Rhododendron Basic Information

Rhododendrons (from the Greek: rhodos = "rose", and dendron = "tree") are in cultivation since the early 1800's.
In cultivation means that professional nurseries started crossing, propagating and selling shrubs.

Almost all (98%?) of the rhododendrons in gardens are either hybrids, or clones/selections
Botanist would rather say 'cultivars' than 'hybrids' (and for botanist 100% of rhododendrons in gardens are 'cultivars' by definition).
The shrubs and trees living in the wild are called species.


Species are very different in form and appearence: some have leaves sized 1/4 inch, others have leaves over 3 feet.
Some species have trouble reaching a maximum height of 1 foot (25 cm.), others can be well over 15 yards (45 ft. or 15 meters).
Most species (about 625) are found in Nepal, Tibet, North India, China, Burma, Korea, Japan and Bhutan.

R. sinogrande
(Not counting the 200+ tropical rhododendrons called vireyas).
Usually well above see-level the majority grows between 5000 ft. to 15000 ft. (1500 to 4500 meters).
So well above the tree-line, reason that rhododendrons are often used as fire-wood by people living at those heights.

There are about 20 species native to Europe, USA and Canada.

Names of species are written in 'lowercase'.

R. nakaharae

So you will find R. fortunei ssp. discolor, or R. wardii var. wardii or R. heliolepis var. brevistylum. (By the way, species is singular and plural, so you can own 1 species or 2 species).

Species-names often include abbreviations like ' ssp. ', ' var. ', ' f. ', or ' x '.
In short this means:
Whenever there are isolated populations of the same species (say 30 miles apart in two different valleys) they might have developed different characteristics. (for instance pure white flowers in one, and bicolored flowers in the other population).
In that case botanist sometimes define a 'subspecies' (ssp.).

When there is variety within a population of species (say there are a few species in the population with slightly bigger leaves that flower 2 weeks before the others) then botanist define a 'variety' (var.).
Defining 'var.'s and 'ssp.'s can generally be seen as a gain of insight, or even scientific progress, for the layman and amateur gardener it can be disastrous. See the story of Mr. Smith and his rhododendron indicum.

When there are different forms of 1 species but not different enough to define a 'var.' or a 'ssp.' then botanist sometimes use the f. (forma). (For instance: R. indicum var. amoeneum f. japonicum).

The 'x' stands for 'natural hybrid'. It is always at the beginning of the name. (great when sorting alphabetically)
So R. x agastum means that agastum is a natural hybrid between 2 other species.
Now here is where my knowledge and belief in botany stops.
In my opinion either all organism are natural hybrids, or none is.
Within 15.000 years from now, a nowadays natural hybrid might have become a species (simply because the parents got extinct). I'm pretty sure that I myself am a natural hybrid, although there are days that I feel quite a species.


Hybrids show less variety than species. That is exactly why they were developed. Hybrids are developed for special purposes. Hybrids are crosses made by hybridizers to combine several favourable properties like: red flower and compact growth.
Or winter-hardiness and low growing habit.
When designing a garden it would be very nice to know that a certain shrub has a maximum heigth of 4 feet.

Hybrids are crosses between species, or species and hybrids, or hybrids and hybrids.
Some hybrids are (in certain area's) so popular that they became more or less synonym with 'rhododendron'.
The 'Catawbiense Grandiflorum' is for most people in Western Europe the 'only' rhododendron they know. ("Ok.. besides the purple ones you have also the white and red rhododendrons....I think...")

R. 'Catawbiense Grandiflorum'
The parentage of hybrids is usually noted as (seed_parent x pollen_parent) = (mama x papa).
In hybridizing 'mama' is the seed-parent surprisingly enough, but that is because she develops the seeds for next generation.

Names of hybrids are written with uppercase and between single quotation marks.
So it is R. 'Nova Zembla', or R. 'Catawbiense Grandiflorum'.

Hybrids can be azaleas, rhododendrons, azaleodendrons or vireyas.

R. 'Nova Zembla'

R. 'Ponticum Roseum'

The problem with hybrids is: how to propagate them? That is, how to get more than 1 ?
(for sure the best way to get more than 1 is: buy 5 extra in your local garden-centre)

Example: Annae x Mr. Thomsom
Suppose you want to create your own hybrid, and name it after your wife/husband or better even your neighbour..who cares?.

