Rhododendron Growing Guide

Big, bright splashes of colour. Glorious blooms blossoming all over your garden. Dazzling bushes that can grow to tower over you – and even your home!

We’re talking about rhododendrons, of course – one of the world’s most beloved garden plants. Read ahead for everything you need to know about growing these lovely flowers in your own home garden.

What Is A Rhododendron?

Rhododendrons are a genus of at least 1,024 species of woody plants. They belong to the heath family (Ericaceae) and can be either deciduous or evergreen.[1] They are originally from Asia, but can now be found growing wild in lowland forests in North America, particularly in the Appalachian Mountains, Pacific Northwest, and California.

These luscious blooms are the national flower of Nepal, the state tree of Sikkim and Uttarakhand in India, the state flower of both Washington State and West Virginia in the US, and the provincial flower of Jiangxi in China.

While you might see Azaleas listed as Rhododendrons, they actually make up two subgenera. They have just five anthers per flower, which differentiates them from ‘true’ Rhododendrons.

Where Can You Buy Rhododendrons?

Head to your local garden centre to peruse the selection of Rhododendrons, or shop online for seeds or plants.

Here are some tips for shopping for Rhododendrons or Azaleas:

• Some varietals blossom in March, while some blossom in July or even September – check out when they bloom before you purchase.
• Make sure your leaves are not yellowed or wilted – they should be deep green.
• Check the soil – don’t buy any plant that is bone dry.
• If you’re planting in the South of England, avoid white Azaleas, as their petals can shatter if the temperature tops 30 C.

When Do Rhododendrons Flower?

Most Rhododendrons flower in the spring, but you can find some varieties that blossom in July or even into the fall. These late-flowering Rhododendrons include Cornielle, Polar Bear, and Cynthia.[2]

When Should You Plant Rhododendrons?

Plant your Rhododendrons in October or March/April, depending on the variety.[3]

How To Grow Rhododendrons?

Follow these steps to grow Rhododendrons in your garden.[4]

1. Dig a hole twice as wide as the root ball, and just as deep.
2. Estimate their mature size, and ensure that the plants have 2 to 6 feet of space each.
3. Place top roots just below the soil level; any deeper, and they may rot.
4. Add soil to fill the hole halfway, and then water well before filling the remainder.
   Set new plants so that their top roots are at soil level or slightly below. If you plant them any deeper, the roots may rot.
5. To retain moisture, mulch plants in the spring with 2 to 5 inches of acidic mulch – Rhododendrons prefer pine bark chips and pine needles.
6. Avoid placing too much mulch around the base of the shrub, as this can encourage rot.
7. Only fertilise Rhododendrons sparingly when the buds swell in spring, as too much fertiliser will burn the plant and potentially kill it.
8. If you receive less than 1 inch of rainfall in a week, water your plants in summer.
9. Deadhead your flowers to promote more growth rather than only seed production. To do so, carefully remove the dead flowers, as the new buds are already present and will start to flower shortly.
10. If you experience severe winters, wrap up your Rhododendrons with burlap and add mulch to the base.

Plant Care Tips / How To Look After Rhododendrons?

Follow these tips to care for your Rhododendrons.

Do they prefer sun or shade?

Like most ericaceous plants, rhododendrons do not like the full morning sun. They prefer dappled shade in a position sheltered from the wind.

Watering advice

Rhododendrons prefer rainwater over tap water, and are hard to grow in dry regions. If you water your Rhododendrons with tap water, the calcium can cause too much alkalinity and lead to yellowing leaves. If you run out of rainwater, tap water will work for a month or two.

Soil tips

Rhododendrons prefer well-drained moist acidic soil between pH 5.0 and 6.0. Add plenty of organic material to help them thrive. If your soil is alkaline, add ericaceous compost to reduce the pH. If you have a rock garden, you will have success with Dwarf Alpine Rhododendrons.

When should you prune?

If you plan to prune your Rhododendron, fertilise in late fall of the year prior. If you delay, you may experience leggy growth. This is because next year’s buds form on this year’s flowers, so you need to prepare them at least a year in advance. Once you are ready to prune, trim around 10 – 15 inches from the most substantial branches, but never more than 20 inches.[5] Cut the plant back and expose the inner branches, and cut ¼ inch above the last cluster of leaves you want to keep. Rhododendrons usually have three main branches – make sure that you prune these back to different lengths so that the shrub remains natural in appearance. If you prune too heavily, you may not experience growth for a two years or more.

