How to grow rhododendrons

Site selection

Choose a site with dappled shade in sheltered conditions. Avoid deep shade beneath other trees. Most rhododendrons will tolerate a more open site if sheltered from cold, drying winds. Dwarf alpine species will tolerate full sun provided the soil does not dry out. Avoid frost pockets and sites exposed to early morning sun.

• Rhododendrons need moist but well-drained, acid soil
  The acidity/alkalinity of soil is measured on a pH scale, which runs from 0 to 14. Neutral soil has a pH of 7. Acid (or ericaceous) soil has a pH of less than 7, and the lower the pH, the more acidic the soil. You can measure your soil pH using a simple testing kit. Acid-loving (ericaceous) plants include rhododendrons, camellias, heathers and blueberries. Adding lime to soil will reduce its acidity. A slightly acidic soil of pH 6.5 is the best general-purpose pH for gardens, allowing a wide range of plants to grow. between pH 5.0 and 6.0 that is rich in organic matter
• If you have alkaline soil
  The acidity/alkalinity of soil is measured on a pH scale, which runs from 0 to 14. Neutral soil has a pH of 7. Alkaline (lime-rich) soil has a pH above 7, and the higher the pH, the more alkaline the soil. Chalky or limestone soils are alkaline. You can measure your soil pH using a simple testing kit. If you have alkaline soil, choose plants that will thrive in it – acid-loving or ericaceous plants such as rhododendrons will not. Club root disease (which affects brassicas) is less prevalent in alkaline soil. , grow rhododendrons as container plants or see our page on more tolerant species and cultivars. Reducing soil pH is not simple
• Dwarf alpine rhododendrons are effective in a rock garden
• Larger rhododendrons are excellent for woodland gardens
• Compact hybrids are ideal for containers on shaded patios

• Plant in October or March/April
• Do not plant too deeply - all rhododendrons are surface-rooting and the roots should be just covered
• Apply at least an 7.5cm (3in) mulch
  Mulch is a layer of material, at least 5cm (2in) thick, applied to the soil surface in late autumn to late winter (Nov-Feb). It is used to provide frost protection, improve plant growth by adding nutrients or increasing organic matter content, reducing water loss from the soil, for decorative purposes and suppressing weeds. Examples include well-rotted garden compost and manure, chipped bark, gravel, grit and slate chippings. of chipped conifer bark
  Bark is the outermost layer of woody plants (trees, shrubs and woody climbers). It is several cells thick and provides protection against physical damage, disease and environmental stresses. Bark comes in a wide variety of colours and patterns, and these can help gardeners when identifying plants. The fissures and crevices of bark on older plants also creates valuable habitat for many garden creatures as well as lichens and small plants. or other acidic material. The mulch should be well- aerated
  Aerating means providing roots with enough oxygen to promote healthy growth. This can be done by ensuring that the soil is well-draining and not waterlogged, or by adding organic matter to the soil to improve its structure and allow for better air circulation. Aerating a lawn means creating small holes in the soil to allow air, water and nutrients to penetrate the roots of the grass. , not packed or firmed down
• Renew or replenish the mulch each spring when the soil is still moist

Container cultivation

Peat-free John Innes ericaceous loam-based potting compost

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own. makes a good compost choice, generally providing easier management of watering and feeding.

RHS research has found that soil-less potting media, including ericaceous peat-free potting composts are suitable for rhododendron growing. However, they can lose their structure over time leading to poor drainage and an airless root environment, causing leaves to brown and die back. Re-potting every other year into fresh potting compost in early spring as soon as signs of growth are seen is recommended. In the intervening year, replace the top 5cm (2in) of compost. You can re-pot back into the same pot if you trim-off up to a third of the roots to make room for fresh potting compost.

Planting, pruning and caring for a Rhododendron

Watering

• Rhododendrons grow best in areas of high rainfall. Even on suitably acid soils they are more difficult to grow successfully in the drier parts of the country
• Tap water, especially in hard water districts, contains too much calcium for rhododendrons, reducing acidity around their roots
• Use rain water for watering rhododendrons, but if rain water runs out, tap water is satisfactory for a month or two in summer

Feeding

When grown in decent, acid soil, rhododendrons are unlikely to need additional feeding. In soil, or potting compost

Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own. , where the pH

This describes how acid or alkaline soil is, and is expressed as a number between 0 and 14. Soil with a pH of 7 is neutral, an acid soil has a lower value, and an alkaline soil has a higher one. Most plants grow best in soil that’s within a specific pH range, so it’s useful to know your soil’s pH. Testing kits are easy to use and widely available in garden centres and online. level isn't sufficiently low, your rhododendron may suffer from magnesium, manganese or iron deficiencies. See our advice page on nutrient deficiencies for information on identifying the symptoms and how to remedy them. Plants in tubs or other containers will beneift from a slow release, granular fertiliser added in spring, or use of a liquid feed formulated for ericaceous plants. Take care not to exceed the manufacturer's recommended amounts, as the roots and leaves of rhododendrons can burn if over fertilised.

Rhododendrons don’t require much pruning other than the removal of dead wood and deadheading of spent flowers if practical.

