Rhododendrons are a hardy flowering shrub grown widely throughout North America, Europe and Asia.
There are thousands of species of Rhododendron with several points of difference including blooms, height, leaf structure and size but also with many aspects in common.
The plant care tips I discuss below will focus on the general growing requirements of shrubs in the Rhododendron genus.
Rhododendron Plant Care Tips
Rhododendrons are extremely versatile, and you can plant them for virtually any landscape use.
Whether you want a ground cover shrub, medium-sized shrub, a ball-shaped shrub or even a tree-like shrub, you’re bound to find the best rhododendron for the job.
Because rhododendrons grow in all sizes and shapes, there is a lot of variety when it comes to rhododendron plant size.
Low-growing varieties stand at around 5-6 feet tall, while tall-growing varieties can grow to over 20 feet tall.
With the exception of some rhododendrons with soft bark that don’t regenerate if they’re cut back, most rhododendrons can be cut back to keep them at a manageable size.
Most rhododendron varieties will enjoy part sun to part shade. Full sun exposure or deep shade are not conducive to healthy growth or blooming, so avoid the extremes.
Rhododendrons should receive direct sun for only a part of the day, and moderate shade in the other part of the day for optimal growth.
Because of their root structure (rhododendrons have dense hair-like roots) and the fact that they’re anchored shallowly into the ground, rhododendrons require extra watering during particularly hot weather.
Don’t allow your rhododendron to dry out in hot weather, but don’t overwater it either. If you notice the leaves of your rhododendron droop in hot weather or brown splotches appear on the leaves, it means that your rhododendron needs watering.
Temperature & Humidity
Rhododendrons are generally cold-hardy, although some rhododendrons can withstand even temperatures of -35°F (-37°C). Other rhododendrons will exhibit leaf curl and droop at around 32°F (0°C).
There’s also a difference in wind and sun tolerance of small-leaf rhododendrons versus large leaf rhododendrons.
Large-leaf rhododendrons have a lower wind and sun tolerance. These rhododendrons should be planted near structures that offer protection from wind or under overhead canopies or shade structures. East and north side locations of the house are the most optimal for these plants.
In dry weather, especially when temperatures go above 95 F, rhododendrons should receive some misting to increase humidity around the plant.
Rhododendrons thrive in acidic, well-aerated soils with good drainage and high in organic matter. Because of the hair-like structure of the rhododendron’s roots, the soil should also be easy to penetrate.
To increase moisture and organic matter content in the site chosen for planting your rhododendron, you should work compost and aged, chopped leaves into the soil, or do the same with sphagnum peat moss or pine or fir bark fines.
To improve aeration and drainage, vermiculite or perlite can also be added to the soil.
During the first year after transplanting, rhododendrons should have their soil kept moist, but not wet or soggy. Placing organic mulch around the base of the plant helps in retaining and adequate level of moisture.
Rhododendrons that are planted in fertile soil do not require any extra fertilizing. If the soil is not as fertile, fertilizer can be applied in late winter or early spring. Choose a fertilizer created for plants that enjoy acidic soil.
Potting & Repotting
In pots or containers, rhododendrons should be planted shallowly; planting them too deep will eventually lead to plant death.
When the plant becomes root-bound, it should be transplanted and the roots should be loosened, while outer roots should be cut.
If you’re keeping a rhododendron in a container, make sure you plant it in well-draining soil to prevent root rot.
Rhododendron Plant Propagation
Rhododendrons can be propagated from seed, cuttings, and layering.
Seeds should be gathered in the fall, when the seed capsules turn brown. The harvested seeds should be dried and sowed in February by sprinkling them on the soil.
The soil should be kept moist, out of direct sunlight, and covered with a clear plastic to encourage germination. Sprouting will occur 3-8 weeks after sowing.
Propagation by stem cuttings should be done by harvesting them in early fall from new growths that are just beginning to harden off.
The cut-end of the stems should be dipped in rooting hormone and planted in moist soil. The pot should be kept at a temperature between 70-75°F. Cuttings will form roots in about 6 weeks.
Layering rhododendrons works by wounding a stem and keeping it down on soil. The scraped part can be dipped in rooting hormone to encourage root development.
Once roots are formed, the cutting can be separated from the mother plant.
In these FAQs, you can learn about the maintenance requirements of rhododendrons and tips on pest and disease control.
How to prune rhododendrons?
Pruning rhododendrons should be scheduled for late summer, early autumn, after the blooming period. When pruning, start with removing dead or diseased branches, then focus on giving shape to the plant. Withered flower clusters should also be removed.
Are rhododendrons toxic to pets?
Rhododendrons are considered toxic to pets. Because all parts of the plant are toxic, it’s best to avoid keeping this plant in your garden if you have pets.
Are rhododendrons prone to pests and diseases?
In some regions, rhododendrons can exhibit diseases and pest problems. In moist, cool weather powdery mildew can be a problem. Weevils, thrips, lace bugs, scale, aphids, spider mites and fungal issues can also appear.
Depending on the issue and its severity, insecticides or insecticidal soaps can help get rid of pest infestations.
Rhododendrons are extremely varied ornamental shrubs that come in an abundance of sizes, leaf patterns, and bloom colors.
Except for their specific soil and lighting requirements, rhododendrons aren’t fussy plants and make an excellent addition to your garden, with the caveat that they’re poisonous to most pets.