Advice For Growing In Pots

Planning to grow a Dwarf Rhododendron in a pot? Follow this advice.[6]

• Choose a wide, shallow container – Choose a pot that is around one third larger than the pot plant in which your Rhododendron came, and ensure it has plenty of drainage holes. You can also add plant feet to keep the base raised so that it is never sitting in excess water. It’s a good idea to repot every two or three years.
• Potting mix – Add potting mix designed for Azaleas to the pot, and set the shrub into the pot with the roots at soil level. To gauge this, assess where it sat in its previous container – Rhododendrons do not like to be planted deep. Add a little bit of mulch, careful not to load it around the base.
• Rhododendrons prefer dappled shade – Set your container in dappled shade, away from direct sunlight. It should get some sun, but never become scorched. Never let the soil become bone dry – keep it moist but never waterlogged.

Common Pests / Diseases

Rhododendrons experience some common pests and diseases.[7]

• Bud blast – The brown buds die, but remain attached to the plant, eventually turning grey and growing black bristles. Remove the leaves immediately or use chemical sprays.
• Powdery mildew – Evergreen Rhododendrons experience this fungus on its lower leaves. It can be hard to see this fungus in its early stages, but it will eventually cause yellow or reddish patches on the upper surface of the leaves. Deciduous Azaleas will experience a white powdery fungus on the upper surface of the leaves. Removing the leaves could cause bigger issues, so tolerate the fungus or remove with an anti-fungal spray.
• Petal blight – The first sign of petal blight is spotting, followed by the petals collapsing into slime. Remove the leaves at first sign of the spots, or treat with chemical sprays.
• Gall – You’ll see the leaves become pale and swollen before a bloom of white fungus spores emerges. Remove the leaves as soon as possible to prevent the fungus from sporulating.

Rhododendrons Summary

• Hardy or non-hardy? Mostly fully hardyx
• Soil types? Acidic soil with a pH between 5.0 and 6.0
• Shade type? They prefer dappled shade
• Evergreen? Some varieties
• Easy to care for? Yes, water them with rainwater and fertilise sparingly.
• Suitable for small gardens? Dwarf varieties are suitable, but beware – many Rhododendrons can grow up to 10 metres in height!
• Can be planted in containers or pots? Yes, dwarf varieties.
• Blooming month? Depending on the variety, they bloom in early spring, July, or even into late fall.

Rhododendrons look lovely in any garden

Relaxing in wooden garden chairs, admiring the fruits of your labour – nothing makes your garden look more bright and beautiful than a Rhododendron shrub. Which variety are you planning to plant this year?

Reference list

Badgett, B. (2019). Rhododendron Care. [online] www.gardeningknowhow.com. Available at: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-care.htm [Accessed 14 Nov. 2020].

Old Farmer’s Almanac (2020). Rhododendrons and Azaleas. [online] Old Farmer’s Almanac. Available at: https://www.almanac.com/plant/rhododendrons [Accessed 14 Nov. 2020].

Royal Horticultural Society (2020). Late-flowering rhododendrons / RHS Gardening. [online] www.rhs.org.uk. Available at: https://www.rhs.org.uk/plants/popular/rhododendron/late-flowering [Accessed 23 Nov. 2020].

Royal Horticultural Society (2019). Rhododendron diseases. [online] www.rhs.org.uk. Available at: https://www.rhs.org.uk/advice/profile?pid=573 [Accessed 14 Nov. 2020].

Royal Horticultural Society (2020). How to grow rhododendrons / RHS Gardening. [online] www.rhs.org.uk. Available at: https://www.rhs.org.uk/plants/popular/rhododendron/growing-guide [Accessed 14 Nov. 2020].

Wikipedia Contributors (2020). Rhododendron. [online] Wikipedia. Available at: https://en.wikipedia.org/wiki/Rhododendron [Accessed 14 Nov. 2020].

Woodie, M. (2018). How to Grow a Rhododendron in a Container. [online] Horticulture. Available at: https://www.hortmag.com/container-gardening/how-to-grow-a-rhododendron-in-a-container [Accessed 14 Nov. 2020].

[1] https://en.wikipedia.org/wiki/Rhododendron

[2] https://www.rhs.org.uk/plants/popular/rhododendron/late-flowering

[3] https://www.rhs.org.uk/plants/popular/rhododendron/growing-guide

[4] https://www.almanac.com/plant/rhododendrons

[5] https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/pruning-rhododendrons.htm

[6] https://www.hortmag.com/container-gardening/how-to-grow-a-rhododendron-in-a-container

[7] https://www.rhs.org.uk/advice/profile?pid=573

Anna Sharples

Anna is the marketing and office manager for Garden Benches – a premium supplier of high-quality wooden benches and other outdoor furniture.