Site Advice

1. SITE ADVICE

Rhododendrons are usually trouble free, providing care is taken in the initial site selection and planting. Their main requirements are a lime free soil, plenty of water and some shelter from scorching sun and wind. Autumn is the best time for planting as this is when most roots grow, but conditions are also good from Christmas to the end of March. Spring and summer planting is fine, but give extra attention to watering.

      1.1 CAN I GROW A RHODODENDRON?

Soil types:

All rhododendrons like moist acid soil (pH3.5 - 6.0). Some varieties such as deciduous azaleas will tolerate pH6.5, while the new INKARHO rhododendrons are grafted onto a special rootstock and will grow in neutral or even chalky soils (pH 7.5). They even tolerate heavy clay soils, and enable Rhododendrons to be grown in many more places than before. See if there are other rhododendrons in your area, or buy a pH meter to measure soil acidity in your garden.

Moisture:

Rhododendrons are moisture loving plants, but they do not like to be planted in damp conditions. When the soil is too damp, consider extra drainage channels, and plant with the rootball above ground and out of the wet. Rhododendron rootballs are shallow, and they only root in the top 40cm of soil, so they can’t compete against nearby deeper rooted trees and shrubs. Ensure that you plant at least 2m away from the trunks of large trees and hedges, remember that tap water is better than no water in the summer.  You might like to consider an automated irrigation system or seep house for large planting schemes.

Sun or shade:

Dappled shade (15-30%) is ideal, but many rhododendrons will grow in full sun, and some in deeper shade. A rule of thumb is the larger the leaf, the greater the shade needed. Many people think that rhododendrons are shade loving plants, but the great storm of 1987 proved that they prefer more sun, because they all performed much better with more light after the storm. In shady conditions, plants will grow taller, and be sparser, with fewer flowers, but these tend to last longer. In sunny conditions, plants will be squatter with more blooms, but the flowers may not last so long in hot weather.

Temperatures:

Rhododendrons vary from tender plants requiring a heated conservatory (Vireyas) to tough plants which will tolerate temperatures lower than -20°C (‘ironclads’). Each of our plants is rated for hardiness by temperature and the RHS hardiness rating (see 2.4), and you can filter for the plants that are best suited to your conditions. Just remember that young plants will need more protection than older plants, and that the micro-climate around your plants can be improved with overhead tree canopies, fleece, and bracken mulch. Further information on hardiness ratings can be seen here: https://www.rhs.org.uk/plants/trials-awards/award-of-garden-merit/rhs-hardiness-rating


      1.3 RHODODENDRONS ON LIMEY SOIL

Rhododendrons are acid loving plants, and require a pH of between 4.5 and 6.0 to grow well. For those on neutral or even slightly limey soils, improvements can be made to the soil, and there are varieties of rhododendrons which are worth trying. However, large scale changing of soil conditions cannot be condoned, is not environmentally sound, and is likely to prove only a temporary adjustment of growing conditions. This page aims to help those with marginal conditions to attempt to grow rhododendrons satisfactorily.

The Chemistry Lesson:

Rhododendrons require a fibrous, organically rich open soil, and will perform best with a pH 4.5 to 6.0 On chalky, limey and alkaline soils, where the water is hard, and where there is a high pH (above pH 6.0), rhododendrons will grow poorly with yellow leaves, or not at all. The reasons are still not fully understood, but high pH causes a toxic intake of calcium, which reduces the availability of nutrients and trace elements, and hinders photosynthesis. Some rhododendrons are found in limestone regions, but usually in areas of high rainfall on dolomitic (magnesium) limestone rather than carboniferous limestone. Recent research indicates the addition of a leaf-mould mulch, which is rich in manganese, is very beneficial.

The pH of soil can be lowered by the addition of very acidic compost in the form of a raised bed, or by the annual addition of sulphur in late winter, allowing time for the pH to lower before growth starts. Flowers of Sulphur is unpleasant but can be used, while Sulphur Chips are more expensive but easier. Preferable is Iron Sulphate which is readily available and also provides a useful source of iron. For each unit of pH that you wish to lower, iron sulphate should be used at the rate of 75 grams per square metre. (eg. To change a soil of pH 7.5 to pH 5.5 would require 150g per m2). Applications are often best split to 35g/m2 twice per year. Iron Sequestrine can also be effective in the short term, but is expensive and has little benefit over iron sulphate.

