Stewartstonian Azalea Plant Profile

All the Red You Could Want From One Plant

Stewartstonian azalea with its red flowers
David Beaulieu

'Stewartsonian' is an evergreen azaleas, a member of the Gable hybrid group, developed by the late Joseph Gable of Stewartson, Pennsylvania. A medium-sized (4 to 5 feet) low-spreading shrub, it has deep-green glossy leaves that may turn a chocolate-reddish color by late fall in regions with cold winters. The older leaves shed in winter, with newer mahogany-colored leaves replacing them. Spring brings clusters of bright orange-red flowers to 'Stewartsonian', drawing pollinators to your yard. This is a four-season shrub that offers more than most azaleas and rhododendrons.

Like nearly all shrubs, azaleas are best planted in the spring, but fall is also an acceptable time—and it can be the most economical time, as garden centers are sometimes closing out stock at this time of year. 'Stewartstonian' is a slow-growing shrub that can take a few years to achieve mature size, but once established it can last for 40 years or more.

Botanical Name Rhododendron 'Stewartstonian' (Gable hyrids)
Common Name Stewartstonian azalea
Plant Type Broadleaf evergreen shrub
Mature Size 4 to 5 feet tall, similar spread
Sun Exposure Full sun to part shade
Soil Type Fertile, evenly moist, well-drained soil
Soil pH 5.0 to 5.5 (acidic)
Bloom Time April or May
Flower Color Orange-red
Hardiness Zones 5 to 8 (USDA)
Native Area Hybrid plant; original parents are probably species from Korea and Japan

How to Grow 'Stewartstonian' Azalea

'Stewartstonian' requires a moderate amount of maintenance. Locating it in part shade and in a soil that drains properly is a good start. But you also have to keep it irrigated, fertilized, and pruned. Like all members of the genus, 'Stewartstonian' likes an acidic soil, and if its health begins to slip, it can be a difficult task to nurse 'Stewarstonian' back to full health.

Light

Plant this azalea in a part-shade location in the southern part of its range. In colder northern regions, it will do well in full sun.

Soil

Enrich the ground (and improve its drainage at the same time) by mixing in generous amounts of decomposed organic matter. These plants prefer acidic soil (pH 5.0 to 5.5), which may require amendment with sulfur, peat moss, or pine needles; or feeding with an acidifying fertilizer. This plant is exceedingly fussy about its soil and will fail to thrive if the pH and basic soil nutrients aren't to its liking.

This shrub will die if exposed to the soil toxins produced by trees in the walnut family.

Water

Make sure the shrub's soil gets consistent moisture—a total of roughly 1 inch per week, preferably in at least two separate waterings. The roots on azaleas should never be allowed to dry out. You must keep the soil moist for this plant throughout the growing season, and applying mulch will help retain the soil moisture it needs.

Temperature and Humidity

'Stewartstonian' is well suited for the climate variations found in USDA zones 5 to 8. These plants don't grow well if subjected to prolonged temperatures over 90 degrees Fahrenheit, and may die out if winter temps fall below about minus-5 degrees. Azaleas typically don't react well to extremely dry environments. In dryer climates, it helps to increase watering, especially as the plant heads into the winter months.

Fertilizer

Fertilize this plant annually with compost. If necessary to lower soil pH, you can use an acidifying fertilizer.

Pruning 'Stewartstonian' Azaleas

Prune 'Stewartstonian' after it is done flowering to shape the bush, as needed. These are shrubs that flower on old wood, so they should be pruned after the flowering period has concluded; this gives the plant time to develop the growth that will create next spring's flowers.

Common Pests/ Diseases

All shrubs in the Rhododendron genus can be susceptible to a variety of insect and disease problems, though it is usually weakly plants that experience such problems. Healthy azaleas can be remarkably problem-free in the right environment.

Insects that may plague azaleas include aphids, borers, lace bugs, caterpillars, leafhoppers, mealybugs, nematodes, scale, thrips, whitefly, and mites. Minor problems can be ignored. Severe infestations should first be addressed with insecticidal soaps and oils before you turn to more toxic chemical pesticides.

Common diseases include blights, canker, crown rot, leaf gall, root rot, leaf spot, rust, and powdery mildew. Some of the leaf fungal problems are largely cosmetic and rarely kill the plant, but root rots, vascular diseases, and some cankers may require that you remove and destroy affected branches or even the entire plant. Fungicidal sprays and powders may prevent or cure some fungal diseases. Pruning to improve air circulation can help prevent some diseases.

If leaves turn yellow, it may indicate an iron deficiency that occurs when soil pH is too high. Treat this by using an acidifying fertilizer or mulching with an acidic organic material, such as peat moss or pine needles.

Landscape Uses

Azaleas are among the most popular spring flowers in North American landscaping and should be given a location where you can easily take in their beauty as you go about your daily chores in spring. 'Stewartsonian' has attractive evergreen foliage that makes it a good choice in almost any four-season application, from woodland borders to foundation planting. This can be a great specimen for mixed shrub borders. Finally, azaleas have shallow root systems that make them relatively safe to plant over septic drain fields.