Sorbus

sorbus

( SOR - buss )

At-A-Glance Information

This genus of deciduous trees and large shrubs, commonly called Mountain Ash, Whitebeam, Rowan or Service Tree, includes species which need full sun to light shade and regular to moderate water for best performance in the landscape. They are natives to mountain areas in Asia, North America, and Europe. Flowers are showy, developing into fruit that varies in color (red, orange, white, pink and golden). Birds will feed on the fruit but not usually until the leaves have fallen off the plant. Foliage is ferny, and some species have great fall color. Use these plants as focal points or specimens, for their wildlife value, as street or urban landscape shade trees, or to naturalize in woodland margins or landscape restoration projects. Low-growing species are sometimes used as groundcover. Take care when planting these shrubs in the landscape, as plant parts are mildly toxic to humans and pets if ingested.

sorbus Species and Varietals

The genus Sorbus includes about 100 distinct species, and is a member of the plant family Rosaceae. Hybrid varieties are common in this genus; many are self-fertile without pollination, allowing them to reproduce clonally without any variation. The plants in this genus are related to those in Pyrus (Pears), Malus (Apple), Crataegus (Hawthorn) and Cotoneaster. This has led to many interesting hybrid species to emerge in areas like western Europe and China. The best growing zones for Sorbus plants are USDA hardiness zones 2-7, though this will vary depending on the species cultivated. For more information, consult the descriptions below. To find your zone, click here: Zone Map.

Sorbus alnifolia (Korean Mountain Ash, Alder-Leaved Whitebeam, Mountain Ash, Water Elm Rowan, Water Elm Mountain Ash, Shuǐ Yú Huā Qiū, 水榆花楸, Adzuki Bean Pear, Azukinashi, アズキナシ, 小豆梨, Red Bean Pear Tree, Patbaenamu, 팥배나무)
This species is a tree, native to Korea, China, and Japan. It reaches 40-50 feet tall and 20-30 feet wide, with undivided dark green leaves that turn orange in autumn. Fruit is red-pink to red-orange. The species is suited for growing in USDA hardiness zones 3-7. Variety submollis has fine, downy hairs on leaf undersides and otherwise does not differ much from the straight species. The main cultivars of the species are 'Redbird' (with an upright columnar habit and showy fruit clusters) and 'Skyline' (also with a fastigiate habit).

Sorbus aria (Whitebeam, Common Whitebeam, Chess-Apple, Quickbeam, White Hazel, Whitebeam Mountain Ash)
This tree is best in USDA hardiness zones 5-9 or Sunset growing zones 3-10 and 14-17, and is native to Europe. It reaches 30-45 feet tall and 20-30 feet wide at maturity. Leaves are dull green on top and white underneath. Fall color is commonly yellow. Fruit is red to orange-red. Cultivars of this species include 'Lutescens' (also called the Silver Whitebeam, with grey-green leaves), 'Majestica' (conical crown to 45 feet tall and tolerance to urban pollution), 'Gigantea' (larger leaves and strong growth rate, to 40 feet tall), and 'Magnifica' (with a conical crown and orange-red berries).

Sorbus aucuparia (European Mountain Ash, Rowan, Rowan Tree, Rowanberry, Quickbeam, Quicken Tree, Common Mountain Ash)
This is a tree, native to Europe to western Asia and Siberia; grows best in USDA hardiness zones 2-9 or Sunset growing zones A1-A3, 1-10 and 14-17. They reach 20-40 feet tall and 15-25 feet wide at maturity with an oval-round crown. Leaves are dull green on top and grey-green underneath. Fall color is yellow-red. Fruit is red-orange. Cultivars include 'Asplenifolia' (with deeply cut ferny foliage and serrated leaves), 'Autumn Spire' (with a columnar growth habit and red autumn foliage), 'Beissneri' (with deeply toothed leaflets), 'Cardinal Royal' (also called 'Michred', with large, bright red berries), 'Black Hawk' (with a narrow, upright habit), 'Rossica' (with a uniform upright shape) and 'Fastigiata' (also with a narrow, upright habit). This species is considered invasive in the northern and Pacific Northwestern United States as well as Canada.

Sorbus hupehensis (Sorbus pseudohupensis) (Hupeh Rowan, Hubei Rowan, Chinese Mountain Ash, Chinese Rowan)
This species takes the form of a tree, to 25 feet tall and wide. It grows well in USDA hardiness zones 5-7 or Sunset growing zones 2-10 and 14-17. Leaves are blue-green, turning orange-red in autumn. Fruit is pure white to pink. Cultivars of this species include 'Pink Pagoda' (to 20-30 feet with bright pink berries), 'November Pink' (with deep pink fruits), 'Rosea' (with a distinct pink flush on white berries), 'Coral Cascade' (blight-resistant with red fruit), and 'Rufus' (with blunt leaflets and rich red-pink fruits).

