How to Grow and Care for Rhododendron English Roseum

With its large clusters of rosy, pink-purple flowers, Rhododendron English Roseum (Rhododendron x 'English Roseum') is a showpiece in the garden.

This hybrid evergreen, dense-growing rhododendron thrives in semi-shade in cooler regions as a foundation or accent shrub. Be mindful that all parts of the plant are toxic if ingested by humans and pets.

Learn how to grow and care for this spectacular plant.

Rhododendron English Roseum Care

Here are the main care requirements for growing Rhododendron English Roseum.

• Plant in an area with dappled sunlight or partial shade that is protected from harsh winds.

• Amend the soil so that it has a pH of <6.0 (acidic) before planting the shrub.

• Plant in an area with well-draining soil. Mulch to help keep the soil moist but it should not over-watered.

• Feed with an acid fertilizer after the blooming period is complete.

Light

This is a woodland shrub that will not thrive in full, harsh sun. Plant in an area with dappled sun or partial shade.

Soil

The soil should be loose and organically rich with good drainage. Test the soil to be sure it is acidic (<6.0) so the plant will produce bountiful blooms or add supplements to increase the acidity.

Water

Rhododendrons like moist soil but can tolerate slightly dry times. Mulch around the plant to help retain moisture but do not overwater.

Temperature and Humidity

Rhododendron English Roseum prefers a cooler climate with lower humidity. It is best suited for USDA Hardiness Zones 4a-7b.

Fertilizer

Fertilize yearly in the spring after the blooming period. The fertilizer should be acid-based. Working compost into the soil will provide the pH needed. If pruning the shrub, fertilization should be done about one month before pruning.

Types of Rhododendron English Roseum

Rhododendron English Roseum is a hybrid of Rhododendron catawbiense or the Catawba Rhododendron. Here are some similar cultivars:

• Roseum Elegans (Rhododendron catawbiense x 'Roseum Elegans'): A broadleaf, dense evergreen shrub with glossy leaves and showy lavender-pink flowers.

• Boursault (Rhododendron catawbiense x 'Boursault'): Upright, spreading branch evergreen shrub with lavender flowers with reddish-brown markings.

• Purpureum Elegans (Rhododendron catawbiense x 'Purpureum Elegans'): Robust, upright shrub with leaves up to 5 inches long and large clusters of light purple blooms.
  

Pruning

Pruning this rhododendron will improve the plant's structure, keeping it full. The best time to prune is right after they have finished blooming and new growth begins. Pruning is particularly important for young plants to increase branching and flower production.

When new growth begins on young plants, snip out any single shoots so that multiple branches will grow. As the plant ages, cut back any overly vigorous branches that extend beyond the desired size of the shrub. This will cause the plant to produce new shoots along the main stems adding foliage where it is needed.

Propagating Rhododendron English Roseum

This rhododendron can be propagated using cuttings or by layering. Layering is the easiest method and can be done in the spring or early summer.

Layering

1. What You'll Need: A healthy lower branch, a small knife, rooting hormone, and a stone or brick.

2. Where to Cut: Cut a slit in the lower branch where it will come in contact with the soil and treat the cut with rooting hormone.

3. How to Propagate: Place the cut on the soil and hold it in place with a stone or rock. Keep the soil moist and the cut area will sprout roots. In about two years, the root system will be strong enough to support the end of the branch. Dig up the new plant, cut the rooted branch away from the parent shrub, and transplant it into the garden.

Cuttings

Cuttings can be taken in early fall after new growth has begun to harden but isn't woody. Softer wood roots more readily than harder wood. Cuttings should be taken in the morning when they are full of moisture.

1. What You’ll Need: Healthy parent shrub, garden shears, plastic bag, a sterile mix of 50% milled sphagnum peat moss and 50% perlite, containers, rooting hormone that contains a fungicide, and acid-based fertilizer.

