Plant Advice

2. PLANT ADVICE

In this section, you can learn more detail and background about the plants we grow.

      2.1 WHAT TO PLANT

There are over 30,000 registered rhododendrons, and Millais Nurseries grow about 800 of these, which is one of the widest ranges in the world!  Our full range can seem a bit daunting if you are not sure what you want, so we have split it into groups to help you narrow your search. This should help with size at 10 years, flowering month and habit. Within these groups, we have split them further to help you make your choice. You can search for these ranges in the search feature by typing ‘Easy’, ‘Plantsman’ or ‘Hedging’.

Easy to grow range:

What it says on the tin! These are generally trouble-free plants that will perform well on most suitable ericaceous soils. They will tolerate sunny and shady positions, and are not fussy growers, so they are the most suitable varieties for novice growers to try.

Plantsman range:

These plants are mainly species and some hybrids which require a bit more attention to detail. The plants themselves may require more careful positioning in a sheltered spot, and care should be taken to ensure that instructions in the description are followed, for instance some species such as neriiflora and taliensia cannot tolerate any fertilizer. These plants are generally better suited to more the experienced gardener or enthusiast who can understand the plants’ needs.

Hedging range:

Easy to grow, dense plants that are well furnished with foliage to the base to give good screening. Good for hedging, but also appropriate if you have a more challenging situation such as a windy hillside, also drier or damper situations.

Tall (1.75 metres after 10 years)

Medium (1.5 metres after 10 years)

Compact (1.2 metres after 10 years)

      2.2 RHODODENDRON AND AZALEA GROUPS

One of the most common questions are get asked is “what is the difference between a Rhododendron and an Azalea?”. So here is the botanical explanation!

All Azaleas are members of the genus Rhododendron. Rhododendron is a genus (a group of plants with shared characteristics) and Azaleas are a group within that genus, rather than forming a genus of their own. This is how you distinguish differences:

- An azalea has approximately 5 stamens in the flower, while other rhododendrons have 10 or more.

- Azaleas may be deciduous or evergreen, but other rhododendrons are evergreen.

- Azaleas are small to medium sized shrubs, but other rhododendrons range from tiny prostrate shrubs to huge woodland trees.

They all require the same conditions and cultural treatment for healthy growth.

So, within the Rhododendron genus, we have separated them into the following categories to help customers with their selections, though these categories are not recognised botanically:

Dwarf:

These are mainly alpine varieties, and range from very tiny plants from 20cm up to around 80cm after ten years. They can be species (found in the wild) or hybrids (bred by man). They mainly have small leaves up to about 30mm long, and mainly flower in April. They look lovely planted in groups of 3 or 5 and are perfect for smaller spaces. Great for welcoming the spring and stunning in rock gardens.

yakushimanum:

This sought-after group of compact hybrids originate from R. yakushimanum, which grows naturally on Yakushima Island in Japan. They grow into mound shaped plants of 80 to 100cm after 10 years, and are ideal for smaller gardens, near the front of border, or for more formal situations in containers. Often they have interesting foliage with nice indumentum (woolly on the underside of the leaf). They flower in May with neat small trusses, which all fade paler with age.

williamsianum:

R. williamsianum hybrids have characterful rounded leaves, and they make an interesting foliage addition to any border. They are generally tolerant of neutral soils which is valuable in many locations. They are relatively compact, 80 to 100cm after 10 years, which suits the front of border or container growing. Bell shaped flowers appear in April, welcoming in the spring ahead of the main flowering season.

Small leaved (cinnabarina and triflora):

Much under-rated in our opinion! Not grown by many nurseries, these are mainly species plants (cinnabarina and triflora) and grow 75 to 150cm in 10 years. They often have an upright habit, soft foliage and stunning bell shaped flowers in April and May. Their small leaves and upright habit make a good focal difference to a landscape of larger leaved rhododendrons.

Hardy Hybrid or tall hybrid rhododendrons:

These are the traditional rhododendrons with large flower trusses, some blooming from January until July. They are ideal for hedging and screening and will sit beautifully in borders, creating a colourful backdrop up to 2 metres tall in 10 years, growing much larger in maturity. Some are also scented. These hybrids have been bred around the world since about 1880, and some historic varieties are still grown today, e.g. R. Pink Pearl (pre 1892). Initially they were bred from wild collected species rhododendrons, but now most hybrids have complex parentage, which has improved vigour, habit and flower quality.

