Plant Image Data Base                                         

  Rhododendron

                                                             Azalea or Rhododendron

• Form
  • small- to medium-sized ornamental shrubs
  • Azaleas generally mature from 2' to 4' in height and width, and Rhododendrons generally mature from 4' to 10' in height and width
  • upright clump growth habit in youth, becoming rounded to spreading with age
  • slow growth rate
• Culture
  • partial shade to full shade
  • performs best in partial shade in moist, well-drained, acidic soils, with the keys to successful growth being the placement of the shrub in a well-drained soil that is moderately high in organic matter and porous media (amend with topsoil, peat moss, mini pine bark chips, leaf compost, aged manure, treated sewerage sludge, and/or sand) and acidity (pH 5.0 to 6.5; amend with aluminum sulfate or very small amounts of crushed elemental sulfur), with adequate moisture and without extended periods of Summer drought, but never in wet or poorly drained soils, never in neutral to alkaline soils (pH 6.5 or above), and never in areas that can receive any Winter salt spray (near roads) or salt water runoff (near roads, driveways, or sidewalks)
  • abundantly available with hundreds of selections, primarily in container form with very porous pine bark or peat-based media, but also in ball and burlap form in soil for the larger-sized specimens
  • use 1" to 2" of pine bark mulch, pine needle mulch, or hardwood mulch to choke out weeds, preserve Summer moisture, and ameliorate Winter freeze-thaw cycles for the shallow and fibrous root system
• Foliage
  • for both Azaleas and Rhododendrons, leaves are medium to dark green, alternate, oblong to elliptical, on short petioles, and subject to chlorosis (yellowing due to a deficiency of iron and other micronutrients) in alkaline soils
  • for most Rhododendrons, leaves are generally lustrous evergreen, from 2" to 6" long, and with subtle scales and/or with dots on their lower surfaces
• Flowers
  • for both Azaleas and Rhododendrons, shades of white, salmon, pink, red, crimson, violet, lavender, or purple are the most common floral colors, but yellows and oranges also exist; most Azaleas and Rhododendrons flower in May
  • Azaleas generally have about 1" to 3" diameter inflorescences that are in small clusters and evenly spread across the top of the emerging foliage, with five stamens per flower
  • Rhododendrons generally have about 2" to 8" diameter inflorescences that usually rise above the evergreen foliage, in bold clusters, with ten stamens per flower

• Assets
  • extremely showy Spring flowering
  • shade-loving
  • some are broadleaf evergreen (most Rhododendrons and a few Azaleas), while others may occasionally have good fall color in semi-shade (some Azaleas)
  • sympodial branching can be subtlety attractive
  • medium to bold texture (Rhododendrons) or fine texture (most Azaleas)
• Liabilities
  • exacting in its requirement for soils that are acidic, organic, and moist, but not neutral to alkaline, sterile, or wet to poorly-drained
  • slow growth rate and establishment
• Habitat
  • zones 4 to 8 for Rhododendrons and the most cold-hardy Azaleas, but most Azaleas are only cold hardy to zones 5 or 6, with some only tolerant of Winters in zones 7, 8, or even 9 (and are restricted to mild Winter regions or are used as indoor flowering plants)
  • native to many regions of the world, but generally from the Eastern United States, China, or Japan

• Varieties
  • Azaleas:
    • Rhododendron 'Delaware Valley White' - white flowers, with ten stamens per flower (atypical for an Azalea), to 3' tall and wide, a Glenn Dale Hybrid, zone 6
    • Rhododendron 'Gibraltar' - ruffled orange flowers, to 8' tall by 6' wide, a Knaphill Hybrid (but often incorrectly listed as an Exbury Hybrid), zone 5
    • Rhododendron "Girard Hybrids" - heavily flowered, to 3' tall and wide, semi-evergreen, zones 5 or 6; the many cultivars include 'Crimson' (crimson), 'Fuchsia' (ruffled red-purple), 'Hotshot' (orange-red to scarlet), 'Renee Michelle' (pink), and 'Rose' (rose)
    • Rhododendron "Northern Lights Series" - the most cold-hardy Azaleas (fully through zone 3), deciduous, with relatively stout stems for an Azalea, to 6' tall and wide, with showy and fragrant flowers before the foliage; several cultivars, including 'Golden Lights' (golden-orange), 'Orchid Lights' (shell pink), 'Rosy Lights' (rose-dark pink), and 'White Lights' (white-yellow)
    • Rhododendron 'Stewartstown' (also known as 'Stewartstownian', 'Stewartstonian', or 'Stewartsonian') - flowers are brick-red, with fall color that is a shiny vibrant red-orange to red-wine on semi-deciduous foliage, to 5' tall by 4' wide, a Gable Hybrid, zone 5
  • Rhododendrons:
    • Rhododendron catawbiense 'Roseum Elegans' - the standard cultivar of Catawba Rhododendron (which has many cultivars and is the most bold-textured and large-foliaged evergreen Rhododendron), with large lavender pink inflorescences, Summer heat tolerance, to 8' tall and wide, zone 4
    • Rhododendron mucronulatum - Korean Rhododendron is the first of the genus to flower, in late March and early April, with pink flowers opening long before the new foliage emerges (on this unusual deciduous Rhododendron), with the petals often damaged by frost and freezes, and best located at northern foundations to delay flowering as much as possible, to 6' tall and wide, zone 4
    • Rhododendron 'Northern Starburst' - the first release of the new "Genesis Series", with stouter stems, heavier burgundy foliage, and larger pink blossoms than 'PJM Compact' (which it is compared to), to 4' tall and wide, zone 4
    • Rhododendron "PJM Hybrids" - a series of hybrids (including 'PJM'), with the original selections bred by Edmund Mezitt and named for his father Peter J. Mezitt, noted for their compact habit (to 4' tall and wide), mahogany-bronzed evergreen foliage, and profuse pink, rose, magenta, lavender, or white flowers, in late April to early May, zone 4

• Purpose
  • Rhododendron species, cultivars, and hybrids are prized for their showy Spring inflorescences in shady situations, and along with Roses are America's most popular flowering shrubs.
• Summary
  • Azaleas and Rhododendrons are very popular small- to medium-sized shrubs that flower prolifically in Spring, but are slow-growing and require acidic, organic, moist, and well-drained soils in shady situations for establishment and optimum performance.

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