Wood specific gravity: 0.69

Culture

Light requirement: full sun, partial sun, or partial shade
Soil tolerances: clay; sand; loam; alkaline; acidic; well-drained
Drought tolerance: high
Aerosol salt tolerance: high

Other

Roots: can form large surface roots
Winter interest: yes
Outstanding tree: no
Ozone sensitivity: tolerant
Verticillium wilt susceptibility: susceptible
Pest resistance: sensitive to pests/diseases

Use and Management

These seeds are widely dispersed by birds and other wildlife, and along with the root suckering and invasive root system, black locust can spread into surrounding landscapes. This feature, along with the thorns found along the branches, should be considered when placing black locust in the ornamental garden. It is probably best saved for the reclamation project or roadside planting where trees receive no maintenance. If left to its own devices, it will form dense thickets, even on the poorest soils, a fact which makes it quite useful in reclamation applications.

Although the wood of black locust is reputed to be extremely strong and durable (pioneers used it to fashion nails for building ships and houses), the branches are brittle and subject to damage in high winds. This may be due largely to the tendency for the branches to form a narrow angle with the trunk and to grow aggressively relative to the trunk forming embedded bark. This can be partially corrected by pruning the major branches so they grow to no more than about half the size of the trunk.

Able to tolerate drought, salt, and poor soil, black locust will grow in full sun or partial shade on almost any soil acid or alkaline except that which is permanently wet. A tough tree which should be saved for the toughest sites. Not for general use in urban areas due to borer problems unless located in a moist, well-drained site with plenty of soil space.

Cultivars include: 'Erecta'—upright form; 'Frisia'—yellowish leaves in early summer; and 'Purple Robe'—lavender flowers. 'Purple Robe' is gorgeous in full bloom

Propagation is by seed or cuttings.

Pests

Locust borer is a serious pest although borer-resistant clones are now being developed. It is also bothered by carpenterworm, locust leaf miner, and scales. Leaf miner is a universal problem, and the trees along the highways in the south can be seen in summer riddled with damage from this pest.

Diseases

Black locust is susceptible to canker, leaf spot, and powdery mildew.

Footnotes

1.

This document is ENH-728, one of a series of the Environmental Horticulture Department, UF/IFAS Extension. Original publication date November 1993. Revised December 2006. Reviewed February 2014. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Edward F. Gilman, professor, Environmental Horticulture Department; and Dennis G. Watson, former associate professor, Agricultural Engineering Department, UF/IFAS Extension, Gainesville, FL 32611.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.