Species:

Millais Nurseries grows one of the widest ranges of species rhododendrons in the world! These are the true rhododendrons, originally collected from the Himalayas, Asia and America by some of the great plant collectors in the twentieth century. Note that species rhododendron names are usually printed in italics and lower case. They are often of Latin origin having been named by botanists. We love our species and you will see that many of them are identified by a collector seed number or a clonal name to represent the best available forms of these plants. A seed number can be cross-referenced with a plant collector’s field notes to show where the plant was collected from. All very exciting for true enthusiasts! We aim to propagate these recognised good forms by cuttings, grafting, seed and occasionally micro-propagation.

Big leaved Species:

These are the ‘show offs’ in our rhododendron world. Although a few hybrids exist, most are species plants with huge leaves up to 60cm long giving a real architectural structure to a woodland garden. They love woodland settings and do need the best moist sheltered positions to thrive. But thrive they will, growing up to 2 metres in 10 years, and up to 5m tall in maturity. The make great foliage plants, but you do need some patience as flowering can take 6-10 years, but so spectacular it’s worth the wait! Difficult to propagate by cuttings or grafting, we grow most from selected seed.

Deciduous Azaleas:

Deciduous, because unlike most rhododendrons, they drop their leaves in the winter, often after a fiery autumn display of red, orange or yellow foliage. The flowering season is mainly from April to early June, but look out for our own ‘Midsummer’ range which has extended the flowering into July and even August. Flowers can range from the delicate and subtle small flowered species in shades of pink and white, to the large flowered Exbury type hybrids in bold and brazen yellows, oranges and reds. Just take your pick, and don’t worry too much about planting plans – they all clash beautifully! Best in a fairly sunny position, they are tough and fast growing up to 150cm and many of them are scented. Deciduous azaleas can be divided into different groups all with distinct characteristics:

Species: Many are scented particularly luteum, occidentale and viscosum.

Ghent: These are some of the oldest hybrids dating from the 1850’s in Belgium. Highly regarded for their subtle, characterful small flowers.

Mollis: These were developed after the Ghents and have larger flowers, often in a trumpet shape with darker markings in the throat. They typically flower from early to mid-May, and for a long time were considered a bit prone to frost damage in cold gardens, but in milder areas they perform well. The plants tend to be a little lower, more compact and spreading in habit. They are reasonably heat tolerant and have a keen following from those that know about them, but they are not widely available at present.

Knaphill/Exbury: The next stage of azalea breeding produced flamboyant large flowered hybrids, some of which are scented. These were originally developed at Knaphill Nursery near Woking in Surrey, and further developed by Lionel de Rothschild at Exbury Gardens. These strong growing plants usually flower from mid-May to mid-June often with big bold coloured flowers.

Evergreen Azaleas:

Sometimes known as Japanese azaleas, these are great little evergreen plants that are happy in a window box or front of border in a large garden. Most grow 60-75cm in 10 years. Some are prostrate in habit, so excellent for cascading over walls and rockeries. With small leaves, many have lovely autumn foliage for that extra bonus. They mainly flower throughout May, with nakaharai types continuing to late June and even July. Flowers are in shades of red, pink, white and mauve, but no yellows (if you find a yellow, it will be a dwarf rhododendron with more stamens!). While it may be tempting to choose a large flowered variety, the small flowered types can give a much more subtle and calming effect in the garden.

      2.3 SUBSECTIONS

The genus rhododendron is divided into nine subgenera. These are further divided in sections and subsections of related species. We have listed the subgenera here together with the sections and subsections for the sake of completeness. However, you will note that we only list the subsection in the plant description as we think all this may well be too much information!

The nine subgenera are:

• Rhododendron: This subgenus contains two Sections called Pogonanthum (where there are ten species in cultivation) and Rhododendron. The Rhododendron section has 28 Subsections and over 200 species in cultivation. These are known as lepidote rhododendrons and have scales on the leaves which can be seen under a hand lens. Effectively, these are dwarf rhododendrons. The subsections are: Afghanica, Baileya, Boothia, Camelliiflora, Campylogyna, Caroliniana, Cinnabarina, Edgeworthia, Fragariiflora, Genestieriana, Glauca, Heliolepida, Lapponica, Ledum, Lepidota, Maddenia, Micrantha, Monantha, Moupinensia, Rhododendron, Rhodorastra, Saluenensia, Scabrifolia, Tephropepla, Trichoclada, Triflora, Unifora and Virgata.

