University of Illinois Extension
University of Illinois Extension - Selecting Shrubs for Your Home
Flame Azalea
Rhododendron calendulaceum
Size: small
Size: medium

Exposure/ Light Requirements:
Full Shade required.
Full Shade
Full Sun not required.
Full Sun
Partial Sun/Shade required.
Partial Sun/Shade
Hardiness Zone: 5 - 7 
Mature Height: 4-9 Feet
Mature Width: 6-12 Feet
Features: Showy flowers (color depends on cultivar selected) in late spring; flowers have no fragrance; autumn color is a very mild yellow; leaves are deciduous (shed in autumn).
Culture: Partial to full shade; moist, well-drained soil is best, but will tolerate some dryness; needs acid soil.

This species is used as a parent in many hybrids, including the Ghent Hybrids, Knapp Hill Hybrids and Ilam Hybrids.

Common Groups of Hybrid Rhododendrons

'Girard' Hybrids: This large group of azaleas includes both deciduous and evergreen cultivars. They vary in flower color and winter hardiness.

'Knapp Hill' Hybrids: The result of crosssing several species of Rhododendron. A large group including 'Knapp Hill', 'Exbury', 'Slocock' and 'Ilam' Hybrids. These azaleas are deciduous. Flower color depends on the cultivar selected. Mildew can be a problem with this group. Hardiness zones 5-7.

'Northern Lights' Hybrids: Developed at University of Minnesota. These hybrids were selected for excellent cold hardiness (to -30 degrees). These azaleas are deciduous. Flower color depends on cultivar selected.

'P.J.M.' Hybrids:A group of hybrids that are known for their compact size (3-6 feet) and their flower color (shades of lavender pink to mauve). These rhododendrons are evergreen, with the foliage taking on a purplish cast in winter. Hardiness zones 5 to 7.

Mature Form: Rounded to slightly broad rounded
Native To: United States
Soil Conditions:
Soil condition: DryDry
Soil condition: Moist, Well-DrainedMoist, Well-Drained
Alkaline Soil
Tolerance: Dry SitesDry Sites
Soil Compaction
Wet Sites
Tolerance: WindWind
Use for: Border.Border
Use for: Naturalizing.Naturalizing
Use for: Specimen.Specimen
Foliage Color: Green
Fall Foliage Color: Yellow
Additional Notes:

Rhododendron and azaleas both fall into the genus Rhododendron. For more information on rhododendrons visit the American Rhododendron Society website

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