Sorbus

sorbus

( SOR - buss )

At-A-Glance Information

This genus of deciduous shrubs and trees, commonly called Mountain Ash, Whitebeam, Rowan or Service Tree, needs full sun to light shade and regular to moderate water for best performance. 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. Take care when planting these shrubs in the landscape, as they have the potential to be mildly toxic 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. This has led to many interesting hybrid species to emerge in areas like western Europe and China. The best growing zones for Mountain Ash vary depending on the species grown. For more information, consult the descriptions below. To find your zone, click here: Zone Map.

Sorbus alnifolia (Korean Mountain Ash)
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.

Sorbus aria (Whitebeam)
This tree is best in zones 3-10 and 14-17, and is native to Europe. It reaches 30-45 feet tall and 20-30 feet wide. Leaves are dull green on top and white underneath. Fall color is commonly yellow. Fruit is red to orange-red. Variety 'Lutescens' has brighter undersides on leaves.

Sorbus aucuparia (European Mountain Ash)
This is a tree, native to Europe to western Asia and Siberia; grows best in zones A1-A3, 1-10 and 14-17. They reach 20-40 feet tall and 15-25 feet wide, with an oval to round crown. Leaves are dull green on top and grey-green underneath. Fall color is yellow to reddish. Fruit is red-orange. Varieties include 'Cardinal Royal' (with large, bright red berries), 'Black Hawk' (with a narrow, upright habit), and 'Fastigiata' (also with a narrow, upright habit).

Sorbus hupehensis
This species takes the form of a tree, to 25 feet tall and wide. It grows well in zones 2-10 and 14-17 and leaves are blue-green, turning orange-red in autumn. Fruit is pure white or tinged with a slight pink color. Variety 'Coral Cascade' is blight-resistant, with red fruit and red fall color.

Sorbus x hybrida
This hybrid species is a tree, growing best in zones 2-10 and 14-17. It has an erect growing habit to 20-30 feet tall with blue-green foliage that is white underneath. Fruit is red.

Sorbus reducta
This shrub species grows well in zones 3-6 and 14-17 and is native to China. It reaches 1-2 feet tall and 3 feet wide, spreading by underground runners. L:eaves are dark green and fruit is pink. Great for rock gardens or bonsai subjects. Fall foliage is bronzey-red.

Sorbus x thuringiaca
This hybrid species takes the form of a tree and does well in zones 3-6 and 14-17. It has an erect growth to 20-30 feet tall and wide, with blue-green foliage that is white underneath. Fruit is red.

Caring for and pruning sorbus shrubs

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. These shrubs 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. Apply a layer of organic mulch around the base of the shrub or tree to increase soil moisture retention and suppress weeds.

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.

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. 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.
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. 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.
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. You can 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.