• Hymenanthes: This subgenus which contains one section, Pontica. These are the larger leaved rhododendrons of which there are over 200 species in cultivation in 24 Subsections. These elepidote rhododendrons have no scales on the leaves when observed with a hand lens. The subsections of Pontica are: Arborea, Argyrophylla, Auriculata, Barbata, Campanulata, Campylocarpa, Falconera, Fortunea, Fulva, Glischra, Grandia, Griersoniana, Irrorata, Lanata, Maculifera, Neriflora, Parishia, Venatora, Pontica, Selensia, Sherriffii, Taliensia, Thomsonia, Williamsiana.

• Azaleastrum: In this subgenus there are two sections, Azaleastrum and Choniastrum. There are only a few species in these sections that are rarely cultivated.

• Pentanthera: This subgenus are the deciduous azaleas. Here there are four sections - Pentanthera (with two subsections and 18 species in cultivation), Rhodora, Sciadorhodion and Viscidula. The section Pentanthera contains two subsections called Pentanthera and Sinensia.

• Tsutsusi: These are the Evergreen or Japanese Azaleas and this subgenus contains two sections called Brachycalyx (with 15 species in cultivation) and Tsutsusi (with approximately 40 species in cultivation).

• Candidastrum: only one species.

• Mumeazalea: one species.

• Therorhodion: one species.

• Vireya: This subgenus contains over 150 species in cultivation and are non-hardy plants requiring a minimum temperature of 5°C. Originating from South East Asia they have stunning flowers and are only suitable for glasshouse production in the UK. Not grown by us.

      2.4 RHS HARDINESS RATINGS/ALTITUDE

RHS Hardiness Ratings:

The RHS revised their Hardiness Ratings for the UK in 2013 and they now run from H1 (tender) to H7 (very hardy). This replaces the previous system which ran from H1 to H4, still found in old reference books. The new ratings are incorporated on this website.

Here is a brief description (shown in degrees centigrade) but more detailed information can be found on the RHS website. https://www.rhs.org.uk/plants/trials-awards/award-of-garden-merit/rhs-hardiness-rating

• H3 (1 to -5) Half hardy, unheated greenhouse/mild winter.
• H4 (-5 to -10) Hardy, average winter.
• H5 (-10 to -15) Hardy, cold winter.
• H6 (-15 to -20) Hardy, very cold winter.
• H7 (<-20) Very hardy.

Please note that the RHS ratings are fairly broad and other factors such as age of plant, site selection, drainage and micro climates will need to be considered.

Altitude guidance:

Plants grown at higher elevations need to be tougher than those grown at lower elevations. This is because it is typically 6°C cooler for every 1000 metres of elevation. In practice, it means that plants growing in the foothills of the Himalayas will be sub-tropical plants not hardy enough for the UK maritime climate. From about 3000 metres you will start to find big leaved rhododendrons growing in the shelter of larch and conifer forest. Progressing further uphill, you will find most of the elipidote (non-scaly) rhododendrons growing up to around 4000m within a woodland setting. Above 4000m, you come above the tree line, and will find scrubby junipers and lepidote (scaly) dwarf rhododendrons growing in fairly tough and unhospitable conditions, often buried under snow for months of the year.

You can use this natural selection process, in conjunction with the Hardiness Ratings to make the correct choice of plant for your garden. So big leaved species will be ideal for sheltered lowland locations in the West Country, but not at the top of a windswept mountain. Likewise, a dwarf alpine variety that may be perfectly suitable for the Highlands of Scotland, may struggle with the heat (and drought) of the South of England.

      2.5 PLANT AWARDS

RHS Award of Garden Merit:

The Royal Horticultural Society publishes its list of 'Award of Garden Merit' plants on its website. The AGM aims to highlight the best plants available to the home gardener. You can spot the plants that have this prestigious award by the letters AGM after their name, and by the AGM logo on the picture.

The list of 'AGM' plants is highly respected, and covers all garden plants, including fruit and vegetables. First compiled in 1992, the lists are on a rolling review using Trials at RHS Wisley, and Committee discussions.