Rhododendrons are shallow rooted plants, so a 25cm raised bed of peaty acidic soil is quite feasible when using the most acidic ericaceous compost. We recommend Bulrush Ericaceous Compost which has a pH as low as 3.5. Mixed with neutral soil, this goes a long way towards acidifying your borders suitable for rhododendrons.

Rhododendron growing on chalky soils can not be recommended, but a membrane can be used to isolate the plant roots from the high pH. Old baths, tubs and plastic have been used by those keen to persevere, but are likely to be only temporary solutions. Problems with poor drainage and limey water seeping in can develop over time.
 

‘Inkarho’ Lime tolerant rootstocks

For more than 20 years, a German research organisation has been developing and breeding lime tolerant rootstocks, onto which normal varieties of yakushimanum and hardy hybrids can be grafted. Propagation is restricted to licensed growers in Germany, and we are pleased to offer these plants to the UK market which represent an exciting development for those struggling with soils up to pH 7.5. These are tough hardy plants that we have been selling since 2005, and we have selected varieties in a range of colours most suited to the UK climate. They are ideal for neutral clay soils and effectively enable 30% more gardeners in Britain to grow rhododendrons. They are strong hungry plants and require more fertilizer to keep their lush green foliage. (see Feeding 3.3)

 

Other rhododendrons suitable for neutral soils:

 

Species

augustinii

decorum

hippophaeoides

hirsutum

lutescens

makinoi

racemosum

rubiginosum

triflorum

williamsianum

yunnanense

 

Hybrids

Cunningham's White

Hybrids grafted onto Cunningham's White (see ‘Graft’ in plant description)

Most williamsianum hybrids


      1.4 HEDGING AND SCREENING

Recent research has proved that rhododendrons make one of the best sound barriers. Their dense, pendulous, evergreen leaves reflect noise pollution away. Few other species are as effective, and none can offer such stunning trusses of flowers.

What size and shape do you want?

Fast vigorous hardy hybrid rhododendrons are normally specified for tall hedges and screens growing to heights of 2 metres in 10 years. However, R. yakushimanum varieties are used extensively in German front gardens to make feature hedges around 1.2 metres tall in 10 years. It is usual to let rhododendrons develop naturally into dome shaped bushes, but trimmed hedges with a square profile are also possible and take up less space. Where this is required, light pruning should take place immediately after flowering, to allow flower buds to develop in time for the following season. Smaller growing varieties can also be planted in front to give more colour and depth to the border.

What is your flowering scheme?

The best colours are red, pink, white and mauve shades. Yellows and oranges are also possible, but are slower growing, require better conditions, and can be thinner and less effective at screening. Hedges can look stunning when just one single variety is chosen. Also effective is a single row of 2 varieties planted alternatively and chosen to flower at different times. However, most informal screens and boundary plantings consist of several or many different varieties. In this way, a flowering season from February through to June is quite possible. Where space permits, planting in groups of 3 is recommended, as this adds more impact to the flowering, particularly when seen from a distance.

 

What conditions are required?

All rhododendrons require an acid soil (pH between 4.5 and 6.0) unless Inkarho rootstocks are specified which are suitable for neutral soils (pH 6.0 - 7.5). Soils must be kept moist in summer, and not waterlogged in winter. The varieties listed here are all tough and well suited for hedging. They will all take full sun or shade. In full sun, the flowers will tend to bloom more readily, but not last so long, and the plants may be a little shorter. In deep shade, early flowering varieties may be protected from late frosts, late varieties will last longer without the sun to burn them, but growth (and hence flowering) can be more sparse. Semi-shade is ideal.

 

Key Points:

Allow room for the plants to grow. A plant which is 2 metre high after 10 years will require 2 metre spacing. (See the plant description for heights at 10 years and use this height as a spacing guide.) Do not plant too close to walls, fences, drives etc. Avoid planting too close to existing strong hedges, trees, and dry banks.

Before planting, thin any trees and remove any low branches.

Do try to plant in the autumn, when the ground is warm and establishment is easier. Planting is also fine up until the end of March.

Rhododendrons hate being planted too deep, and it is the most common cause of failure. Do not plant any deeper than the top of the pot or rootball.

Plant them well, and they will reward you well. Use plenty of organic leaf mould, or well-rotted compost, with a light feed of fertiliser (e.g. Millais Ericaceous Slow-Release Fertilizer). Avoid animal manures, mushroom compost and bonemeal which are all unsuitable for rhododendrons.