2. Where to Cut: Use garden shears to cut a 4 to 6-inch long stem of this year's growth. Cut just below a node.

3. Maintaining the Cutting: Remove the leaves from the bottom two inches and any flower buds. Dip the stem in water then in rooting hormone before inserting about two inches deep into a container of mixed peat moss and perlite. Place a plastic bag over the container and use sticks to keep the bag away from the foliage. Place in an area with indirect light and keep the cutting warm and moist. but not soggy. Fertilize monthly with an acid-based fertilizer. Rooting should take place in 3 to 4 months. Remove the plastic bag once the rhododendron is rooted and keep the soil moist until it is time to plant.

4. When to Plant the Cutting: Transplant the cutting into the ground in late spring or early summer.

How to Grow Rhododendron English Roseum From Seed

Patience will be required to germinate rhododendron seeds and grow this plant but you can do it. Start the seed indoors in late January or February.

1. Sprinkle the seeds in a small pot containing 50% milled sphagnum moss and 50% horticultural perlite. Do not cover the seeds with potting medium.

2. Place the pot in a plastic bag and place in a warm area (65-75 degrees F.) with indirect light. Rotate the pot once or twice a week to give all surfaces equal light.

3. The seeds should sprout within three to eight weeks. Leave the pot in the bag to maintain humidity levels until the plant has a set of true leaves. The seedlings can then be transplanted into larger pots. Do not transplant into the garden until the plants are around two years old and can withstand outdoor conditions.

Overwintering

The only extra attention these shrubs need in the winter is if there is heavy snow and ice that can break the limbs. If those conditions are predicted, wrap the shrub in burlap. Don't forget to remove the burlap once the weather warms.

Common Pests and Plant Diseases

Several insects can cause damage to Rhododendron. The Rhododendron borer, Synanthedon rhododendri, and stem borer (Oberea myops) cause wilting of leaves and twig dieback. The borer can then become an adult beetle that feeds on the undersides of the leaves and causes them to wilt. Black vine weevils (Otiorhynchus sulcatus) and Rhododendron gall midge (Clinodiplosis rhododendri) feed on the leaves and can defoliate the shrub.

Botryosphaeria canker, a fungal disease, is the most common disease of rhododendron in the landscape causing scattered dying branches on an otherwise healthy plant. There are many common fungal leaf spot pathogens of Rhododendron spp. but most do not cause serious harm and are cosmetic.

How to Get Rhododendron English Roseum to Bloom

Protecting the shrub from full sun and keeping the acidic soil cool and moist will keep these plants blooming for many years. Do not overfertilize mature plants, as you'll get more leaves than blooms.

Avoid pruning in late winter or early spring, or you risk removing the buds that will become flowers. Deadhead the spent blooms to encourage new bud production. Protect the shrubs during bitterly cold weather and heavy snows or ice storms.

Bloom Months

Most English Roseum bloom in late spring or early summer, depending on your hardiness growing zone.

How long does Rhododendron English Roseum bloom?

The shrub can bloom for two to five weeks depending on weather conditions.

What do Rhododendron English Roseum flowers look and smell like?

The flower heads are held in dome-shaped trusses of 8-10 flowers. The rosy, lilac pink blooms are funnel- or bell-shaped with 10 stamens. While some rhododendrons are fragrant, the English Roseum hybrid is not highly perfumed.

Common Problems with Rhododendron English Roseum

Even with proper care, there are a few common problems.

Leaves Turning Yellow

Soil that is too alkaline can cause leaves to turn yellow. Rhododendrons need acidic soil. To help correct the problem, amend alkaline soil with compost or acid-based fertilizers.

Wilting Leaves

Overwatering or drought can cause wilting or drooping leaves. Keep the soil moist and be sure to mulch plants well.

Frequently Asked Questions

How long can Rhododendron English Roseum live?

Rhododendrons that are well-tended can live for decades. English Roseum was introduced in the 1950 and many original specimens are still thriving.

Where should I place a Rhododendron English Roseum?

Choose a spot that is not located in full sun; the shrub thrives in dappled or partial shade. The location should have good drainage and avoid low-lying areas. The shrub works well as a specimen plant in the garden or a foundation plant.

Can I grow Rhododendron English Roseum in a pot?

While some rhododendrons can be grown successfully in a container, English Roseum is a large variety and should be planted in the garden. Choose a dwarf variety such as Rhododendron x 'Cunningham's White' or Rhododendron x 'Landmark' instead.

Read the original article on The Spruce.