rhododendron seed starting

Submitted by Sadauskas on Tue, 02/02/2010 - 21:42

hello all,

just received my seeds :o and was looking for some seed starting basics on rhododendron kinsai x showa-no-homare and r. shiryu-no homane x mine-no-hoshi(hope I got all that right). I am quite unfamiliar with rhododendrons, but wanted to give some a try. I read something about spahgnum medium, and just wondered if any rhododendron growers might want to chime in?



Submitted by Boland on Wed, 02/03/2010 - 14:25

I hybridize rhodies so know all about sowing them.  Here is my technique...others may differ!  I use peatmoss (horticultural grade) that has been sterilized.  I fill the pots, water and let drain for a while.  I then surface sow the seeds and place syran wrap over the pot surface, held in place with an elastic band.  I then place the pot under flourescent lights (or a bright window shaded from direct sun) then wait.  The seeds normally sprout in about 2-3 weeks.  I leave the plastic in place for another 3-4 weeks before I remove it.  In about 2-3 months, the seedlings will be large enough to prick out (wait until about 2-3 true leaves have formed)


Submitted by RickR on Wed, 02/03/2010 - 15:23

Todd's method is essentially the same as what I have done.  I too, used pasteurized peat, surface sow and enclosed pot and all in a clear plastic bag, set on the north sided of my house in open shade.  The warm cozy inside temperature that he uses is certainly a plus.  Seedlings grow slowly in my experience.  Now if I could just keep the chipmunks away . . .

Submitted by IMYoung on Wed, 02/03/2010 - 16:42

Todd is a great Rhodo man, I know, and he and Rick are living in North America so their methods are surely pertinent.
Michele mentions the sphagnum method and I thought you might like to see a couple of old articles from the SRGC journal on the subject?
I have made pdf files, very rough and ready, but  readable I hope, of these... the first by the late Alec Duguid, (the man who did great things  as manager at Edrom Nursery in the Scottish/ English border country) from the SRGC Journal #72 of June 1983  and the second, a response to that article, detailing the method that Ian and I used, from the "Rock Garden" #90 of January 1992 .

At that time we were sowing in the moss alone but nowadays we would use a layer of sphagnum moss over a compost mix rich in leaf mould for ericaceous subjects.

Hope these "blast from the past" items are of interest!

Submitted by Sadauskas on Wed, 02/03/2010 - 18:22

It was nice to open up the forum and see what tidbits of info I received :)  Thank you everyone for your input.  Todd, I will add my rhodo seeds into my line-up of containers. I set my pots up exactly as you describe...just will do the peatmoss instead of my normal mix.  Thank you to RickR and Iam too.  Looking forward to the nargs forum to be as wonderful as the srgc forum ;D

Submitted by Rimmer on Thu, 02/18/2010 - 17:50

Maggie your post from the SRGC was wonderful to read.
I have been using sphagnum as a mulch when starting cuttings of rhododendron (Ledum) and Daphnes with great results


Submitted by Gene Mirro on Fri, 04/09/2010 - 11:18

I have two lots of R. occidentale, one of which I collected myself, so I know the seed was fresh.  Both have been sown for two months now, with no sign of germination.  Aren't these plants supposed to be "immediate" germinators at room temperature?  I am an experienced grower, so I'm pretty sure I haven't done anything boneheaded. 

Submitted by Boland on Mon, 04/12/2010 - 05:33

Gene, rhodo seeds should germiate within 2 weeks.  The key is surface sow and keep at 100% humidity.  I cover my pots with saran wrap until I see the first true leaf emerge.

Submitted by Gene Mirro on Wed, 06/02/2010 - 19:26

I tried another batch, and they came up like grass.  They definitely need to be surface-sown.

Submitted by IMYoung on Sun, 06/06/2010 - 05:02

I'm pleased the old info proved interesting. :D

We always reckon that the finer the seed (and rhodo seed is pretty dustlike) the shallower it should be sown, so the surface sowing is perfect and, as Gene has shown, works a treat.  :D

Submitted by Moyles on Fri, 12/31/2010 - 16:10

Just an additional note:  I think we all germinate rh seed in pretty much the same way: surface sown on milled sphagnum covered under lights with germination generally in 30 days or less. If one is interested I have a paper in which I detail my approach  (A Vireya Seed Ody ......... etc) that is viewable in the archives of Callard's site: Vireya.net.  All ericaceous seed are easily germinated in this way. In any case Vireya.net is worth the trip!  A marvelous site. 

Submitted by RickR on Fri, 12/31/2010 - 23:06

A wealth of other info on that site, too.  Thanks for sharing it.

And welcome to the forum, Bill!

Submitted by Tingley on Fri, 01/11/2013 - 10:35

I have a question about Rhododendron seed longevity. Since my efforts to find plants of Rhododendron augustinii for sale in Nova Scotia have been fruitless, I went looking for seed sources. Van Dusen Botanical Garden in Vancouver, BC has packets for sale, but they were collected in January of 2009. R. augustinii is high on my wishlist, so I am tempted to order a packet anyway, but wonder if the seed would simply be dead at this late date. Does anyone have any advice, or a source of fresh seed I could contact?

Submitted by Diane Whitehead on Fri, 01/11/2013 - 18:55


I have about four clones of augustinii.  I don't think I got them all deadheaded so there ought to be seeds for you.  Or, since you're in Canada, I could just mail you a batch of cuttings.


