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

University of Illinois Extension

Hort Answers

Shrub

Flame Azalea
Rhododendron calendulaceum

 
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. 
 
Cultivars

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
 
Size
Small
Medium
 
 
Mature Width
6-12 Feet
 
Mature Form
Rounded to slightly broad rounded 
 
Native To:
United States 
 
USDA Hardiness Zone
5 - 7 
 
Soil Conditions
Dry
Moist, Well-Drained
 
 
Tolerance
Dry Sites
Wind
 
 
Exposure/Light Requirements
Full Shade
Partial Sun/Shade
 
 
Uses
Border
Naturalizing
Specimen
 
 
Foliage Color
Green
 
 
Fall Foliage Color
Yellow
 
 
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

 

 
Related Resources
Home, Yard & Garden Pest Guide
Illinois Commercial Landscape and Turfgrass Pest Management Handbook
U of IL - Distance Diagnosis through Digital Imaging
U of IL - Plant Clinic