AGM plants have all been recognised for their ability to meet the following criteria:

• Excellent for ordinary use in appropriate conditions
• Of good constitution
• Essentially stable in form and colour
• Reasonably resistant to pests and diseases
• Available to buy in the UK

As part of the overhaul, all existing AGM plants are reconsidered to ensure that they still meet the stringent AGM criteria. Those that fall short are removed from the list, while new awards are made after round-table assessment by relevant committees or during RHS trials. David Millais, as Chairman of the RHS Rhododendron Camellia and Magnolia Group is involved with the Committees and Trials that produce these lists. For more information on awards please see the RHS website:

https://www.rhs.org.uk/plants/trials-awards/award-of-garden-merit/agm-lists

Other RHS Awards:

The AGM is now the dominant Award for plants and acts a guide to performance in the garden. Some older books give reference to the following awards, which are based mainly on flower quality at exhibition:

• PC: Preliminary Certificate
• HC: Highly Commended
• AM: Award of Merit
• FCC: First Class Certificate.

Other Awards:

The Bundesgartenschau (abbreviated to BUGA) is a biennial Federal horticulture show in Germany. It takes place in different cities around the country and features plants and landscaping. Many of Hans and Holger Hachmann's stunning Rhododendrons that we grow have achieved Bronze, Silver or Gold Medals from these shows.

      2.6 PLANT COLLECTORS

The first rhododendrons to arrive in the UK were R. ponticum and azalea luteum from the Caucasus mountains of Turkey, and Northern Spain during the eighteenth century. This was followed by a selection of deciduous azaleas from the Appalachian mountains in North America. It was not until the early twentieth century that the great plant collectors such as Frank Kingdom-Ward, George Forrest, and Ernest Wilson were sponsored by large estates to travel to the ‘unknown’ Himalayas to bring back fascinating rhododendrons which had never been seen before. This started a frenzied competition amongst garden owners to grow these new plants, and to breed new hybrids from the progeny.

There was then a large gap in collections due to the closure of communist China, until 1985. Ted Millais was at the forefront of a new wave of plant collectors, and he managed to gain access through a reciprocal agreement with Kunming Botanic Gardens so that he showed their staff the best of the British gardens and collections. Ted introduced a number of new rhododendrons for the first time including R. coeloneuron, huianum, irroratum ssp yiliangense and ochraceum.

Now, with the introduction of the Nagoya Treaty, the Convention on Biological Diversity (CBD) and the Convention on Trade in endangered Species of Wild Fauna and Flora (CITES), the collection of wild collected seed is not possible without permits, which are extremely difficult to obtain.

      2.7 RHODODENDRON BREEDERS

Since the first species from the mountains of the Himalayas began to be grown in Europe, keen landowners and their gardeners have tried to breed more and more choice plants with different flowering characteristics and flowering periods. Here we list a few prominent breeders:

Ted Millais:

(1918 – 2003) Ted and his wife, Romy, started Millais Nurseries in 1970 offering retail and mail order plants, turning their lifetime's hobby into a small business. Ted's interest was kindled by his uncle, J. G. Millais, a great naturalist, botanist and author, who identified and described many Rhododendrons for the first time and published his great two volume series 'Rhododendrons' (1917 and 1924).

As septuagenarians, Ted and Romy organised many exciting plant hunting trips to the Himalayas, and introduced several wonderful new species, some of which are now offered by the nursery. Over the years they developed what was former heathland at Crosswater Farm into a stunning woodland garden, which is open to the public in May (see opening hours). Today, their son David manages the nursery, which grows over 35,000 rhododendrons every year. Many of Ted's breeding trials are still being followed by David, and many of Ted's best plants continue to be named and introduced. Ted was keen to develop good new yellow flowering varieties, disease resistant cinnabarinum hybrids, and later flowering varieties to extend the flowering season and avoid the frost in this cold garden.

Hans Hachmann:

Without doubt, the most important German Rhododendron breeder ever, Hans Hachmann was born in 1930 and died in 2004. Starting as a gardener his passion for rhododendrons began at an early age. During his career he introduced hundreds of new varieties from his nursery based in Barmstedt in Germany. The nursery is today headed by his son Holger Hachmann, another excellent breeder of rhododendrons.

Lionel de Rothschild:

(1882 – 1942) was an English banker and Conservative politician best remembered as the creator of Exbury Gardens. He was the eldest of the three sons of Leopold de Rothschild (1845–1917) and a part of the prominent Rothschild banking family of England. Lionel de Rothschild developed an interest in horticulture at a very young age and is said to have planted his first garden at the age of five. In 1919, he purchased the Mitford estate at Exbury in Hampshire where he devoted a great deal of time and money to transform it into one of the finest gardens in all of England with more than one million plants. He co-sponsored plant-hunting expeditions to the Himalayas to collect seed for propagation and breeding. In all, he developed 1,204 new hybrids of rhododendron and azalea, a number of which we offer today.

