Flame Azalea (Rhododendron calendulaceum)-Hort Answers - University of Illinois Extension
University of Illinois Extension

University of Illinois Extension

Hort Answers


Flame Azalea
Rhododendron calendulaceum

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). 
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 Height
4-9 Feet
Mature Width
6-12 Feet
Mature Form
Rounded to slightly broad rounded 
Native To:
United States 
USDA Hardiness Zone
5 - 7 
Soil Conditions
Moist, Well-Drained
Dry Sites
Exposure/Light Requirements
Full Shade
Partial Sun/Shade
Foliage Color
Fall Foliage Color
Pests and Problems
Additional Notes

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


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