• If pruning is necessary to restrict size, follow the guidelines in our profile on pruning evergreen shrubs
• Many rhododendrons respond well to hard cutting back; response is best from deciduous
  Deciduous refers to plants (mainly trees and shrubs) that lose their leaves seasonally, usually during autumn. This is a natural process triggered by the season's shorter days and lower temperatures and allows plants to conserve energy during the winter months. azaleas and rough-barked rhododendrons such as Rhododendron ponticum, rather than from smooth-barked kinds. After cutting back, mulch
  Mulch is a layer of material, at least 5cm (2in) thick, applied to the soil surface in late autumn to late winter (Nov-Feb). It is used to provide frost protection, improve plant growth by adding nutrients or increasing organic matter content, reducing water loss from the soil, for decorative purposes and suppressing weeds. Examples include well-rotted garden compost and manure, chipped bark, gravel, grit and slate chippings. , feed if soil conditions are poor, and keep well-watered to encourage new growth

Only species rhododendrons come true from seed. Named cultivars must be propagated from semi-ripe cuttings, grafting or layering.

• Rhododendron cuttings are taken from the current year’s growth during late summer or autumn, once the shoots have completed their seasonal growth and the bud
  A bud is a small, undeveloped shoot that contains the potential for new growth. Buds are typically found on stems, where they can be apical (found at the tip) or axillary (found between leaf axils) and may develop into leaves, shoots or flowers. is fully developed. Take cuttings
  A method of growing new plants from parts of an existing plant, such as sections of root, stem, leaf or bud. When prepared correctly and planted in the right conditions, they can produce roots and eventually become independent plants. There is a wide range of different methods for taking cuttings, depending on the plant and time of year. with a heel, and then wound the stem to encourage rooting
• Cuttings of evergreen azaleas are taken in mid- to late-summer. Deciduous
  Deciduous refers to plants (mainly trees and shrubs) that lose their leaves seasonally, usually during autumn. This is a natural process triggered by the season's shorter days and lower temperatures and allows plants to conserve energy during the winter months. azaleas are harder to propagate from cuttings; commercial growers take cuttings in early spring, and grow plants on in frames for up to two seasons before planting out
• Grafting is used where cuttings cannot be rooted. Rootstocks are taken from R. ‘Cunningham's White’. Saddle grafting
  This involves joining the roots of one plant to the stem of another related plant, so they fuse together to form one plant. There are various methods, all requiring great skill. Grafting is mainly carried out by commercial growers to combine one plant’s qualities of flowering or fruiting (the upper section or scion) with the vigour or resilience of the other (the roots or rootstock). Fruit trees are often grafted, along with a few ornamental shrubs and trees, and in recent years grafted vegetable plants (mainly tomatoes, but also chillies, cucumbers, aubergines, etc) have become available. (which involves cutting an inverted V in the scion
  A shoot (or bud) cut from a chosen plant for the purpose of grafting (or budding) onto a rootstock, in order to produce a new plant. stem) is done from January to March, and spliced grafting is done in spring
• Layering can take 15 months to two years (some smooth-barked rhododendrons may take three years until rooted). Check layered shoots in the autumn of the second year. If not well-rooted, partially sever and lift in the following spring as growth begins
• Seed propagation: sow into pots in January and leave the seed uncovered on the surface of the compost
  Can refer to either home-made garden compost or seed/potting compost: • Garden compost is a soil improver made from decomposed plant waste, usually in a compost bin or heap. It is added to soil to improve its fertility, structure and water-holding capacity. Seed or potting composts are used for growing seedlings or plants in containers - a wide range of commercially produced peat-free composts are available, made from a mix of various ingredients, such as loam, composted bark, coir and sand, although you can mix your own. . Alternatively, scatter the seed over the compost and ‘plough’ in using a pencil. Place the pots in a heated propagator
  A heated propagator is a portable heated structure with a vented lid and adjustable, constant, thermostatic temperature control. It provides a warm, humid environment to help seeds germinate and cuttings to root quickly.

Non-flowering and bud drop

• Rhododendron flower buds
  A bud is a small, undeveloped shoot that contains the potential for new growth. Buds are typically found on stems, where they can be apical (found at the tip) or axillary (found between leaf axils) and may develop into leaves, shoots or flowers. start forming in late summer. A short period of dry conditions at this time may cause flower buds to fail to form, or buds may only partially form, drying up and dropping unopened the following spring
• Prevent this by mulching and watering thoroughly and regularly during dry periods from July onwards

Leaf drop

• Leaf drop can occur following a period of drought and is usually preceded by drooping and rolling of the leaves. The oldest leaves are the first to drop. Higher leaves may show browning at the leaf tip or edge. Leaf drop can also occur following extended periods of waterlogging
• As with all evergreen shrubs, each year a proportion of the foliage (mainly older leaves) are shed in spring and summer. This is normal and not a cause for concern

Leaf droop

• Rhododendron leaves may droop in severe cold, but usually recover with a return to milder conditions

Other problems

• Windy, cold or wet weather can cause leaf scorch and flowers can be damaged by rain and frost
• Nutrient deficiencies may cause yellowing foliage known as chlorosis
• Vine weevil can be troublesome (especially for container-grown rhododendrons), as can rhododendron leafhopper, scale insects and the diseases that affect rhododendron such as bud blast, azalea gall and honey fungus