This tree is 40-80' tall at maturity, forming a trunk about 1½-3'
across and an irregular crown with spreading to ascending
branches. Trunk bark is variable, depending on the age of a tree; young
trees have brown bark that is slightly rough and shallowly furrowed,
while older trees have thick gray bark that is very rough and deeply
furrowed with forking ridges. The bark of branches is gray to brown and
more smooth, while twigs are brown to reddish brown, smooth, and
glabrous with scattered white lenticels. Pairs of stout thorns about
¼-½" long occur along the twigs and branches near the axils of former
leaves. Young shoots are light green and slightly pubescent.
Alternate compound leaves about 6-14" long occur along the twigs and
shoots; they are odd-pinnate with 7-19 leaflets. The stalks and
leaflets of young compound leaves may be slightly pubescent, but they
become glabrous with age; they are initially yellowish green. At
maturity, individual leaflets are 1-2" long and ½-¾" across; they are
medium green or blue-green, oblong or oblong-elliptic in shape, and
smooth along their margins. Each leaflet has a tiny bristle-like tip,
while at its base there is a short petiolule (basal stalklet)
less than 1/8" (3 mm.) long.
The flowers are produced in drooping racemes about 4-7"
long. The central stalk and pedicels of each raceme are pale green and
glabrous to slightly pubescent. Individual flowers are ¾-1" long,
consisting of 5 white petals, a tubular calyx with 5 teeth,
several hidden stamens, and an ovary with a single style. The petals
are arranged in a pea-like floral structure consisting of an upright
banner and a pair of forward-projecting wings that enclose a keel.
There is a small patch of yellow at the base of the banner. The calyx
is light green, yellowish green, pinkish green, or brownish
green; it is either
glabrous or slightly pubescent. The blooming period occurs from late
spring to early summer for about 2 weeks after the leaves are fully
developed. The flowers have a strong sweet fragrance. Afterwards,
fertile flowers are replaced by drooping seedpods that become 2-4" long
at maturity during the autumn. Mature seedpods are dark brown,
oblongoid in shape, flattened, and either straight or slightly curved;
each seed pod contains 4-14 seeds. The seeds are a little less than ¼"
long, reniform in shape, dark brown, and somewhat flattened. They are
dispersed primarily by gravity, falling to the ground not far from the
mother tree. The woody root system produces lateral roots that are
usually shallow and widely spreading, although at dry sites it can
develop a taproot with deep lateral roots. The root system also
produces long underground runners, from which clonal offsets are
produced. As a result, this tree often occurs in clonal colonies. The
deciduous leaves usually turn yellow during the autumn.
The preference is full sun, mesic to dry-mesic conditions, and a
relatively loose soil containing loam, silty loam, or sandy loam. The
root system of this tree fixes nitrogen into the soil. Black Locust
develops rapidly while it is young, producing seedpods in as
little as 6 years. However, it is relatively short-lived with a maximum
longevity of about 90 years.
Locust is occasional throughout Illinois (see Distribution
tree is native to the southern tip of Illinois, otherwise it is either
adventive from the south or an escape from cultivation. Habitats
include upland woodlands, well-drained areas of bottomland woodlands,
disturbed open woodlands, wooded slopes, savannas and sandy savannas,
thickets, stabilized sand dunes, strip-mined areas, and roadsides. In
Illinois, this tree is an invader of sand prairies and sandy
savannas. It is also cultivated as a landscape tree in both suburban
and urban settings. Because of its shade intolerance, Black Locust is
more common in secondary growth woodlands than old growth forests;
habitats with a history of occasional disturbance are preferred.
The flowers are cross-pollinated primarily
although the Ruby-throated Hummingbird, honeybees, butterflies, and
moths also visit the flowers. The floral reward of these visitors is
nectar. According to Robertson (1929), butterflies and moths are
not effective at cross-pollination, unlike the bees and hummingbird.
The foliage of Black Locust is a food source for caterpillars of
the skippers Erynnis
(Funereal Duskywing), Erynnis icelus
(Dreamy Duskywing), Erynnis
(Zarucco Duskywing), and Epargyreus
(Silver-spotted Skipper). The caterpillars of
several moths also
feed on the foliage; they include such species as Dasylophia
(Black-spotted Prominent), Euparthenos nubilis
(Orange Wing), Pero
(Honest Pero), Sciota
(Locust Leafroller), and Zale
(Black Zale). See the Moth Table
complete list of
these species. Other insects that feed on this tree include the
, and Vanduzea
; the aphid Aphis
the plant bugs Lopidea
(Black Locust Plant Bug) and Orthotylus
; larvae of the wood-boring beetles Agrilus egenus
(Locust Borer); larvae of the seed beetle Amblycerus robiniae
Seed Borer) and the leaf beetle Odontota
(Locust Leafminer); larvae of Dasineura pseudacaciae
(Black Locust Gall Fly); and many others. See the Insect Table
more complete list of these species.
Notwithstanding their toxicity,
the seeds are a minor food source of the Thirteen-Lined Ground
Squirrel and some upland gamebirds, including the Bobwhite and
Ring-Necked Pheasant. While White-Tailed Deer occasionally browse on
the foliage of Black Locust without apparent ill effects, it is toxic
to such domesticated animals as horses, cattle, and sheep. Because
Black Locust is vulnerable to heart-rot fungi, older trees often
develop cavities that become dens for bats, screech owls, and
woodpeckers. Clonal colonies of Black Locust can provide significant
cover for the White-Tailed Deer, Coyote, Red Fox, Striped Skunk,
Pheasant, Bobwhite, and other wildlife in semi-open areas.
Along an abandoned railroad track near Urbana,
Illinois, and border of a yard in NW Ohio. The photographs were taken
during early to late spring.
a result of the last glaciation, the range of Black Locust
was confined to relatively small areas in or around the
mountains of the east and the Ozark mountains further to the west.
Since the time of European settlement, however, the range of this tree
has expanded significantly into many areas of the United States,
including central and northern Illinois. This dramatic expansion of
range is largely the result of its cultivation as a landscape tree,
from which it occasionally escapes and naturalizes in surrounding
areas. The flowers of this tree are quite showy and fragrant, but
short-lived. Other Robinia
in Illinois are shrubs less than 30'
tall that are more hairy or glandular and their flowers are pink. It is
possible to confuse Black Locust with another native tree, Honey Locust
but this latter species differs by having less
showy flowers, smaller leaflets, and more prominent thorns along
its trunk, branches, and twigs. Honey Locust also differs by having
longer seed pods (exceeding 4" in length) with larger seeds. The wood
of Black Locust is heavy, strong, and durable, but it is often damaged
by the larvae of Megacyllene
(Locust Borer) and heart-rot
fungi before reaching commercial size. Nonetheless, the wood has been
used to make fence posts, rail ties, ship timber, boxes and crates, and
pulp for paper products.