Sorbus x hybrida (Hedlundia hybrida, Sorbus x thuringiaca) (Hybrid Rowan, Oakleaf Mountain Ash, Swedish Service-Tree, Finnish Whitebeam, Hybrid Mountain Ash)
This hybrid group forms trees that grow well in USDA hardiness zones 3-8 or Sunset growing zones 2-10 and 14-17. They are a cross between Sorbus acuparia and Sorbus aria, and originate in Scandinavia. Trees have an erect growing habit to 20-30 feet tall. Blue-green foliage is white underneath. Fruits that follow white blooms are orange-red and persist into fall and winter.

Sorbus reducta (Dwarf Chinese Mountain Ash, Dwarf Rowan, Mat Forming Rowan, Mat Forming Mountain Ash, Carpet-Forming Rowan, Pù Dì Huā Qiū, 铺地花楸, တရုတ် တောင်, ကရမက် အပုမျိုး)
This shrub species grows well in USDA hardiness zones 5-9 or Sunset growing zones 3-6 and 14-17 and is native to China and Myanmar. It reaches 1-2 feet tall and 3 feet wide, spreading by underground runners. Leaves are dark green and fruit is pink. Great for rock gardens or bonsai subjects. Fall foliage is bronze-red.

Caring for sorbus

Take care when planting these shrubs in the landscape, as plant parts are mildly toxic to humans and pets if ingested. Plant your Sorbus in full sun to light shade. Mountain Ash shrubs and trees should be getting between 6 and 8 hours of sun each day. Space shrubs and trees between 20 and 25 feet apart if growing as a specimen, 10 to 15 feet apart if they are growing as a hedge or cluster, and between 6 to 8 feet apart for tight hedges or smaller saplings. These plants prefer soil that is well-draining with a pH of between 5.5 and 7.5 (though this varies somewhat depending on the species grown; check individual species requirements for more information). Water new shrubs regularly until established. Once mature, these plants are considered drought-tolerant, but will benefit from deep waterings during the warmer, drier months of the year.

Pruning these shrubs is relatively straightforward. They require minimal pruning; simply remove dead, diseased, damaged, or crossing branches in late winter or early springtime before new growth pushes out for the year. These shrubs and trees do well without fertilization, but if it seems to be struggling in your yard, it may benefit from a feeding with a balanced, water-soluble fertilizer in springtime. If your area gets particularly cold in wintertime, the trunks of these trees and shrubs when they are young or newly planted may need to be protected with burlap to prevent sunscald and frost cracks. Sorbus shrubs and trees are long-lived, able to survive in the landscape for over a century. In urban settings however, their life span is shortened to between 30 and 40 years or so. Proper care will ensure the longest-lived trees.

Propagation

Sorbus 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. To propagate your shrub or tree, follow the instructions below:

Propagating Sorbus 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 Sorbus by Seed

Keep in mind that the plant you get from seed will not necessarily mimic all of the characteristics of the parent plant, and that young Sorbus plants grown from seed are not always viable. 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. 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.
Harvest your seeds. The seeds are contained within the fruits produced by these shrubs, which are usually red, pink. golden, orange or white. You'll know that they are ripe when they are a bit squishy and the birds have started to eat them. At this time, cut entire bunches off the plant and place them in a paper bag in a cool, dark, dry place until you're ready to work with them. When you're ready, remove the seeds from the berries by either cutting them open or grating them against and abrasive surface. Ensure that all fruit has been removed from the seed, rinse them, and set them out to dry.
These seeds are dormant until they are exposed to cold. This happens in the garden naturally, but it can be helped along indoors by placing the seeds in a plastic bag filled with moistened peat moss. Place the bag in the refrigerator and leave them in there for between 60 and 180 days (4 months is a good average, though some seeds may take a little longer). Do this with as many seeds as you can; they will probably not all sow, and the ones that do will be of varying characteristics, especially if you are dealing with a hybrid variety. The more seeds you plant, the more chances you will have to get the resulting shrub you want. After your seeds have been cold-stratified, place them in lukewarm water for six hours. Viable seeds will sink to the bottom, and non-viable seeds will float to the top. Discard any seeds that float.
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 a few months 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.

Sorbus seeds are contained within fruits; there are typically one or two seeds in each.

Sorbus seeds are small, brown and have a curved tip.

Problems and Pests

Certain Sorbus species are considered invasive in some regions; see the species descriptions above for more information. Check with local authorities before planting. Sorbus shrubs are affected by the following issues: