JARS v39n3 - Rhododendron Companion Plants

Rhododendron Companion Plants
R.T. Johnson and R.E. Lyons
VPI & SU, Blacksburg, VA

Rhododendrons and azaleas deserve special attention as some of the most popular garden plants in the world. They are members of a single genus Rhododendron in the Ericaceae. It is a large genus, with over 900 species. Their popularity as garden plants led to great breeding programs and the release of many cultivars and interspecific hybrids.

Rhododendrons and azaleas are not difficult to grow but have rather specific growing requirements. Their soil should be acidic, organic, moist, fertile and well-drained. Each of these conditions are important. In neutral pH soils, the plants are unable to take up sufficient iron for normal green leaves and will become chlorotic. Rhododendrons do not tolerate drought or wet soils (with a few exceptions): organic soils provide more suitable moisture control than mineral soils. The need for good moisture control suggests partial shade in summer. Partial shade in winter is also advisable for the evergreen species, to avoid sunscald and leaf desiccation: sunny winter days can damage plants when the leaves warm and lose moisture while the frozen earth prevents replacement of the moisture. In any case, a protected site is advisable because the flower buds are less hardy than vegetative buds and may be killed in severe winters or exposed locations.

Although beautiful, rhododendrons and azaleas are showy for only a relatively short season when in bloom. The showiness can be enhanced by selecting species or cultivars with different blooming times or by using companion plants for greater diversity. The companion plants are selected for early or late blooming periods, showy fall color, persistent showy fruits or attractive textural contrasts with the rhododendrons. They must be able to thrive under the same growing conditions and must not compete with the rhododendrons' own shallow roots.

The over-story which provides partial shade can contribute to the ornamental value of the garden. Most pines are suitable choices as they cast a medium shade and are deep rooted. The oaks with tap roots are also good, but shallow-rooted species like the willow oak ( Q. phellos ) will compete with the rhododendrons. American Holly ( Ilex opaca ) is a good choice, as it has a finer leaf texture than the evergreen rhododendrons as well as showy fall and winter berries. Blackgum ( Nyssa sylvatica ), Sweetgum ( Liquidambar styraciflua ), and Sourwood ( Oxydendrum arboreum ) are desirable companion plants for the over-story since each has very showy fall color. The blackgum also produces abundant berries in the fall, which are attractive to songbirds. Sourwood's assets are its racemes of white flowers in midsummer and bright red fall color. All of these species thrive in the same soil conditions as rhododendrons and azaleas.

Possibly the two most popular companion plants are Kalmia latifolia , Mountain laurel, and Pieris japonica , Japanese andromeda. Each is a medium shrub with great flower interest. Mountain laurel usually has rosy pink flower buds which open white, although several new cultivars from Connecticut have red or deep pink blooms. Japanese andromeda is valued for its terminal clusters of waxy-white, lily-of-the-valley-like flowers. Both species are evergreen and provide a textural contrast to rhododendrons and azaleas. The soft growth of Pieris is usually bright red, adding early summer interest. The related species Kalmia angustifolia , sheep laurel, and Pieris floribunda , Mountain Pieris are also good choices.

Many of the species of Vaccinium are suitable companion plants, as they often have attractive orange to scarlet fall color. The choice of species will depend on desired height and location, as some species are tall and heat tolerant, others are short and not heat tolerant. Several species to consider are V. angustifolium , low bush blueberry; V. corymbosum , high bush blueberry; V. ashei , rabbiteye blueberry; V. arboreum , tree huckleberry; and V. vitis-idaea , lingonberry. There are less well-known species and a closely allied genus, Gaylussacia , huckleberry, which are suitable as well.

The oakleaf hydrangea, Hydrangea quercifolia is valuable for its large leaves (textural contrast) and bright red fall color. The flowers have large sterile bracts which are white in summer, turning to pink and then brown in autumn. This concentration of visual interest in summer and fall is a desirable addition to the rhododendron and azalea spring interest.

If large shrubs to small trees are desired, the witch-hazels ( Hamamelis mollis , japonica , x intermedia , vernalis and virginiana ) are also appropriate companion plants. Neutral summer foliage and good fall color are combined with winter flowering interest. Witch hazels bear masses of tiny flowers with ribbon-like petals close to their stems. The time of bloom varies from late autumn for H. virginiana to late winter for the other species. Both H. mollis and H. virginiana bear fragrant flowers. Flower color varies from pale yellow to orange, depending on species and cultivars. Their flowers are prominent when combined with evergreen rhododendrons.

Summersweet ( Clethra alnifolia ) is a good choice for summer flowering interest, with terminal spikes of fragrant white or pink flowers. It is a medium shrub and can have attractive yellow-orange fall color in good years. Summersweet blooms well in partial shade and provides flower interest in July and August.

Spicebush ( Lindera benzoin ) is a excellent companion shrub for several reasons: very early, tiny yellow flowers on the bare stems and good medium textured summer foliage followed by red berries and yellow fall color. It is difficult to transplant but will tolerate heavy shade or drier soils, and is thus a good plant for trouble-spots in the garden.

Two groups valuable for fall color and fruit interest are Barberry ( Berberis koreana or thunbergii ) and Cotoneaster ( Cotoneaster , many species). Both genera often have reddish to purplish fall color and persistent red fruits. Flower and fruit production are best in sunny locations. Many heights (prostrate ground covers to medium shrubs) and forms are available through species and culture selection. The evergreen barberries ( B. julianae , verruculosa ) may be valuable for their foliage, which is much finer in texture than the evergreen rhododendrons. Barberries are spring flowering plants and bear yellow flowers attractive to bees. These two genera are less shade tolerant than some of the other plants suggested, and should be reserved for sunny locations.

Drooping leucothoe ( Leucothoe fontanesiana ) is a good small shrub with glossy evergreen foliage. It's fountain-like growth habit contrasts nicely with the rounded habit of rhododendrons and azaleas. The foliage turns bronze in winter: 'Scarletta' is more compact than the species and turns deep purple in winter.

Red veined Enkianthus ( Enkianthus campanulatus ) is useful for its subtle, bell-like clusters of flowers in May, and most importantly, for its scarlet fall color. The flowers hang below the foliage and are cream with red markings - not vibrant, but attractive. The fall color is almost neon-red, definitely eyecatching among evergreen rhododendrons.

For a low evergreen shrub, consider Oregon Grapeholly, Mahonia aquifolium . The glossy, holly-like compound foliage is attractive year-round, and fragrant yellow flowers in April result in terminal clusters of waxy blue fruits, which are showy in mid to late summer. The winter foliage is bronze and may be damaged in an exposed location.

There are a great many other shrub species which will thrive under these conditions, but few besides the ones discussed above are available in the nursery trade. Others, such as heath ( Erica ) and heather ( Calluna ) are widely available but somewhat more difficult to grow.

Don't limit yourself by considering woody tree and shrub species as the only appropriate companions for rhododendrons and azaleas. Plant habits closer to ground level, which include many herbaceous materials, complete the landscape composition and are aesthetically complimentary. In fact, many possibilities originate from the associations found within the rhododendrons' native environments, and often lend themselves to cultivation. Natural companions could include the following plants, which are linked by a common requirement for moist, organic, acidic soil, even though they may be members of very different botanical families.

Gaultheria procumbens , or teaberry, is a low growing semi-woody perennial which forms loose mats in the shade of an upper story. It is particularly desirable due to its relatively extended season of visual interest. White, bell-shaped, nodding flowers appear very early in spring, generally in April and May. Although these are short-lived, the bright red berries which follow often hang for months amongst the plant's shiny evergreen foliage. They can even last throughout the following winter and be quite striking after a light snow fall if not eaten by birds or squirrels. Teaberry spreads by means of underground stems which can be trained or rerouted to form denser clumps. Any damage to either the foliage or berries results in the release of an unmistakable wintergreen fragrance.

Bearberry ( Arctostaphylos uva-ursi ) is also a fine choice for placement within the rhododendron collection, especially in sandier, well-drained locations. It is similar in many respects to teaberry given its glossy, evergreen foliage, floral characteristics and production of very visible red berries. Yet it flowers from May through July and is a woody, creeping shrub which attains a maximum height of one foot.

A true herbaceous plant which also yields conspicuous red fruits is the Partridgeberry ( Mitchella repens ). Partridgeberry blends nicely with Gaultheria and Arctostaphylos , and forms a lovely groundcover in the shade of surrounding taller plants. The white veins of its evergreen foliage provide excellent contrast against the prevalent solid green of its companions. Tubular, white flowers appear in pairs during June and July, and are uniquely fused, producing a single red fruit whose two dimples are evidence of its origin as two flowers. Aside from this characteristic, partridgeberry roots readily at each node and forms a mat of intertwined stems which hug the ground. All three of the above plants are found within established North American woodlands, but there are many other cultivated choices to consider. Several cultivars of Yellow Archangel and Spotted Dead Nettle are receiving considerable attention lately due to their excellent adaptability to shady sites. Although scientific names can get confusing here, you will often see these plants available as Lamiastrum galeobdolon 'Variegatum' and Lamium maculatum . Probably the best selection for placing beneath rhododendrons is L. maculatum 'Beacon Silver'. This plant literally lights up the surrounding area with its vivid, green-edged, silvery leaves. Rarely will it grow greater than 8" high, and its habit is much less rampant and uncontrollable than some of its relatives. Beacon Silver is perfect for limited as well as larger gardens and it is fast becoming an extremely popular ground cover. This is one of those herbaceous plants whose foliage value far surpasses that of its flowers; which incidentally, appear in spring as spikes of pink amidst the silver background.

The genus Epimedium is a superior choice for the prescribed conditions of the rhododendron collection. Hybrids of the species rubrum are most commonly offered commercially and highly desirable for their flowering in the spring, ground cover qualities during the entire season, and red foliage in the fall. Attaining heights of 9-12", epimediums do well under the protection and shade of taller shrubs and trees. Some of their foliage may persist during the winter, but it is now being suggested that it be removed prior to the appearance of the stalks of columbine-like flowers in May and June.

To this point, all of the semi-woody or herbaceous plants suggested have exhibited a spreading behavior. Many other herbaceous perennial alternatives, however, either form dense clumps, or simply increase in size more slowly. Ferns, for example, are a group with a great variety of height, texture, and color. Most Adiantums, or maidenhair ferns, are excellent specimens and always in demand for their arching, graceful fronds. This delicate group will last for years, once established. Maidenhairs must be moist at all times, especially after transplanting, since they are absolutely intolerant of wilting. Although sometimes a little difficult to obtain, the Japanese silver painted-fern is an unusually colored plant. Fronds reach a maximum of 18" after unfolding their neatly splashed, silver and green leaflets positioned on wiry, wine red stems. This combination is a welcome rarity among the prevalent greens of non-flowering rhododendrons and azaleas.

A final collection of plants worth mentioning is the Hostas. This single genus possesses such enormous diversity of foliage color, shape, size and texture, and plant habit, that its flowers seem to be of secondary importance. Hostas thrive in the shade in almost any type of soil, as long as it is moist, and better if it is highly organic. The Plantain Lily, as Hosta is also known, is available in variegated forms, blues, pale and intense greens, prostrate and erect forms, and with wide or linear leaves. Your choice will be difficult due to the assortment, but some of the cultivars currently available are Elegans (a blue tone leaf type), Golden (gold, puckered leaves reaching 15"), and Francee (wide, predominantly green leaves with a thin white edge and lavender flowers). Check the latest garden and nursery catalogs for the newest introductions, and other tried-and-true standard varieties.

By selecting companion plants from this group, one can increase the beauty of the rhododendron and azalea collection through color and textural contrasts, and broader seasonal effects. The collection may be as beautiful in late summer, fall, and winter as in spring.