Companion Plants for
Rhododendrons & Azaleas
Rhododendrons and Azaleas are ericaceous plants, members of the heath family, which is called the "acid loving" family. Their special cultural requirements and shallow root structure make them incompatible with some plants. This section will give examples of some plants that have been grown very successfully with Rhododendrons and Azaleas. Astilbe is an Asian perennial herb that grows well with Rhododendron and Azaleas. Kalmia, Mahonia, Pieris, and Viburnum are evergreen shrubs that compliment Rhododendron and Azaleas. Many of the viburnum will grow tall and can be used behind Rhododendrons or Azaleas.
 Astilbe
Prized for both foliage and flowers, perennial astilbes grow
well in both open and dappled shade and even tolerate deep shade.
The plants grow best under deep-rooted trees such as ash, oak,
sweet gum and honey locust. Although astilbes will grow in sun,
shade intensifies the color of the flowers and makes them last
longer. The lustrous, finely divided leaves have a fern-like appearance.
Mounds of green-to-bronze foliage become 12 to 18 inches tall.
In midsummer, feathery plumes of flowers are produced. Each 8-
to 12-inch plume tops a stalk 2 to 3 feet tall. Blooms last for
several weeks. Of the A. arendsii hybrids, 'Peach Blossom' is
light pink, 'Rosy Veil' is dark pink, and 'Red Sentinel' is rich,
bright red. A white variety is also available. Astilbes can be
used in flower borders, massed as a ground cover, or planted with
spring bulbs to hide their fading foliage. They produce excellent
cut flowers and can be dried for winter arrangements.
How To Grow Astilbe
Astilbes can be grown throughout Zones 4-9, hardy to -25°
F. They do best in moist soil that has been enriched with organic
matter such as peat moss, leaf mold or compost. The soil must
be moist in summer but well drained in winter. Water frequently
if the soil is light and sandy. Astilbes should be fertilized
each spring with a high-phosphorus formula such as 5-10-5. Plants
multiply rapidly, exhausting the soil around them; as they become
overcrowded, they produce few flowers. To rejuvenate them, divide
clumps in the spring or fall after two or three years of flowering.
When you do so, work organic matter and a dusting of 0-20-20 fertilizer
into the soil before replanting. Set plants 15 to 18 inches apart.
Astilbes are relatively pest free.
 Kalmia
latifolia (mountain laurel)
Mountain laurels are unusually cold-resistant shrubs that grow 4 to 8 feet high in about 10 years but can easily be kept smaller by pruning. Their 3- to 5-inch lustrous dark green leaves are attractive at all seasons, but they are nearly hidden beneath 4- to 6-inch clusters of small cup-like blossoms in late spring. The flowers range in color from nearly white to a pink so deep as to seem almost red; brownish flecks inside the cups look like freckles or sprinkles of nutmeg. The structure of the flowers is unusual--each stamen is held in a tiny slot under tension until it is released when touched by bees so that it catapults pollen onto them. The plants make handsome ornaments when planted near a foundation or in a shrub border, and can be massed to create a spectacular springtime effect.
Mountain laurels are hardy to Zone 5, hardy to -20° F. They thrive in light shade and moist acid soil. They will tolerate deep shade but bear fewer flowers there than in brighter light. Do not cultivate the soil around the plants to keep roots moist and cool; instead, use a permanent 2- to 6-inch mulch of wood chips, oak leaves or ground bark. If faster-than-average growth is desired, dust the soil under the plants with rhododendron-azalea-camellia fertilizer or cottonseed meal in spring. Remove seed capsules after the flowers fade to ensure an abundance of blossoms the next season. Pruning is rarely needed, but to lower the height of a plant, prune immediately after flowering; new stems will sprout even from large branches. If an old plant has become too large, cut the plant back so that only 2-inch stubs remain above the ground; it will soon grow into a small, thickly foliaged shrub.
 Mountain laurel is an ideal plant for shade, preferring subdued light to sun; it will even do well in deep shade, although its bloom will be less abundant. This evergreen shrub, indigenous to the mountains of the eastern United States, grows 4 to 8 feet high but can be kept lower by pruning. Its glossy, oval, dark green leaves, 3 to 5 inches long, are almost hidden in late spring by masses of rose to white cup-shaped flowers that are borne in loose clusters 4 to 6 inches across. Inside these flowers the petals are marked with brown dots that look like sprinklings of nutmeg. A hardy plant, mountain laurel will keep its leaves unfurled even in severe cold. Mountain laurel spreads by sending up suckers. It does well near rhododendrons, since both plants need acid soil. Use them for foundation plantings on the north side of a house, in woodland settings or behind spring-flowering bulbs in garden borders.
