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Choosing Rhododendrons and Azaleas

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Rhododendrons and azaleas come in all shapes and sizes from a few inches tall to giant forest trees.

There are over 900 species of rhododendrons and over 25,000 named hybrids of rhododendrons and azaleas.

Glendoick have around 5000 varieties in their collection and propagate around 500 species and hybrids.

Our online catalogue can be daunting. But don’t worry, it is easy to narrow down what you want to a few sections. If you know broadly what you want, then go to our Glendoick mail order webshop arranged in categories. 

Rhododendron and Azalea Hardiness

We use divisions of hardiness which have now been adopted by R.H.S. In the UK, it is rarely extreme cold that causes damage so don’t pay too much attention to minimum temperature ratings. Damage from late or early season frosts when plants are soft or not hardened off is much more common and can be very damaging or fatal. In continental Europe the minimum temperature ratings are more relevant. 

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H6 Hardiest, -24C (-10F) and below suitable for inland areas in N. Europe: 
H5 Very hardy: -18C (0F) and below, coldest UK 
inland areas, amongst hills, much of N Europe. 
H4 Hardy: Suitable for most of UK areas, including fairly well inland, coastal Europe. 
H3 Fairly hardy: Low elevations and sheltered gardens, 
fairly near east coast. Limit of hardiness at Glendoick. Such plants are damaged in our severest winters or by late or early frosts. 
H2 Rather tender. Low elevations fairly near west coast and on east coast. Not reliably hardy outdoors at Glendoick but reliably hardy in Argyll, Cornwall, West Wales, Ireland and similar climates.

H1 Tender (mild frosts only). Mildest U.K. west-coast islands and south/western coastline. Greenhouse culture for colder areas.

Size: how large will rhododendrons and azaleas get?

Spacing and Size

Rhododendrons have no ultimate size, they can carry on growing indefinitely. The key is the annual growth rate, up to 60cm per year for the most vigorous and less than 1cm for the slowest growing. Moisture and length of growing season greatly affect rate of growth: mild wet areas may have twice the growth rate of cold dry ones.

We recommend planting the larger growing and large leaved species 3-4m apart as they look best as individuals rather than blocks. Plant larger hybrids at 2m spacing for quick effect to fill in 5-8 years and 3 m apart for more long term plans. Deciduous azaleas can be planted 2m apart and are fine if they grow into oneanother. They can be pruned. Medium rhododendrons will grow to 1/2m x 1/2 in 6-10 years so 1 1.5m spacing is good. Dwarf rhododendrons and evergreen azaleas are best planted in clumps 60cm apart so that they carpet the ground. 

Can I prune rhodendrons and azaleas to keep them small? 

Yes! Most varieties can be cut back if they get too big, but some (especially species with smooth bark) will not regenerate. All lepidotes (small leaved rhododendrons) and deciduous and evergreen azaleas can be cut back or pruned. After flowering is the best time. Beware of pruning old (pre 1950s) plants grafted on R. ponticum which tend to sucker. These R.ponticum suckers are easy to see at flowering time (with purple-magenta flowers) Break the suckers off.  The Picture > shows some bushes of 'Fastuosum Flore Pleno' which have been cut back hard and regrown.                                                                        

Sizes used in Glendoick webshop

Rhododendron wardii x macabeanum

Dwarf:   to 40cm in 10 years, usually spreading, wider than high.
Semi-dwarf   to 40-80cm in 10 years, often spreading wider than high.
Low  80-1.3m in 10 years, will eventually reach 2m x 2m or more (20 years)
Medium 1.3-2m in 10 years up to 3-4m+ in time.
Tall  2m+ in 10 years, ultimately 4-6m x 3-6m or more.


Flowering Time

In mild and maritime climates, in most of the UK, France etc, the flowering season can cover 7 months or more, from Christmas onwards, while in severe climates, Sweden and Northern Germany for example, the flowering season is compressed into a 3 month period starting in April. We find with the vagaries of the British climate, there is considerable variation from year to year, especially with early-flowering varieties.


Flowering time



Very Early










Early May


Late Mid

Late May





Very Late



Very Late



With around 900 species and over 25,000 named hybrids, it can be hard work to decide what you want. You’ll be lucky if your local garden centre stocks more than a handful of common varieties, and then, probably only in Spring, at flowering time. For species and more unusual hybrids you will have to go to one of the many specialist nurseries like this one at Glendoick.

Glendoick Garden Centre offers around 250 different varieties.

Cuttings and Grafted Rhododendrons

Most rhododendrons and azaleas are produced from cuttings but some are hard to root and have to be grafted. Grafted rhododendrons had a bad reputation,  largely due to the traditional use of R. ponticum as an understock, until the 1950s. This species is so vigorous that it constantly sends up suckers, which if not removed, gradually take all the sap from the plant, eventually causing the variety grafted onto the rootstock to die off. Many old hybrid collections have now reverted to dense plantings of R. ponticum with the hybrids grafted on top lost long ago.

Glendoick never use R. ponticum as an understock and only use ‘Cunningham’s White’, other rootstocks which throw very few suckers. Some species such as R. lacteum, R. wightii, and R. bhutanense are much longer lived grafted than on their own roots, and likewise most yellow hybrids require sharp drainage and often perform best when grafted.

Seed-raised plants.

Rhododendron species (but not hybrids) can be raised from wild collected or hand pollinated seed 

Collectors’ numbers are (or were) assigned to seed collections which document where the plant was collected, how large it was and what grew with it. Often a specimen of the plant is pressed and sent back to a botanic garden herbarium.

As an example:

K.W. 7724 was collected by Frank Kingdon Ward on his 1927-28 expedition to India, Tibet and Burma. In his field notes, he records it as ‘A forest tree with handsome foliage, truss with 12-18 flowers…it goes right to the summit where it forms forests…it should be hardy’. He records the location as Mt Javpo, Naga Hills, Assam, occurring from 8-10,000ft and he collected the seed on December 1st 1927. 


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