Submitted by Tingley on Mon, 01/14/2013 - 16:23

Thank you Diane, I am looking forward to germinating seed from your plants, and possibly attempting cuttings as well! Hopefully a few plants will come of all this... love seeing new plants work their way into the landscape. Last years' seedlings were Digitalis obscura and Kniphofia triangularis. A few Enkianthus sp. seedlings have germinated, along with Poncirus trifoliata.

Submitted by Gene Mirro on Tue, 01/15/2013 - 22:48

Gordon, if van Dusen stored their seed in the freezer, it will be as good as the day they froze it.  If they stored it in the fridge, it might still give some germination.  If they stored it at room temperature, it will give very poor germination, if any.

See http://nargs.org/smf/index.php?topic=635.0 for my method of growing Epigaea repens.  I now grow Azaleas, Rhodies and Oxydendrum arboreum using exactly the same method.  It's very similar to Todd's method, except my mix is a lot "dirtier".  The plastic bags can be replaced by transparent propagation domes if you are using 10" x 20" flats.  I really like the 4" tall domes made by NGW, model number 726239: http://www.sunlightsupply.com/p-11762-ngw-propagation-domes.aspx .  Ignore the part number in the photo; it's a typo.

The method with the plastic bags and fluorescents is so bulletproof that I am now growing almost everything that way.  I don't see any damping off using this method; I can't explain it.  But keep a close watch for fungus gnats.  For some interesting control methods, google "fungus gnats kill".

I like to start fertilizing with dilute soluble fertilizer as soon as a good stand of seedlings is up.  Make sure to water the fertilizer into the mix thoroughly with a gentle spray of water.  If you don't fertilize, the plants will grow very slowly.

For a very detailed description of growing Haberlea and Ramonda using this method, see here:  
See also http://nargs.org/images/stories/plants/Gentianopsis.pdf  and

Submitted by Jeddeloh on Wed, 01/16/2013 - 21:41

I sterilize peat pellets for fern spore sowing by putting them in a heat proof plastic container (like a plastic shoebox), pouring boiling water in, and slapping the lid on.  The steam seems to do a good job of sterilizing it.  When it's cool I peel back the plastic mesh and sow my spores. I imagine you could just pack moist peat moss into any heat proof container  and nuke it for several minutes until it's good and steamy.

I think it might work to sow rhododendron seeds onto peat pellets the same way.  When I pour the boiling water in I try to avoid pouring it directly onto the pellets since that can wash out the peat.

My favorite plastic boxes for this venture are the clear plastic accessory boxes from The Container Store.  They hold 9 pellets which is enough for me of anything.

Good luck.


Submitted by RickR on Thu, 01/17/2013 - 08:43

Using boiling water pasteurizes, and has always been my choice if I need to do it.

But in view of your "I'm serious" comment, the medium's temperature must be raised above the boiling point to be considered sterile.  Using a pressure cooker, or in an oven in a covered but not sealed pan would be good methods.  :)

Submitted by externmed on Thu, 04/25/2013 - 17:09

Just to make things a bit complicated: I had 2 identical  pots of Rhododendron maximum (supposedly from the one red (chimeric?) plant still lingering in the Appalachian Mountains.  One pot I placed under a bright fluorescent plant light at 67F, the other I temporarily forgot in an unlighted 62F room.  Got rapid complete germination in the unlighted pot and slower 20% germination in the warmer bright light plant.  Transferred the lighted plant to the unlighted room and got, eventually, good germination.
These were special fluorescents that are two are 3 times brighter and also warmer than standard bulbs.  I don't know if standard 40 watt 4 foot fluorescents would have seemingly yielded poorer germination as well.
Charles S  MA USA
(one seedling has reddish stems-so far) 

Submitted by RickR on Thu, 04/25/2013 - 18:42

That's interesting and useful.

My thermometer, placed 6 inches under the lights with the plants reads about 5 degrees F warmer than not under the lights.

67F is which?

Submitted by Gene Mirro on Thu, 04/25/2013 - 21:19

Externmed, where the pots in baggies or under propagation domes?  How close to the lights were the seeds?  Did you keep the potting mix moist in the pot under lights?  There are a lot of variables.  I get my best germination of Ericaceous plants by surface sowing, and with the top of the potting mix about six inches below my 40-watt cool white lamps, at a room temp of about 55F (the temp under the lights is around 60F).  The pots are fully enclosed in a sealed freezer bag.  When they germinate under the lights, they don't have a chance to stretch and weaken.

Submitted by externmed on Fri, 04/26/2013 - 21:48

Hi Gene,
The two identical pots with the same surface sown seeds germinated better in the dark at 60F, than in the light at possibly 80F in a baggie. 
I was just guessing that 80F and bright light was too much for this Rhodendron; but I was surprised with near total germination in near darkness.  They warm seeds germinated at least 60% when I moved them to the dark room
I wouldn't have normally used the bright light, but I had had a recent failure with the standard fluorescents--so it just went under there.
I have a room that I keep at 42F for the winter; if I have the time and ambition, I'll hopefully start a lot of seed in there next fall.
Thanks for your comments and suggestions,
Charles S   

Submitted by Gene Mirro on Sat, 04/27/2013 - 09:28

I haven't tried a 42F room, but I'll bet that fluorescents in a 42F room will be magical for germinating alpines, bulbs, etc.  But growth will be slow at that temp.  I suspect that would be a perfect setup for the European alpine Gentians.

Submitted by Hoy on Sat, 04/27/2013 - 16:07

This was one of my firstsuccesses with rhodo seed: R suchuenense, about 20 years ago. I used soil from the spruce woodland nearby and didn't cover the seeds. Kept the pot in a semi cold greenhouse during the winter. Now the shrub is 3m tall and 4m wide.