How To Grow Kalmia Latifolia
Mountain laurel is hardy in Zones 5-9 and does best in a cool,
moist, acid soil with peat moss or sand added. To retain moisture,
give it a year-round 2- to 6-inch mulch of wood chips, ground
bark, well-rotted oak leaves or pine needles. For greater bushiness
and more abundant flowers, remove the mulch briefly in the spring
and spread cottonseed meal or a fertilizer prepared for acid-loving
plants around the base; then replace the mulch. The plant's flower
production for the following year can be improved by pinching
off the seed capsules when the flowers fade, but this is not essential.
If pruning is needed, do it immediately after flowering stops.
If the plant dies back in winter, cut it to the ground; it will
send up new growth.
 Mahonia
The holly grapes, which include some of the most cold resistant
of the broad-leaved evergreens, have leaves that look like holly
leaves and fruit that looks like grapes, but they are related
to neither. On most species, the feather-shaped leaves, composed
of tough, leathery, spiny-edged leaflets, turn an attractive bronze
in cold weather. In the spring holly grapes bear short clusters
of bright yellow flowers at the tops of their stems. The flowers
are followed by bluish black edible berries that can be made
into jelly. Holly grapes are excellent for use near foundations
and in shrub borders.
Mahonia aquifolium
Mahonia aquifolium, the Oregon holly grape, usually grows about
2 to 3 feet tall in three to four years and has glossy leaves
with five to nine leaflets; M. aquifolium 'Compacta' is a fine
low-growing variety that seldom exceeds 18 to 24 inches in height.
The Oregon holly grape is hardy to Zone 6, -5° F.
Mahonia bealii
Mahonia bealii, the leather-leaved holly grape, which grows
8 to 10 feet tall in five to six years, has horizontal leaves,
often 12 to 16 inches long, that are composed of 9 to 15 leaflets,
each up to 4 inches long; unlike those of other holly grapes,
they usually do not become bronzy in cold weather. The leather-leaved
holly grape is hardy to Zone 7, +5° F.
Mahonia repens
Mahonia repens, the dwarf holly grape or creeping mahonia,
which generally grows only 12 inches tall, spreads vigorously
underground, making it a fine evergreen ground cover; its leaves
are composed of three to seven leaflets. The dwarf holly grape
to Zone 5, -15° F.
How To Grow Mahonia
Holly grapes prefer light shade and moist soil but will also
grow in the sun except in very hot, dry areas. Set out balled-and-burlapped
plants in spring or fall. Holly grapes are rarely bothered by
pests but should be planted in a sheltered place in northern areas
to prevent winter winds from dehydrating foliage. Growth can be
stimulated by cottonseed meal or a rhododendron-azalea-camellia
fertilizer applied lightly beneath plants in the spring. To control
size, prune tall canes to the ground in early spring.
 Pieris
In spring andromedas bear 3- to 6-inch clusters of tiny flowers
that resemble lilies of the valley. Buds for the following spring
form in summer, and are decorative all fall and winter. Andromedas
are effective planted in a border or singly on a lawn.
Pieris japonica (Japanese andromeda)
Native to central and southern Japan where it has traditionally
been a favorite for tea gardens and temple gardens. Japanese andromeda
is a broad-leaved evergreen shrub with handsome foliage and graceful
flowers. Japanese gardeners often plant it in groups, either under
large trees or beside rocks. Andromedas become 8 to 12 feet tall
and have pleasantly curving branches. The shiny oblong leaves,
1 1/2 to 3 inches long, are bronze colored when young, but then
turn green in the summer. In spring, the branch tips bear drooping
clusters of white flowers, 3 to 6 inches long. After the blooms
fade, new pink-green buds form and remain on the bushes until
the following spring, giving the plants the appearance of being
in bloom throughout most of the year. Japanese andromedas grow
slowly, reaching their maximum height in 10 to 15 years. If sheltered,
they tolerate cold relatively well, and they thrive in the salty,
breezy air of the seashore, provided the climate is not excessively
hot.
Pieris floribunda (mountain andromeda)
Mountain andromeda is hardy to Zone 4, hardy to -25° F.
Plants usually grow 2 to 3 feet tall in four to five years. They
have dull green leaves 2 to 4 inches long and white flowers. They
thrive in full sun or partial shade.
How To Grow Pieris
Japanese andromedas grow in Zones 5-8, hardy to -15° F,
in the East and in Zones 8 and 9 in coastal areas of the Pacific
Northwest. They thrive in light shade and will grow in full sun
if kept moist at all times. Japanese andromedas require a moist,
well-drained soil enriched with 1 part peat moss or leaf mold
to 2 parts soil. Plant andromedas in spring or early fall, spacing
them 3 to 4 feet apart if grouped. In colder areas, give them
a location sheltered from strong wind. Fertilize in early spring
with a light scattering of cottonseed meal or rhododendron-azalea-camellia
fertilizer. Mulch permanently with pine needles, wood chips or
chunky peat moss to prevent roots from drying. Japanese andromedas
are by nature shapely plants, so little pruning is required. Propagate
from stem cuttings taken in late summer or from seed.
 Viburnum
Examples of Viburnum include: V. burkwoodii (Burkwood viburnum); V. carlcephalum (fragrant snowball); V. carlesii (Korean spice viburnum); V. cassinoides (withe rod); V. dentatum (arrowwood); V. dilatatum (linden viburnum); V. lantana (wayfaring tree); V. lentago (nannyberry); V. opulus (European cranberry bush); V. plicatum var. tomentosum (Japanese snowball); V. prunifolium (black haw); V. sieboldii (Siebold viburnum); V. trilobum (American cranberry bush). All are called viburnum.
Viburnums are such a varied and important genus of flowering, fruit-bearing shrubs that few generalizations apply to all of them. Various types grow from 2 to over 20 feet in height, and are thus valuable for many garden uses such as shrub borders, informal hedges, screen plantings, plantings near a house or as single plants that are grown to stand by themselves.
Like hydrangeas, viburnums have two kinds of blossoms- flowers, incapable of producing berries, and fertile ones that can produce berries. Unlike hydrangeas, the fertile flowers are usually as attractive as the sterile ones. Viburnums offer an added attraction- handsome fruit of fertile flowers is appealing to birds and that of some species makes delicious jellies and jams. Flowers are usually white or pink and many are delightfully fragrant. The foliage of most viburnums turns some shade of red in the fall. Burkwood viburnum grows 4 to 6 feet tall and bears 2 1/2- to 3 1/2-inch round clusters of very fragrant pink or white star-shaped fertile flowers in late spring. Its berries change from red to black as they ripen in late summer; the glossy 1 1/2- to 4-inch leaves are dark green above and whitish underneath during the summer and become dark red in the fall.
Fragrant snowball, a plant that looks best standing by itself, grows 6 to 7 feet tall and bears ball-shaped 4- to 5-inch clusters of sweetly fragrant white star-shaped fertile flowers in late spring. Berries change from red to black as they ripen in midsummer. The fragrant snowball's toothed 3- to 4-inch leaves turn red in the fall. Korean spice viburnum is a 3- to 5-foot species that bears 1 1/2- to 3-inch clusters of fragrant pink star-shaped fertile flowers in mid-spring. It has 2- to 3 1/2-inch toothed leaves that turn purplish red in the fall. V. carlesii 'Compactum' grows to a dense bush 2 1/2 to 3 1/2 feet tall.
Withe rod, whose canes were once the weapons of stern schoolmasters, grows 5 to 6 feet tall and has 3- to 4-inch flat-topped clusters of masses of tiny creamy white fertile flowers in early summer. Its berries change from green to pink, then from red to blue before becoming black when they ripen in late summer. Its 1 1/2- to 4 1/2-inch leaves turn bright red in the fall.
It is easy to see why Indians used the stems of arrowwood as shafts for their arrows. Mature plants may send curving branches 10 to 15 feet high, but always have many straight young stems coming up from ground level. This species has 2-inch clusters of tiny creamy white fertile flowers in early summer, followed by dark blue berries in the early fall. Its shiny nearly round 1 1/2- to 3-inch leaves are coarsely toothed and become red in the fall.
Linden viburnum grows 6 to 9 feet tall and bears 3- to 5-inch flat clusters of pure white fertile flowers in early summer. The flowers are followed by bright red berries in early fall, when the broad 2- to 5-inch leaves begin to turn rusty red.
 Wayfaring tree grows 10 to 15 feet tall and has 2- to 4-inch flat clusters of tiny white fertile flowers in mid-spring, followed by berries that turn from red to black in early fall and cling to the shrub through part of the winter. The 2- to 5-inch round gray-green leaves turn red in the fall. Nannyberry may grow as a large shrub or small tree to a height of 15 to 20 feet or more. Its long arching branches bear 3- to 4 1/2-inch clusters of tiny white fertile flowers in late spring, followed by 1/2-inch black berries in early fall. The slender glossy 2- to 4-inch leaves turn shades of red and orange in the fall.
