Flame azalea (Rhododendron calendulaceum) is among the most spectacular North American native shrubs you can grow. It lights up the spring garden with its colorful blooms and shines brightly in the fall with its brilliant foliage. It attracts butterflies, hummingbirds, and other pollinators.
I. Appearance and Characteristics
Rhododendron calendulaceum, the flame azalea, is a species of Rhododendron. It is a deciduous shrub that grows up to 120–450 cm tall. This species of Rhododendron is native to the Appalachian Mountains in the eastern United States, ranging from southern Pennsylvania and Ohio to northern Georgia. It may be extirpated from Pennsylvania and Alabama. It occurs naturally in mixed deciduous forests and is typically found in woodland slopes and mountain balds in the Appalachians, where it prefers dry and rocky mountain woods. The inflorescences of Rhododendron calendulaceum are visited by many animals such as bees, butterflies, hummingbirds and small mammals. It is a popular cultivated plant due to its bright yellow, orange or red flowers.
It is a deciduous shrub, 120–450 cm tall. The leaves are simple, 3–7 cm long, slightly dull green above and villous below. The arrangement is generally alternate, however they appear whorled towards the tips of the branches.
The flowers are 4–5 cm long, usually bright orange, but can vary from pastel orange to dark reddish-orange. These non-fragrant flowers have 4-5 lobes and grow in clusters of 5–10. It typically blooms in late May and early June.
Rhododendron calendulaceum is a highly variable species. The flowers of Rhododendron calendulaceum can range from light yellow to orange and rarely, a scarlet red. Some flowers appear to be a blend of multiple colors, and others seem to have secondary pigments such as pink or red. Some have even reported a white flower in Rhododendron calendulaceum, however these records cannot be verified. The buds of Rhododendron calendulaceum are typically a darker color than the mature flower, however some flowers deepen in color with age. In some plants, a full range of colors such as yellows, oranges and reds can be present on the same plant. It is possible that color change is affected by light, with flowers in shadier areas exhibiting less color change compared to plants in open spaces.
The flower sizes in Rhododendron calendulaceum are also variable, with a typical flower size of 2-2.5 inches. Some flowers have blossoms of less than an inch, while others can be up to 3 inches.
II. How to Grow and Care
Sunlight
Flame azalea likes environments with partial shade and can be planted in places that are partially covered with shades. They can also grow in plentiful sunlight, but they need to be protected from intense sun exposure. It is recommended to shade off 75% of the sunlight in spring to prevent new leaves from becoming sunburnt.
Temperature
Flame azalea does not like to be too cold or too hot. It is suited to grow in an environment with a temperature of 12 to 25 ℃. When the temperature exceeds 30 ℃ or is lower than 5 ℃, the plants grow slowly and will enter dormancy. When the temperature is lower than 3 ℃, flame azalea will suffer frost damage. They need a sufficient amount of slightly acidic water. They cannot sustain too much accumulated water.
Watering
It is best to use rainwater or snow water to water flame azalea. Do not use alkalic water or tap water. There are many ions in tap water that can worsen soil permeability and ventilation. Purchase some pH test strips to test the water at your house. If it is slightly alkalic, add a little white vinegar.
When growing vigorously, give flame azalea plenty of water. In winter, water them less. At the end of winter when temperatures rise, flame azalea is ready to grow, so increase watering frequency. In summer, the soil tends to dry easily, so water plants often and spray water onto their leaves. During summer, water flares azalea in the early morning. In winter, water plants before noon. During spring and fall, water plants any time other than noon. Reduce watering frequency during rainy seasons to avoid waterlogging.
Soil
Flame azalea enjoys rich, loose, and slightly acidic soil. Because the plants do not like too much accumulated water, the soil must be well-drained. Leaf mold, pine needle mulch, peat soil, or fully decomposed groundwood are good choices. These are all acidic culture mediums with strong draining ability. Alkalic or sticky soil cannot be used.
These plants are suited to soil with a pH of 4.5-6. Before planting, it is best to test the pH level of your soil. If it is slightly alkalic, you can moderately add an acidic fertilizer like aluminum sulfate. Mix it with the original soil, and ensure the soil is within the suitable pH range before planting flame azalea.
Fertilizing
The best time to fertilize flame azaleas is just after they finish blooming. Apply a slow-release granular fertilizer for acid-loving plants following the packaging directions, or spread high-quality compost around the base of the plant.
Planting Instructions
Plant flame azaleas in spring or fall. Dig a hole about twice as wide and equal depth as the pot the plant is growing in. Remove it from the pot and loosen the root ball to encourage the roots to spread. If it is rootbound, spray the root ball with a stream of water from a hose to knock off some of the soil and then loosen the roots.
Set the root ball in the hole so that it sits slightly higher than it was at in the pot, which allows for some settling (you can use some of the soil you dug for the hole to raise the root ball, if needed). Backfill the hole with soil and then water thoroughly. Spread a couple of inches of organic mulch over the surface of the planting area and keep the plant watered until it is established.
Pruning
When the flame azalea has adequate space, little pruning is necessary other than to remove broken or diseased branches. Cut these back to a healthy branch or bud as soon as you notice them. If the azalea outgrows its space or you want to modify the shape, prune immediately after it finishes blooming, cutting back to a healthy branch or bud.
Propagation
The easiest way to start new flame azalea plants in the home garden is layering. Bend a lower branch so that part of it rests on the soil surface. Scrape away a bit of the outer bark where it touches the soil and apply rooting hormone. Hold it in place in direct contact with the soil using a rock or landscape pin. After two years, the new plant will develop sufficient roots to be separated from the parent plant and moved elsewhere.
Flame azaleas can be propagated by softwood cuttings of new growth taken from mid-April through early May. Prepare a small pot with amended garden soil or a soilless mix. Take 6-inch cuttings and dip the bottom in a rooting hormone to increase root development before inserting it into the planting medium. Water as needed to keep the medium moist. Rooting should begin in four to six weeks, but young plants should be overwintered in a cold frame or a protected area before planting in the garden.
Flame azaleas can also be grown from seeds collected when the seed pods turn brown but have not opened yet, typically about the time of the first fall frost. When the temperature is at least 50°F, sprinkle the seeds thinly on prepared soil in a pot and barely cover them. Keep the soil moist but not soggy and place the pot in bright light. For even faster germination, keep the temperature close to 70°F and add a grow light. The seeds germinate in two to three weeks. Overwinter the seedlings in a cold frame or protected area before transplanting them to the garden. The young plants will flower in three to four years.
Potting and Repotting
Flame azaleas often are sold in 3-gallon pots, but the shrub grows quickly when young and isn’t well-suited to home garden container plantings. You can grow the shrub for a year or so in a large container (preferably with wheels for mobility) filled with well-draining potting soil designed for acid-loving plants, but you’ll need to leave the container outdoors year-round. Flame azalea doesn’t thrive as a houseplant and benefits from being transplanted into the ground.
Pests and Diseases
Although flame azaleas are not susceptible to many pests, they are occasionally infested with lace bugs, borers, or weevils. Keep the garden free of weeds and provide good air circulation to avoid problems. Deer browsing may be a problem where deer populations are high.
Root rot can occur when azaleas are grown in heavy soil or soil that doesn’t drain well. If root rot occurs, do not replant an azalea or rhododendron in that spot.
Powdery mildew—a plant disease that creates a powdery coating on leaves and flowers—occasionally occurs where air circulation is limited. Improve air circulation by pruning to thin the branches. When watering, aim water at the base of the plant rather than the leaves to prevent the spread of the disease.
III. Types of Flame Azalea
- Chatooga
Rhododendron calendulaceum ‘Chatooga’ bears flowers that are 2 inches across. They are attractive, ruffled, pink flowers with a yellow blotch. The shrub grows 8 to 12 feet tall and 5 to 8 feet wide and offers excellent fall color. Zones 5–9
- Golden Sunset Flame
Rhododendron calendulaceum ‘Golden Sunset Flame’ produces large flowers in orange, yellow, and gold shades and has great fall foliage color. It grows to 12 feet tall and 8 feet wide. Zones 5–8
- Smokey Mountaineer
Rhododendron calendulaceum ‘Smokey Mountaineer’ offers upright growth up to 12 feet and is covered with 2-inch, orange-red blooms in late spring to early summer. In fall, the foliage is spectacular. Zones. 5–9
IV. Uses and Benefits
Flame azalea is a very popular garden choice for borders and woodland gardens, because its flowers are so showy and come in many colors. As a shrub, it can hold its own against smaller plants and so works for defining different areas of the garden. It is also useful in very acidic soils, where it will grow quite happily. Mountain laurel is a classic planting combination.
V. Harvesting and Storage
Collect the flowers of flame azalea for cut flowers, focusing on ones that have not yet fully bloomed. These flowers are both more beautiful and can be enjoyed for a longer period of time. Use a sharp knife to cut the bottom of the branch diagonally; this increases the area where the cut flower can absorb water. To extend bottle life, do not let the flowers receive direct sunlight. Change the water frequently and cut the bottom of the cut flower obliquely when changing water.
