Robinia

( rob - INN - ee - uh )

At-A-Glance Information

Commonly called Locust or False Acacia, this genus of flowering deciduous trees and shrubs includes species which need full sun and little to moderate water to grow well in the landscape. These plants are vigorous and are well adapted to hot, dry regions. Leaves are divided and ferny and flowers are either pink or white, blooming in clusters in spring and summer and attracting pollinators like bees and butterflies to the garden. Seed pods follow blossoms. These trees and shrubs can tolerate poor soil and drought conditions, but the wood is brittle, the roots are aggressive, and the plants often spread via suckers, making them a bit hard to control in gardens where they are in close proximity to other plants. Use Robinia in the landscape for fast shade, ornamental foliage, or as erosion control in sloped areas. Take care when planting these in your garden, as the bark, leaves and seeds are toxic to humans and pets if ingested.

Robinia Species and Varietals

The genus Robinia includes between 4 and 10 species, and is a member of the plant family Fabaceae. The plants in this genus are closely related to those in Olneya (Desert Ironwood). Locust growing zones vary depending on the species; for more information, consult the descriptions below. To find your zone, click here: Zone Map.

Robinia x ambigua (Idaho Locust, Hybrid Locust, Ambiguous Locust, Idaho Locust, Purple Robe)
This tree is best in USDA hardiness zones 3-10b or Sunset growing zones 2-24. Flowers are pink in the straight species, though cultivars are more commonly grown as ornamentals in landscapes. These hybrids can be invasive in regions of North America and central Europe, where they escape garden cultivation and form dense thickets, crowding out native plants.

'Bellarosea'
This species forms a rounded open crown with large pink flower clusters in summer.

'Decaisneana'
40-50 feet tall and 20 feet wide, with pale pink fragrant flowers.

'Idahoensis' (Idaho Locust)
To 40 feet tall and 30 feet wide, with bright magenta-rose flowers.

'Purple Robe'
Dark purple-pink flowers and red-bronze new growth. Suckers and reseeds heavily.

Robinia hispida (Bristly Locust, Rose-Acacia, Moss Locust, Mossy Locust)
These plants are native to the southeastern United States, and grow to reach about 10 feet tall when mature. Compound leaves are green, and flower inflorescences bear pink to purple pea-form flowers. Bristly Locust is suited for growing in USDA hardiness zones 5-9 and is considered invasive in Michigan, New Jersey, Ohio, Pennsylvania, Wisconsin, and Washington in the United States as well as regions of Europe, Asia, and South Africa.

Robinia neomexicana (Desert Locust, New Mexican Locust, Rose Locust, Pink Locust, New Mexico Locust, Rusby's Locust, Uña de Gato, Southwestern Locust)
This shrub or small tree is native to USDA hardiness zones 5-8 or Sunset growing zones 2, 3, 7-11, 14 and 18-24. It is native to California, Arizona and New Mexico. It is a thorny shrub about 6 feet tall and just as wide, though it can reach as much as 30 feet tall as a tree. Flowers bloom in drooping pink clusters.

Robinia pseudoacacia (Black Locust, False Acacia, Yellow Locust, Locust)
This species takes the form of a tree and is native to eastern and central United States, growing well in USDA hardiness zones 3 through 9. It quickly reaches 40-75 feet tall and 30-60 feet wide at maturity with an open branching habit and deeply ribbed brown bark. Branches have thorns. Flowers are white and bloom in clusters. A valuable timber tree in its natural habitat. Suckers freely; makes a good street tree. This species is considered invasive in the Midwest, Great Lakes, and New England regions of the United States as well as Europe, Asia, and South Africa.

'Frisia'
Reaches 50 feet tall and 25 feet wide, with orang new growth and yellow mature leaves. In summer, foliage takes on green tones. Thorns on young wood are red.

'Lace Lady' ('Twisty Baby')
8-10 feet tall, 12-15 feet wide dwarf variety with twisting branches. Not known for showiness of blooms.

'Pyramidalis' ('Fastigiata')
Narrow, columnar growth to 50 feet tall and 10 feet wide.

'Tortuosa'
This is a slow grower that reaches 50 feet tall and 30 feet wide with twisting branches and few flowers.

'Umbraculifera' ('Inermis', 'Mop Top')
Grafted, dense round tree to 20 feet tall and wide.