European cranberry bush is resistant to the smoke and grime of cities. It grows 8 to 12 feet tall and has 2- to 3-inch clusters of tiny white fertile flowers ringed by a circle of 3/4-inch flat sterile flowers that open in late spring. In early fall the fertile flowers are followed by translucent red berries that remain on the shrub through most of the winter. The berries resemble cranberries but are so tart that even birds do not eat them. There are a number of varieties of the European cranberry bush, all with three-lobed leaves that turn red in the fall.
V. opulus 'Compactum' grows only 5 to 6 feet tall and has heavy crops of fruit. Dwarf European cranberry bush, V. opulus 'Nana', which does not bear flowers or fruit, grows about 2 to 2 1/2 feet tall and has very dense foliage, making it an especially good low hedge. European snowball, V. opulus 'Roseum', also called V. opulus 'Sterile', is so named because it has 3-inch ball-shaped clusters of sterile white flowers in late spring, but has no fruit. It is very susceptible to attack by aphids. Yellow-fruited European cranberry bush, V. opulus 'Xanthocarpum', has bright yellow berries.
Japanese snowball grows 5 to 7 feet tall and bears 2- to 3-inch ball-shaped clusters of sterile white flowers in late spring. The 2- to 4-inch leaves are whitish beneath and turn purplish red in the fall. Doublefile viburnum, V. plicatum var. tomentosum, has broadly spreading horizontal branches, which in late spring are lined with a double row of flat-topped white flower clusters 3 to 4 inches wide. Each cluster of this variety is composed of a center of tiny fertile flowers surrounded by 1- to 1 1/2-inch flat sterile blossoms. Marie's doublefile viburnum, V. plicatum var. tomentosum 'Mariesii', is similar except that its sterile flowers are more numerous and larger, reaching up to 2 inches across. The ordinary species does not bear fruit, but all varieties do: colorful berries that turn from red to black as they ripen in midsummer.
 Black haw grows 10 to 15 feet tall and has 2- to 4-inch flat clusters of tiny white fertile flowers in mid-spring. The 1/2-inch blue-black berries that ripen in early fall, are good for jam and jelly. The dark green 1 1/2- to 3-inch leaves turn wine red in the fall. Siebold viburnum, a plant that is very attractive standing by itself, usually grows 6 to 10 feet tall. Its 3- to 4-inch clusters of tiny creamy white fertile flowers appear in late spring and are followed by fruit that, beginning in midsummer, changes from green to pink, then from red to dark blue and finally ripe black in early fall. The coarsely toothed dark green shiny leaves, 2 to 5 inches long, take on a rich red color in the fall.
American cranberry bush grows 8 to 12 feet tall and has 2- to 3-inch clusters of tiny white fertile flowers surrounded by a ring of 3/4-inch sterile flowers in late spring. The blossoms are followed by bright red berries that resemble cranberries and ripen in early fall when the leaves turn red. The berries cling most of the winter-are too tart for birds, but make delicious jams. The variety V. trilobum 'Compactum' grows 4 to 5 feet tall.
How To Grow Viburnum
Burkwood viburnum, fragrant snowball and linden viburnum grow in Zones 5-9; Korean spice viburnum, Siebold viburnum and Japanese snowball grow in Zones 4-9; withe rod, arrowwood, nannyberry and American cranberry bush grow in Zones 2-9; wayfaring tree, European cranberry bush and black haw grow in Zones 3-9. All do best in full sun, but will tolerate light shade. Nearly any well-drained garden soil is suitable. For informal hedges 3 to 8 feet tall, plant them 2 1/2 to 4 feet apart. Korean spice and Burkwood viburnums and fragrant snowballs are sometimes grown as grafted plants- that is, their stems are grafted to the roots of stronger-growing species. Do not buy such grafted plants since the stronger species tend to sprout and overwhelm the desired variety. Bare-rooted Japanese snowballs and fragrant and Korean spice viburnums are difficult to establish. Buy only plants that are balled and burlapped -that is, sold with their roots in their original soil ball wrapped in burlap - or container-grown plants; set them in the garden in spring. The other viburnums can be purchased either bare-rooted, balled and burlapped, or container grown and may be planted at any time without difficulty. To preserve the natural graceful shape of viburnums, limit pruning to the removal of any dead branches. New plants can be started from softwood cuttings of young growth in late spring or early summer, from semi-hardwood cuttings of more mature growth in mid- or late summer, or from hardwood cuttings of dormant leafless growth in late fall or early winter.
Directory of Contents for Henning's Rhododendron & Azalea Pages
•• Providing Information about the Genus Rhododendron since 1996
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Articles Published by the American Rhododendron Society
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Last Updated:
July 6, 2020
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