Rhododendron from seed

[Rodger Whitlock]

Rhododendrons sown on January 19 are beginning to germinate. Time from sowing, 21–24 days.

Many years ago when I first started growing society exchange seed, I grew a plant of Rhododendron calendulaceum, one of the eastern American azaleas with colors in the yellow-orange-red spectrum. I have it to this day and every spring it produces a fair crop of its sweetly scented yellow flowers.

Remembering that success, I dug into my library and hauled out a now-ancient third edition of Hartmann & Kester, "Plant Propagation, Principles and Practices" where I originally found the sowing technique I'd used on R. calendulaceum with such success lo, those many years ago.

It's a very simple technique. As used this year, the steps are:

• Fill pot with suitably peaty soil.
• Top dress with a layer of fine sand.
• Hydrate soil and sand from below by immersion.
• Sprinkle seeds on sand.
• Cover loosely with transparent plastic film.
• Keep at room temperature.
• Spritz with water from a hand sprayer every once in a while.

Of course, it's a long journey from a newly germinated seed to a flowering plant, but at least the first step is successful.

The species that have germinated so far are

• Rhododendron atlanticum
• Rhododendron pachysanthum, collected Taiwan
• Rhododendron smirnowii, collected nr Vladivostok

[Diane Clement]

This is useful, Rodger, I have had some success in the past with this sort of method.  What is difficult is how long to leave the cling film on after germination as the seedlings are very small?  Also how to harden them off to outside temperatures?  I usually try and germinate them later in the year so it is a bit easier to get them outside.  If you germinate them now, what do you do with them for the next few weeks? 

[Rodger Whitlock]

Notice that I said "loosely". Hermetically sealing these pots strikes me as a bad idea, so loosely draping the flat with film helps keep the seeds from drying out, yet allows a modicum of ventilation.

Your other questions are on point and I'm afraid I don't have answers for you. Yet. For the moment, it's a matter of keeping the seed pots damp (not wet), as well ventilated as possible, and on the cool side. In the strongest possible indirect light, too.

If I'd realized how quickly these seeds would germinate, I'd have held off sowing them until the weather warmed up some. I do that with all seeds in the Asteraceae and Lamiaceae in the belief that they prefer warm conditions to germinate well. Sow them now and put them outside in the weather, and the seed will merely rot.

Previewing this message before clicking "post", I see that I've waffled on the subject. Welcome to the world of weasel words! I guess my main point is that those reading who have shied away from trying rhododendrons from seed need not be so coy: the seeds want to germinate given the right conditions, and the right conditions aren't difficult to provide.

[Kristl Walek]

My preferred method for sowing Rhododendron (and many other small seeded ericaceous species) is thus:

-moist sphaghnum moss inside a clear plastic take-out container
-no holes on bottom of container or in (clear) lid.
-sprinkle seeds on top
-pop on lid and place in warm spot with light available.
-i often do not take off lid (except to very occasionally check moisture levels) until seedlings reach the lid.

advantages of method:
-provides light most rhodos need to germinate
-rarely a need to add moisture
-sphaghnum is absorbent, antiseptic--never one case of damping off
-because rhodos are slow from seed but also continuous growers during their infancy, the plants can be grown inside these containers for quite some time without a need for dormancy during their first couple of years until they are large enough to leave the nest.

[Diane Whitehead]

I have used the same method as Kristl. I had a fridge which
vented its heat at the top, so I would germinate all my rhodo
seedlings on top.

The seedlings will stay almost in a state of suspended animation
for a very long time.

Since rhododendrons produce hundreds of seeds, and therefore
hundreds of seedlings, far more than I wanted to grow, I would
remove as many as I wanted and pot them up singly, but leave
the rest in the mother pot in case something went wrong with
the potted ones.

I'd grow them under lights in a cool spot in the house (lights on
for 24 hours a day) and water them with a spray bottle.

I didn't keep track of the earliest I had flowers, but most of the
large hybrids I produced flowered from 6 to 8 years from the
time I pollinated them.  I had "grandchildren" in flower from one
on my favourite crosses within 15 years.

[Wallace]

I have sleep and memory problems so I forget to spray, which we cannot afford to do.  I take 3/4 liter polypropylene deli containers.  I cut a 1 cm hole in the lids.  I spray cotton balls with QAC's (Quarternary Ammonium Compounds a.k.a. fabric softener) and let dry.  I plug each hole with 1/2 cotton ball.  This allows adequate ventilation without much evaporation or contamination

I moisten the vermiculite (flat) sand (round) mix with 50% vinegar.  I mixed in sphagnum peat since I could not find sphagnum.  I used one container to tamp down the soil.  Sprinkle the seeds and spray thoroughly with hydrogen peroxide.  It sterilizes the seed and the top surface of the soil.  Iron in the soil and light above will split the peroxide. (It is always in dark bottles.) cover and place on a heat mat on a timer or with a temperature control.

I got impatient after a couple of weeks and sprayed Gibberellic Acid 3 with a needle and syringe thru the cotton.  Two days later I had hundreds of Begonia grandis, the next day hundreds of Knap Hill Azaleas (close to 100% germination).  Let us see what tomorrow sprouts.

They can go three weeks between watering.  The water is added to the top and drips thru the cotton.

Larry Wallace
Cincinnati, Ohio
El Norte