Caring for robinia

Take care when planting these in your garden, as the bark, leaves and seeds are toxic to humans and pets if ingested. Plant these shrubs and trees in full sun to light shade. Make sure the location gets at least 6 hours of direct sun per day for best growth rate and flower production. Ensure that soil is well-draining, rich in nutrients, and has a pH of between 4.6 and 8.2 (acidic to slightly alkaline). If local soils are particularly poor in nutrients or do not drain well, make sure to amend the planting area with plenty of organic compost before installing. Apply a layer of mulch around the planting area to suppress weed growth, maintain consistent soil temperatures, and to increase soil moisture retention. Water new plantings regularly as they establish a strong root system in their new home. Mature trees and shrubs are drought-tolerant, and only need supplemental water during extended dry periods.

Fertilize yearly in early springtime with an application of 2 inches of compost around the base of plants. Pull mulch back before applying, work the compost gently into the soil a bit, water it in, and then replace mulch. Pruning is simple; remove any damaged, diseased, or dead foliage at any time during the growing season, and remove spent flower spikes to neaten up the appearance of plants and encourage reblooms. Pruning is not generally necessary except to trim off dead or damaged foliage and branches. The best time to prune is in late wintertime or early springtime. These trees can be expected to survive for 60 years to over a century or more in the landscape before needing to be replaced.

Propagation

Robinia can be propagated either by using cuttings or from seed. To ensure that your propagated plant will have the exact same qualities and characteristics as the parent plant, use the cutting method. If you would like to experiment and see what plant you can grow from seed, just keep in mind that it will not necessarily be the same as the parent plant. Robinia shrubs are known to spread via suckers; these 'volunteer' plants can be dug up and replanted elsewhere, negating the need to propagate. To propagate your shrub, follow the instructions below:

Propagating Robinia by Cutting

Gather your materials. You'll need a clean, sanitized, sharp pair of garden shears, rooting hormone, a pencil, and a 3-inch pot filled with well-draining compost or other organic material. A good substrate for propagation of cuttings is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
In spring or early summertime, choose the plant that you'd like to propagate. Ensure that this plant has had a good watering the night before, as this will improve the chances your cutting will take root. Cut one or more 6 inch portion(s) of the end of a non-blooming branch and strip the leaves off of the lower half. Make a hole in your planting medium by pushing your pencil into the soil. Dip the end of the stem in rooting hormone, and plant it in your pot. Fill in soil around the cutting to support it, but do not pack it down too much.
Moisten with water, and ensure that the pot has good drainage. Your pot should be kept in warm, sunny conditions indoors (and not in direct sunlight). Water whenever your soil is dry under the top layer. After about four to six weeks have passed, your cutting should be rooted. When you notice new growth, you'll know you have roots.
Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring, when you can transplant it to its new home. Ensure that the last frost of the year has occurred before doing this, as it can seriously damage your new shrub.

Propagating Robinia by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant. To propagate by seed, you will need a paper bag, a pot or tray filled with well-draining compost or other organic material, a spray bottle and a fork. The best potting medium for seed germination is a light blend of 1/3 pasteurized soil or compost, 1/3 sand, vermiculite or perlite, and 1/3 coconut coir or peat moss.
Water your prepared seed tray or pot filled with soil, and ensure that good drainage is occurring. Place your seeds in your prepared pot or seed tray about a quarter inch to a half-inch under the soil surface. Cover them with soil and water with your spray bottle. Place your tray of seeds in a warm area that receives bright, indirect light. A greenhouse is the most ideal climate, but a planting tray inside a window will do as well. Germination usually takes 7-14 days from sowing. Once you begin to see seedlings sprout and they are able to be handled, transplant them to their own individual 3-inch pots, filled with the same potting medium as described in Step 1. Loosen the seedlings from the potting medium by digging down with the tines of your fork about an inch and a half, gently working the seedling up until it is out. Then, replant the seedling in its individual pot.
Keep the seedling's potting medium moist, but not wet, by spraying the soil with your spray bottle. Keep your new plant in an area with bright light (though not direct sunlight) and temperatures between 55 and 70 degrees Fahrenheit. Ensure that the soil stays moist, but not wet, until the next spring (after the danger of frost has passed), when you can transplant it to its new home.

Robinia seed pods are generally 2-5 inches long and each contain 4 to 10 seeds.

Robinia seeds are hard, small, bean-like, and brown to black in color.

Problems and Pests

Certain Robinia species are considered invasive in some regions; see the species descriptions above for more information. Check with local authorities before planting. Although Robinia plants are generally pest and disease resistant, they can have issues with the following: