Robinia

( rob - INN - ee - uh )

At-A-Glance Information

Commonly called Locust, this genus of flowering deciduous trees and shrubs needs full sun and little to moderate water. These plants are vigorous growers that are well adapted to hot, dry regions. Leaves are feathery and flowers are either pink or white, blooming in clusters in spring and summer. Seed pods follow. 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. Take care when planting these in your garden, as the bark, leaves and seeds are poisonous if ingested.

Robinia Species and Varietals

The genus Robinia includes between 4 and 10 species, and is a member of the plant family Fabaceae. It was introduced to Europe by royal French gardeners Jean Robin and Vespasien Robin in 1601, and the genus was named after the father-son duo. 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)
This tree is best in zones 2-24 and is not commonly grown. Flowers are pink. Varieties are more popular than the species. They are listed here.
'Decaisneana'
40-50 feet tall and 20 feet wide, with pale pink fragrant flowers. 
'Idahoensis' (Idaho Locust)
This variety can be grown as either a shrub or tree, and grows quickly to reach 40 feet tall and 30 feet wide, with bright magenta-rose flowers. This is one of the showiest varieties of Locust in bloom.

Robinia hispida (Bristly Locust, Rose-Acacia, Moss 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. 

Robinia neomexicana (Desert Locust, New Mexican Locust, Rose Locust, Pink Locust)
This shrub or small tree is native to 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)
This species takes the form of a tree and is native to eastern and central United States. It quickly reaches 40-75 feet tall and 30-60 feet wide. It has 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.
'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')
Dense, round-headed tree 20 feet tall and wide. This variety is usually grafted onto another tree. Does not produce many flowers.

​Robinia 'Purple Robe'
This tree grows well in zones 2-24 and is a hybrid with dark purple-pink flowers and red-bronze new growth. Suckers spread and reseed heavily.

Caring for and pruning robinia shrubs

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. Make sure that the soil drains well. Robinia is tolerant of many different soil types but thrives in moderately rich, loamy soils. Water young plants to help them establish a strong root system. Mature trees and shrubs are drought-tolerant, and only need supplemental water during extended dry periods. Apply a layer of organic mulch around the base of the plant to increase soil moisture retention and to regulate soil temperature. 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. Although these plants do not normally require any additional feedings, but if they are struggling they might like a feeding in springtime with a balanced, slow-release fertilizer. 

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
  1. 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. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand.
  2. 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.
  3. 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. 
  4. 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
  1. 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. If you don't have any potting medium on hand, you can make some. Just mix together equal parts (by volume) moss peat and horticultural-grade sand or grit sand. You will also, after a few weeks, need individual 3-inch pots for each seedling that grows.
  2. 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.
  3. 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 seeds are contained inside seed pods that form after flowers have faded.
Robinia seeds inside their seed pod.

Problems and Pests

Although Robinia plants are generally pest and disease resistant, they can have issues with the following:
  • Aphids
  • Caterpillars
  • Boerers
For information on these pests, including instructions about how to eradicate them, consult the pages below.
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Gallery

These trees are beautiful additions to landscapes.
The flowers on these shrubs are the only parts of the plant that are not toxic.
These shrubs have great movement in the landscape, and can tolerate arid conditions.

Videos

All about Black Locust trees.
Learn about these wonderful trees.
How to propagate Black Locust.
A great video on the history and background of these plants.

Resources

Robinia pseudoacacia: Facts. Benefits, Grow and Care Tips
Robinia pseudoacacia - False Acacia
How to Grow Robinia
Robinia - Growing Guide
Wikipedia: Robinia
The New Sunset Western Garden Book: The Ultimate Gardening Guide (2012) - Page 567, Robinia (Locust)
If you have any questions, comments, concerns, or photos you’d like to add to the gallery, please let me know by sending me a message using the button below!
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