Rhododendrons are a large group of flowering trees and shrubs numbering over 1,000 species. While celebrated as ornamental shrubs for gardens, rhododendrons are also classed as invasive plants. This article will cover everything about rhododendrons.
What are Rhododendrons?
Rhododendrons form a group of 1,024 flowering woody shrubs and trees. Members of this large genus include both evergreen and deciduous shrubs and are divided between four major subgenera. As domestic plants, rhododendron are prized by many horticulturalists for their vibrant blooms.
These showy flowers can range from pink or purple to orange, red, or yellow. Rhododendrons typically bloom in late spring and early summer. The wider rhododendron group includes subspecies such as azaleas.
Most Common Species of Rhododendrons
Rhododendron luteum “Yellow azalea”
The deciduous Rhododendron luteum is a hardy azalea variety with yellow flowers. It grows in southeastern Europe and parts of nearby Asia like Turkey. It typically reaches up to 4 metres tall.
This hardy evergreen variety is quite a small species, only growing to about 2 metres. Rhododendron “Scintillation” makes a good container plant as it has shallow roots.
Rhododendron calophytum “Beautiful-face rhododendron”
The Beautiful-face rhododendron is a large species that can reach up to 10 metres tall. The flowers are pink or white and emerge in the early part of spring.
Classification: Deciduous & evergreen
Native to: Asia & North America
Care Level: Easy to moderate
Common Problems: Leaf spot, Scale insects
Toxicity: Extremely toxic to dogs, cats, and people
How do you identify Rhododendrons?
Rhododendrons are woody shrubs that can be evergreen or deciduous. True rhododendrons have broad, tough foliage that feel similar to the texture of leather. Each bloom forms a stunning cluster of petals and true rhododendrons produce ten stamens.
Where do Rhododendrons come from?
A large amount of the rhododendron species are indigenous to Asia, typically in areas around the Himalayas. Rhododendrons are also found in North America, inhabiting regions ranging from California and the Pacific Northwest along with the Appalachian mountains. Various types of rhododendrons also grow in parts of Australia and Europe.
Ecology of Rhododendrons
Rhododendrons have various ecological impacts, both positive and negative. Rhododendrons are frequently celebrated for their ability to attract bees and other pollinators. Many rhododendrons, especially deciduous varieties, produce stunning scents that lure these insects.
But rhododendrons can also be voraciously invasive plants, especially in the British Isles. Rhododendron ponticum is the main culprit. Rhododendrons can rapidly overpower native flora, impacting biodiversity.
Seasons for Rhododendrons
The typical flowering season for most rhododendrons runs from mid to late-spring and into mid-summer – usually April through to July. In some climate zones, a few varieties can flower earlier or later. Deciduous rhododendrons will lose their leaves during the autumn and winter, while evergreen varieties will retain their foliage all year round.
Basic Care Tips for Rhododendrons
Even though rhododendrons are ornate statement shrubs, they don’t actually require too much care. Here are the basics of cultivating rhododendrons.
Rhododendron shrubs utilise shallow root systems, so don’t need to be planted too deep into the soil. The chosen location should be prepared with organic matter, preferably acidic mediums such as decomposing pine needles.
Dig a hole that has the same depth as the rootball but is slightly wider. Fill in around the rootball with acidic or ericaceous compost and organic matter such as leaf mould.
Rhododendrons thrive when their soil is kept slightly moist. Never let them dry out completely. Use rainwater rather than tap water wherever possible. Tap water makes the soil too alkaline for rhododendrons, who need acidic growing mediums.
Most rhododendron varieties grow best in shady areas with indirect, dappled sunlight. This is similar to the conditions in jungles and forests where rhododendrons typically grow. Direct morning sun is beneficial, but direct afternoon sun is too intense for rhododendrons.
Acidic soil is best for rhododendrons, preferably with a pH level that ranges from 4.5 to 6.0. Neutral soil can be acceptable but alkaline soils will not support rhododendrons well.
The soil needs to provide nutrients for the rhododendron shrub, so incorporate acidic organic matter. These additions can include decomposing conifer or pine bark and needles or leaf mould. Acidity can also be provided through watered-down black tea or coffee grounds.
If given enough organic matter through the soil, rhododendrons don’t need regular fertilising. However, container-grown rhododendrons benefit from fertilising with acidic or ericaceous fertilisers. Refreshing the upper layer of compost regularly also provides fresh nutrients for rhododendron shrubs grown in containers.
Soils with low nitrogen levels can hamper rhododendrons. Use a soil test to identify the soil composition to check the nitrogen levels. If the amount is too low, use blood meal or manure to increase the amount of nitrogen in the growing medium.
Mulching is very important for rhododendrons. Adding mulch helps preserve moisture in the growing medium, reducing the risk of the rhododendron drying out. It’s also a way to add extra nutrients to the soil.
Use acidic sources of mulch like leaf mould and composted bark chippings or pine needles.
Rhododendrons grow fairly slowly and won’t need regular pruning. While the shrub is flowering, feel free to deadhead any flowers that are past their best. This helps divert energy and resources into younger blooms.
The main reason to prune rhododendrons is to shape them. These plants are usually used as ornamental statement plants, so shaping them is something that many gardeners do. After the shrub’s flowering period is over, trim away any older branches or those that are dead or diseased.
Avoid drastically cutting back the rhododendron as the shrub will find it hard to recover. Trimming away older stems helps more air circulate around the roots and young stems, keeping the rhododendron healthy.
Rhododendrons do well as container plants, but they require repotting every one or two years in the spring. Repotting into a larger container helps the rhododendron continue to grow.
When it comes time to repot, transfer the rhododendron to the new pot with some of its previous soil. Then add fresh growing medium, such as ericaceous compost, around it to provide new nutrients. This allows the rhododendron to acclimatise to its new pot.
Rhododendrons can be easily propagated in two ways – taking traditional cuttings or layering the cuttings. For the traditional method, take a young heel cutting from the shrub towards the end of summer and pot it up.
The layering technique similarly involves choosing a semi-ripe branch during mid-summer. This stem should be fairly low down on the shrub. Trim away secondary leaves but keep the main bushel of foliage at the tip of the stem.
Make a half-cut into the mid-point of the stem and gently bend the stem down to the ground. Use some strong wire to hold the branch against the ground. Layer some compost over the top of the branch.
Use a stone or something similarly heavy to keep the branch pressed into the ground. The shoot will develop roots in the autumn, and after this it can be fully divided from the original plant and given its own pot.
Leaf spot – This is a fungal disease that attacks unhealthy rhododendrons. The main symptom is the appearance of dark blotches on the foliage. All affected leaves should be pruned and extra care should be taken to provide the shrub with the correct growing conditions.
Scale insects – These pests siphon sap from rhododendron leaves. Their presence is indicated by scaly spots on the foliage as well as black, sooty mould. Scale insects can be removed using insecticidal soaps or neem oil solutions.
Rhododendrons are an easy way to add a show-stopping ornamental plant to a garden. There are over 1,000 of these deciduous or evergreen shrubs. Rhododendron shrubs can be grown directly in the ground or as container shrubs.
Garden varieties need minimal maintenance as long as they are placed in a shady location with slightly acidic growing medium. Rhododendrons also require consistently moist soil.