Your neighbour is called mr. Thomson and his wife is called Anna... so..logically you will cross the 2 species: R. thomsonii x R. annae.
That makes sense (yes ?), or better both: R. annae x R. thomsonii as well.
That'll be fun!.
Here is what you do. (this is not for the 'weak-of-heart')

You'll need to start with:
  • plastic transparent bags
  • rubber bands
  • an almost blossoming R. annae
  • an almost blossoming R. thomsonii
  • nerves of steel
  • patience like a statue
  • the fore-mentioned neighbour's

Ok, now you will be in competition with bee's and bumble-bee's as soon as the flowers open. So be prepared to get up at 5 o' clock in the morning.
Take one flower of R. annae, take some pollen of it, bring that to one flower of R. thomsonii. Then seal this thomsonii flower with plastic bag and rubber bands to prevent insects interfering with your 'good' work.
(this cross will thus be R. thomsonii x R. annae, because the new seeds will grow on the mama-plant = R. thomsonii).
Do the same with thomsonii and vice versa.
(This is called hand-pollination, you will see that abbreviated as 'hp' in some books).
Now wait for 5 months.
Untill the seeds are brown, not green.
The seeds, by the way, are very small, on the photos on the right you see 8 tot 10 'seed-boxes' or 'seed-capsules' (per flower) each containing up to 500 seeds.
Collect the seed-boxes from the plastic bags. Do not forget to mention the parentage:
either annae x thomsonii, or thomsonii x annae. Now let the seed-boxes dry for a while on a normal indoors temperature.
Open them and start seeding.

seed-boxes ok

seed-boxes not ok yet
For the seeds to do well, you need a soil-temperature that is higher than the air-temperature. (a condition normally found high up in the mountains).
Do not cover the seeds with earth or whatever...they need day/sun-light to start.
When lucky you will get about 200-500 new plants. Perhaps even more. All about 1 inch tall in 4-7 weeks

Ok, now you start selecting 'the best' from all those 500.
They will be, in fact all 500, different. Some of them have definitely hairs on the leave-surface, others don't.
Some of them are twice as big as others, some of them have a different color on leaves lower-surface ...and so on.
So take your pick and select the 58 (?) best plants. Try to sell the rest.
Put all those 58 in seperate small containers and wait 5 months.
Now replace them in bigger containers.
Wait another year.
Plant all 58 in your garden (get rid of the lawn). Wait 2 years.

Seeds come up like a lawn

now 1 to 3 in seperate containers
Unfortunately your new gardner while mowing the remainder of your lawn, has mixed up the labels of your crosses, so you'r no longer sure what is annae x thomsonii and what is thomsonii x annae.
Shit happens.

Now in next spring, the 58 5-year-old plants might blossom for the first time.
You can hardly sleep and wait....
Then the moment is there:
In fact 52 do not blossom at all, 5 have very small flower-buds, that hardly open, and only 1 has a real flower... a white pale pinkish flower, not very spectacular (in fact just like Mr. Thomson)..

So you get rid of all the other 57, (and fire your gardner) and keep this one rhodo with flowers that you call 'Mrs. Anna Thomson'. (Because a name like "Mrs. Thomson Anna" would be silly, although perhaps botanically more correct, who knows?).

Register your new hybrid with the ARS.

Now... how to get more 'Mrs. Anna Thomson''s?
NOT by repeating the cross you did 5 years ago...chances are about 1 out of that you will get the same shrub.
So you need cuttings; you start cutting up your sole specimen of 'Mrs. Anna Thomson' and create 12 new rooted cuttings.
Cuttings are in fact 'clones': they will have the exact genetic material as the rhododendron they were taken from. This is the only true duplication of plants.

Wait another 4 years.
Now you have 12 'Mrs. Anna Thomson''s blossoming (if the rabbits did not beat you). So you can spare 1 or 2 and offer one to your neighbours.
Unfortunately, the nextdoor's Thomson's have moved 6 years ago, and your new neigbours are very surprised to receive a Rhododendron "Mrs. Anna Thomson" that flowers pale pinkish.
But they can see that you are very proud of your new rhodo, and they accept it with a smile.
They know that you had a rough time since your divorce (7 years ago) due to your extravagant interest in nursing rhododendrons.

Still there?
It is wonderfull to be able to create something new in nature, but playing 'God' implies also being able to take the burden.

For more information about propagation, cuttings, crafting, layering and seeding see cultivation (Steve Henning's page) of rhododendrons.
Imagine that all 'Nova Zembla''s, or 'Lems Monarchs's' are both in fact originating from 2 original shrubs.
Two incidental hybrids.
And then propagated by cuttings.... till there were 35000, or even more.
In fact some registered hybrids might be 'extinct': no one knows whether there are still specimens of that hybrid growing somewhere.
That is one of the goals of hirsutum.info: inventarisation of living rhododendrons (hybrids).

Selections of species

Selections are a special topic.
If you have read the above section about hybrids, you can understand the significance of 'picking' 1 rhododendron from a population in the wild.
That is exactly what early plant-collectors did.
They took from a population of species in the wild the best pink, the one with the biggest leaves, and so on, and shipped those home.
Home these plants where propagated by cuttings, or by pollinating them with themself ('selfed').

Very likely these home-species will be different from the original population. (certainly in the long run). So they became known under different names.
We now have the R. keiskei 'Yaku Fairy'. Or the R. aberconwayi 'His Lordship'.
We see both ways of notation: species and hybrid in one name.
Usually selections are listed under hybrids, although they are much more species than hybrids.
On hirsutum.info we follow this botanical 'logic'.
You will find selections under hybrids, but also listed with the concerning species. Unfortunately selections are often called 'clones'... from what I understand of botany this is in fact incorrect.
Hirsutum.info     Project started 03 June 2009