Fairly fast-growing plants, well adapted to hot, dry regions. Leaves are divided like feathers into many roundish leaflets. Clusters of white or pink, sweet peashaped flowers bloom from midspring to early summer, followed by beanlike pods about 4 in. long. Locust trees tolerate poor soil and can get by on little or no water, but they do have some drawbacks: Wood is brittle, roots are aggressive, and plants often spread by suckers.
R. x ambigua. Tree. Hybrid between R. pseudoacacia and R. viscosa, a seldom-grown pink-flowering locust. The following are the best-known selections.
'Decaisneana'. To 4050 ft. tall, 20 ft. wide. Flowers are like those of R. pseudoacacia, but color is pale pink.
'Idahoensis'. IDAHO LOCUST. Shapely tree 2540 ft. tall and 1530 ft. wide. Reddish bronze new growth and bright rose-pink flowers in 8-in. clusters make it one of the showiest locusts. Bears no seedpods. Thrives under arid conditions; good choice for western parts of Texas and Oklahoma.
R. hispida. ROSE ACACIA, BRISTLY LOCUST. Shrub. Native from Virginia and Kentucky to Georgia and Alabama. Small, showy shrub that will form colonies from root suckers. Extremely invasive in good soil. Grows to 7 ft. (sometimes 10 ft.) tall, 8 ft. wide. Bristly stems; blue-green leaves to 10 in. long, with 7 to 15 leaflets. Rose or pale purplish pink flowers in dangling, 4-in. clusters in late spring. Tolerates dry, poor soils. Use on dry banks, in naturalized settings.
R. x margaretta. Tree. Hybrid between R. hispida and R. pseudoacacia. Grows at a moderate rate into an open, rounded tree 1530 ft. high and wide. Bristly stems; leaves to 8 in. long, with up to 19 leaflets. Fragrant, -in. pink flowers hang in clusters to 7 in. long. 'Flowering Globe' reaches 18 ft. tall and wide, with 8- to 10-in. clusters of dark pink flowers. 'Pink Cascade' ('Casque Rouge') reaches 15 ft. tall and wide, has purplish pink flowers and pinkish new growth.
R. pseudoacacia. BLACK LOCUST. Tree. Native to eastern and central U.S. Fast growth up to 4075 ft. tall, 3060 ft. wide, with rather sparse, open branching habit. Deeply furrowed brown bark. Thorny branches. Leaves divided into 7 to 19 leaflets, each 12 in. long. White, fragrant, - to -in.-long flowers are held in dense, pendent clusters 48 in. long.
Black locust is little valued in its native territory, but it is a favorite in Europe. Rot-resistant wood is sought after for fence posts. Bees make delicious honey from the nectar of its flowers. The tree manufactures its own fertilizer through nitrogen-fixing root nodule bacteria and can colonize the poorest soil. Given some pruning and training in its early years, it can be a truly handsome flowering tree, but locust borer limits its usefulness in many regions; locust leaf miner is also a damaging pest, especially in the Upper and Middle South.
Often used as street tree, but not good in space between sidewalk and curb or under power lines. Wood is extremely hard; suckers are difficult to prune out where not wanted.
Recommended selections include the following.
'Frisia'. To 50 ft. tall, 25 ft. wide. New growth is nearly orange; mature leaves are yellow, turning greener in summer heat. Thorns and young wood are red.
'Lace Lady' ('Twisty Baby'). Dwarf to 810 ft. tall and 1215 ft. wide. Picturesquely twisted branches; few flowers.
'Pyramidalis' ('Fastigiata'). Narrow, columnar, to 50 ft. tall, 10 ft. wide.
'Tortuosa'. Slow grower to 50 ft. tall, 30 ft. wide, with twisted branches. Few flowers.
'Umbraculifera'. Dense and round headed, to 20 ft. tall and wide. Usually grafted 68 ft. high on another locust to create a living green lollipop. Very few flowers.
R. 'Purple Robe'. Tree. Hybrid of uncertain parentage often sold as R. pseudoacacia 'Purple Robe'. Resembles R. x ambigua 'Idahoensis' but has darker, purple-pink flowers, reddish bronze new growth; blooms 2 weeks earlier and over a longer period. Suckers from the rootstock spread and reseed heavily.
R. viscosa. CLAMMY LOCUST. Tree. To 40 ft. tall, 20 ft. wide. Rare. Native to eastern U.S., from North Carolina to Alabama. Leaves are divided into 12 to 24 leaflets. Deep pink flowers in 3-in. clusters appear in late spring. Common name refers to sticky glands on the stems, leafstalks, and seedpods.