Robinia pseudoacacia 'Purple Robe': 'Purple Robe' Black Locust

Introduction

This cultivar of black locust probably grows to about 40 feet tall with a spread of 20 to 35 feet. The upright, rounded growth and short, irregular branches form a tighter canopy than the species and cast medium shade below the tree, allowing a lawn to grow. 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 four to eight-inch-long, dense clusters of extremely fragrant, one-inch rose to pink blossoms (similar to sweet-peas) which are literally "alive" with the bustling activity of visiting bees. The honey which is produced from the species is quite delicious and sought-after. The two to four-inch-long, dark red to black, leathery seeds pods which follow will persist on the trees throughout the winter.

General Information

Scientific name: Robinia pseudoacacia

Pronunciation: roe-BIN-nee-uh soo-doe-uh-KAY-shuh

Common name(s): 'Purple Robe' black locust, `Purple Robe' 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

Description

Height: 30 to 50 feet

Spread: 20 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

Flower

Flower color: lavender

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; acidic; alkaline; well-drained

Drought tolerance: high

Aerosol salt tolerance: high

Other

Roots: can form large surface roots

Winter interest: yes

Outstanding tree: no

Ozone sensitivity: unknown

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, 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, black locust 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 of the species 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. This will probably not be as much a problem on this cultivar as it is on the species.

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.

Other cultivars include: 'Erecta'—upright form; 'Frisia'—yellowish leaves in early summer.

Propagation is by cuttings.

Pests

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

Diseases

This tree is susceptible to canker, leaf spot, and powdery mildew.