Mulch with bark or leaf mould, and ensure the roots are kept free of weeds and grass.

Dead head old flower heads for the first few years.

Recommended Varieties:

We can recommend the following varieties which are all dense to the ground and give plenty of privacy. We normally keep good stocks, but please remember that we do sell out, and we recommend early ordering. We do not stock or recommend R. ponticum which can prove particularly invasive, and seems prone to the new disease Ramorum Dieback. R. Fastuosum Flore Pleno is more appropriate.

 

Tall (1.75 metres after 10 years)

Albert Schweitzer

Pink

Late May

Beau Brummel

Red

June

Fastuosum Flore Pleno

Lavender

 Late May

Gomer Waterer

Pinky white

 Early June

Horizon Monarch

Yellow

May

Jean Marie de Montague

Red

 Early-mid May

Lem's Monarch

Pink

 Late May

Lord Roberts

Red

 Early June

Madame Masson

White

 May-June

Mrs Charles Pearson

Pinky white

 May-June

Mrs T H Lowinsky

Pinky white

 June

Van

Pink

 May


Medium (1.5 metres after 10 years)

Caucasicum Pictum

 

Pink

 April

Christmas Cheer

 

Pink

 March

Cosmopolitan

 

Pink

 Early May

Cunningham's White

 

White

 May

Nova Zembla

 

Red

 Late May-June

Polarnacht

 

Purple

 May-June

Taurus

 

Red

 April-May

 

Compact (1.2 metres after 10 years)

Dreamland

Pinky white

 May

Fantastica

Pink

 May

Hoppy

Pale lavender

 May-June

Hydon Dawn

Pink

 May

Marlis

Pink

 May

Millennium Gold

Yellow

May

Percy Wiseman

Creamy pink

 May

September Red

Red

May

Silberwolke

Pinky white

May


      1.7 HOW TO PLANT A RHODODENDRON

Site selection: 

Most rhododendrons will take sun or shade, but dappled shade is best. Select a position with moist acidic soil (in the range pH 3.0-6.0, but pH4.5-6.0 is best), and avoid planting close to large trees, hedges and shrubs which will steal all the soil moisture. Choose a position with enough room for the plant to grow – the height we give at 10 years is also a guide for your plant spacing. Rhododendrons need to be planted at least 2m from large trees and hedges. Dry banks and disturbed soils can be particularly inhospitable. Avoid open lawn situations or too close to walls and fences. Also ensure there is good winter drainage, as rhododendrons cannot survive with their roots in saturated soils. Yellow varieties in particular need good drainage.

Site preparation: 

Always clear any perennial weeds and grasses from the site, which may require several applications of herbicide during the previous growing season. Check to make sure that you have removed any overhanging tree branches before starting, and that the planting hole is clear of invasive roots. Your position may need to be modified if you come across tree roots or other obstructions. The planting position should be in good topsoil, not disturbed subsoil which has insufficient organic matter. On any hard compacted soils, break up and loosen a wide area to allow the roots to grow. Dig a wide hole, at least twice the width of the rootball, but there’s no need to go deeper than about 25cm.

Compost:

Choose a good ericaceous compost, such as the Bulrush Forest Gold Ericaceous compost that we stock. Many well-known brands from garden centres and multiples are not good quality and have been milled too fine, giving poor aeration and drainage. Mix in plenty of ericaceous compost around the roots, adding your own well-rotted leaf mould, pine needles or compost as a mulch. Never use animal manures which are too strong for sensitive rhododendrons, or mushroom compost which is too limey. Even on good soils we recommend at least 10 litres of compost for a dwarf plant, and 20-30 litres for a larger hybrid. Specimen sizes need more compost, say 60-120 litres per plant. Remember, the better you plant, the more you will be rewarded.

Planting: 

Do not plant too deep (see diagram). Rhododendrons hate being planted deeper than the top of the pot. The number one reason for failure is when the lower stems have been buried below soil or mulch level, and this needs careful checking when planting on a slope. On damp sites, planting on a mound is recommended to raise the rootball above any wet. Give your plant a dose of slow release ericaceous plant food (we recommend Millais Ericaceous Slow Release Fertilizer).

Mulching:

Add 25-50mm mulch of leaf mould, composted bark or wood chips to conserve moisture, insulate the soil, and reduce weed growth, but ensure that any mulch is not applied around the stem. Rhododendrons are surface rooting and we never recommend any weed suppressing matting, slate or gravel mulch, which often cause problems with aeration and drainage for the surface roots of rhododendrons. Matting spoils soil aeration and drainage. Stones and gravel are too heavy, and transmit excessive heat in summer and cold in winter.

