Purple Robe Locust

Robinia pseudoacacia ‘Purple Robe’

Leaves: Deciduous. Entire leaf is 6-14” long. Leaves are divided into 7-19 leaflets, 1-2” long and are arranged in pairs. Leaflets are oval or elliptic in shape with a smooth leaf edge. Dull medium green to dark bluish-green color. Fall color is yellow-green.

Bark/Twigs: Gray-black to almost black colored bark. Deeply furrowed into rounded, interlacing, fibrous ridges. Although it is advertised as thornless, branches can have short, stiff spines ¼- ½” long.

Flowers/Fruit: Extremely fragrant, pea-like flowers are pink, sometimes white, and 1” across. Flowers grow in 4-8” long, hanging clusters and bloom in late spring (May to early June). Flat hanging pods are 2-4” long and contain 4-8 seeds. Pods start out green with purple-brown blotches and dry to a light brown.

Mature size and shape: Medium to large. 30-50’h x 20-35’w. Upright and rather irregular shape. Relatively oval overall. Upright tree with a straight trunk and a narrow oblong crown, becoming ragged and scraggly with age.

General information/special features: Plant in full sun. Grows in practically any soil. Does not tolerate constantly wet soils. Drought and salt tolerant. Has the ability to fix-nitrogen, so can survive in depleted or sandy soils. The wood of black locust is exceptionally dense and resistant to decay. Settlers planted it for long lasting fence post wood and used it in shipbuilding for nails.

Landscape use and Maintenance: Shade tree. Good for difficult sites, but not highly recommended for the home landscape because of its invasive nature, thorns, and weak wood. A tough tree that can be used in areas where other trees will not grow.  Fast growing rate. High maintenance. Can spread quickly by seed or by shoots from roots (sucker sprouts).  Susceptible to borers. Transplants very easily. Prune in late summer or fall. If pruned in the spring it will “bleed.”

USDA Hardiness Zone:  5-8

Family/Origin: Fabaceae – Legume. Native to the central-eastern U.S.

Campus use: Somewhat common. Can be found along South Campus Drive at the Behavioral Science bus stop and the traffic circle.

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Gleditsia triacanthos

Leaves: Deciduous. 20-30 leaflets with blunt points grow along one leaf stem (petiole). Each leaflet is narrow and oblong, but broadest towards the base and is about ⅓” long. Leaf edges are not quite smooth, but have very light serrations. The entire compound leaf is 6-8” long. Bright green color. Fall leaves are a yellow color and one of the first to change in September.

Bark/Twigs: Bark forms long panels with slightly curled edges. Smooth reddish-brown bark on younger branches. Older bark becomes gray brown to nearly black with vertical ridges and deep furrows. Sometimes thorns appear, but most nursery varieties are thornless.

Flowers/Fruit: Green to brown flowers hang down in 2” long clusters resembling furry caterpillars. Blooms late spring or early summer (May to June). Dioecious- species has both male and female trees. Long, brown seed pods 12-18” long, 1” wide, persist through the winter.

Mature size and shape: Large. 30-70’h by about the same width but varies greatly. Open spreading crown shape that provides filtered light shade.

General information/special features: Plant in full sun. Adapts quite readily to extremes in soil conditions.

Landscape use and Maintenance: Good shade tree with filtered light so grass can still grow underneath. Very hardy and drought tolerant. Also tolerant of salty soils. A tough tree for urban conditions including along streets. Fast growing rate. Average to high maintenance. Leaves are really small which can be good in lawn areas, but challenging in pavers and patios. Easy to transplant.

USDA Hardiness Zone:  3-9

Family/Origin: Fabaceae – Legume. Native to most of the eastern U.S.

Campus use: Extremely common. Can be found east of the Bookstore (Bld 67) and south of Union Building (Bld 53).