Edmund de Rothschild:

(1916 – 2009) Son of Lionel de Rothschild, and recipient of the Victoria Medal of Honour (VMH), given by the Royal Horticultural Society. At the outbreak of WW2, Edmund joined the British Army; in May 1946 he was demobilised and returned to the family bank. He inherited Exbury Gardens on his father’s death but the estate had fallen into severe disrepair as a result of the War. While pursuing a career in banking, he also set about restoring the renowned 200-acre gardens. Edmund's expertise became such that in the 1950s and 1960s he served on the Council of the RHS. Today Exbury is managed by a charitable trust set up by Edmund involving his children.

      2.8 HOW WE PROPAGATE OUR PLANTS

We use several different methods of propagation such as cuttings, seed, grafts and micro-propagation.

Cuttings:

This is our main method of propagation. All our evergreen azaleas and dwarf rhododendrons are propagated by cuttings. Some of the tall hybrid rhododendrons and the deciduous azaleas are much harder to root by this method. Timing of taking cuttings is crucial, and ranges from early June to January to get the best rooting results. Most of our cuttings come from our own garden, trials area, and nursery but we do occasionally get material from other large Rhododendron gardens, particularly if we want to introduce a new or different variety to our range. We try to take semi-ripe cuttings, when new plant growth has hardened but is still flexible. We have modern propagation facilities with the latest automatic mist, humidity, heating and ventilation controls.

If you want to try taking cuttings yourself, start with something easy like evergreen azaleas. Cut the top 75mm of the current year’s growth, strip off the lower leaves, dip in hormone rooting powder, and stick them into a good fibrous peaty compost in a pot or tray. Water, and cover with a polybag to retain moisture and humidity, and place on a North-facing windowsill. Once you have mastered this, try something more difficult such as dwarf rhododendrons, yakushimanum hybrids or some of the hardy hybrids. Bear in mind these are more difficult to root and it will take three years to produce most plants, so you may just prefer to buy a ready grown plant from us!

Seed:

Many of our big leaved species rhododendrons are sown from selected seed, as this is often the only way to grow them. Seeds can be collected from rhododendrons that flower in your garden, and while these will not come true to their parent, it can be an exciting exercise. Rhododendrons are very promiscuous and they can be cross pollinated by birds and insects flying from one plant to another. To grow species or to propagate new hybrids, one has to carry out controlled pollination to avoid insect pollination. This process is best left to the experts! Seedling variation means that there can be differences from one batch of plants to another.

Seedpods are collected in the autumn, dried briefly to open the pod, sieved clean and then stored dry in brown envelopes in the fridge until we are ready to sow them. We sow onto moist spongey compost at 15°C in December or January, and cover with polythene to maintain humidity. Regular misting to keep the emerging cotyledon moist is critical. Germination usually takes 2 to 3 weeks in our propagation glasshouse and then we give extra lighting to encourage growth before the light levels improve in spring. The whole process takes 3 years until your plant is ready in a 3 litre pot.

Grafts:

Some varieties of Rhododendrons are particularly difficult to propagate from cuttings and we get better results by grafting. The process is similar in that we collect cuttings but instead of planting them in compost, we graft them onto the rootstock of another rhododendron during the autumn. In simple terms, we root Cunningham's White cuttings, and then make some accurate cuts to the base of that plant and splice the desired variety on top of it. By tying the two pieces together with rubber bands, the parts will fuse together forming a choice plant growing on the roots of Cunningham's White. We use Rhododendron Cunningham’s White as it is a reliable, disease resistant plant which rarely produces suckers from below ground because it is not too vigorous. In the past, grafted plants had a bad reputation for throwing out suckers because R. ponticum was used as the rootstock, and in time some branches escaped, so the mauve ponticum became the dominant part of the plant.

Micropropagation:

In other words, test tube rhododendrons! Since about 1985, this method of propagation has proven to be successful, particularly for bulking up new varieties. Buds are excised and sterilized, and with the use of growth hormones in agar, they are encouraged to multiply and produce many shoots in a growth jar under lights within a laboratory situation. Tiny pieces of cutting are rooted on agar gel, before being weaned into small cells of compost. This is a useful method to propagate the varieties that are hard to root by conventional means, but it is not something we do here.