Feeding: 

Just like us we all need regular meals, so do young rhododendrons! Give your plants a dose of slow release ericaceous plant food in March and again in June. We do not recommend bone-meal which has too much calcium. When planting in the autumn, (the best time) a small dose of superphosphate will encourage good root growth.

Maintenance: 

Ensure that your plant has plenty of water during the summer, especially on dry sandy soils. Rhododendrons are shallow rooting, and will not be able to compete with nearby deep rooted trees and shrubs without additional water. Avoid perennial weeds and grasses growing around the plant. Straight after flowering, pinch out the old flower heads to encourage healthy new growth and energy for next year’s flowers, and to nicely ‘groom’ your plant for the summer. In the autumn consider a light mulch (around 30-40mm) to prevent weeds, conserve moisture and feed the plant. Enjoy!


      1.8 PLANTING IN CONTAINERS

Rhododendrons and Azaleas can be grown very successfully in containers providing you are prepared to maintain them properly, and pot them on into a larger pot with fresh compost every 2-3 years. In some locations, this may be the only way to grow these beautiful plants. We do not recommend planting the taller hybrids in pots as these can become pot-bound and top heavy, nor our Plantsman range that may need a little more TLC. The most successful varieties are Compact and Dwarf Rhododendrons, and Evergreen and Deciduous Azaleas. The addition of bulbs gives extra interest, but avoid planting other plants which could complete with your rhododendrons. Follow these tips and your plants should flourish.

Pot Selection:

It’s best to choose a pot that is not too much larger than the original, and to pot on again every 2-3 years. Choose a pot that is about 50-75mm wider on each side than the original pot, to allow for a few years’ growth. Remember Rhododendrons are shallow rooted so choose a pot that is wide, rather than a deep bucket style pot. Pots can be plastic, ceramic, wood or stone, but wooden ones will offer useful insulation to the roots of Camellias. Drainage is essential, preferably in the base and at the sides to allow excess water to drain out.

Compost:

Choose a good ericaceous compost, such as the Bulrush Forest Gold Ericaceous compost that we stock. Many well-known brands from garden centres and multiples are not good quality and have been milled too fine, giving poor aeration and drainage. Add extra composted bark at the base of the pot, and mix through the compost for extra drainage. Follow our tips on How to plant a Rhododendron – see 1.7 to ensure that you don’t plant too deeply. Finally, mulch the top with bark chippings or pine needles, but make sure that you do not bury the stem over time. Avoid using pea shingle, slate or stone mulches, as these conduct heat and cold to the shallow roots of the plant. Roots don't like the weight of stones above them.

Feeding:

You wouldn’t survive long without food, and neither will your rhododendron. The easiest way to provide essential nutrients is to use a good granular fertilizer (such as Millais Ericaceous Slow-Release Fertilizer). Add a sprinkling to the top of the pot in March and again in June. All the nutrients will be taken into the root of the plant over the course of the growing season. Water soluble ericaceous feeds are very effective at greening up plants quickly, but provide no longevity through the season, and can scorch some of the more sensitive dwarf varieties. Use during the growing season and follow the instructions on the pack. See Feeding 3.3 for further information.

Watering:

The magic ingredient! Your plant will not survive for long if you allow the roots to dry out, and small pots will dry out quickly in the summer. Equally, if the compost is soggy in winter, roots will rot and this will lead to poor growth and disease. Don’t stand the pot in water, but ensure that you water regularly, especially during dry spells. A substantial drink once a week is better than a light dribble. Rain-water is best, particularly in hard water areas, but tap water is better than no water! See Watering 3.2 for further information.

Winter protection:

Some of our tender plants love being outside for the summer months, but really benefit from moving into shelter for the winter. Some ceramic pots are also vulnerable to frosts, but damage can be reduced with pot ‘feet’ which also aid drainage. Camellias are especially vulnerable to root death in cold weather, but foliage will tolerate colder weather.  During very cold spells, foliage will often droop and look a little sorry for itself, but don’t worry it will perk up again once the temperatures warm up. We ‘bulk up’ our container plants during the winter (huddle them together to keep warm!). If the temperature is likely to drop below –10°C then cover them with fleece or better still dig the container into the ground or transfer them to a cold greenhouse.