Growing Rhododendrons from Seed

John K. Weagle

Rhododendrons are easily grown from seed. The seeds will remain viable for more than a year if they are stored cool and dry in glassine envelopes. In most cases they can be expected to germinate close to one hundred percent. Choose hybrid seed or open-pollinated seed of hybrids or species with great care. Many years can be wasted growing worthless seedlings whose merits could have been determined with a little research before sowing. Do not hesitate to ask any of our more experienced members which reasonable choices to make from our local Seed Exchange.

First and foremost, you must label things properly and keep the labels with the seedlings throughout their lifetime. With no records you will be unable to use your unknown plant sensibly in future breeding.

The following methods are ones that work for me. I am certain there are better methods.

To sow the seeds, I prefer 3 1/2″ wide x 4 1/2″ deep square pots. Drainage is very important and the deeper the pot the better the drainage. There are two sowing methods that I use. Method #1 or #2 can be used for any rhodo seeds. Method #2, I use as a back-up method for the more difficult species and hybrid crosses. With less free time, I find I am using this method more often as it allows me to skip one transplanting step.

Method #1

1. Collect fresh green sphagnum moss growing at the surface of a bog; the type found in the woods is unsuitable.
2. This moss can be chopped rather fine with scissors or a cleaver or left whole. It seems easier to disengage the fine rhododendron roots later if the sphagnum has been chopped.

3. Pack the sphagnum tightly in the pots or flats and soak thoroughly.
4. Allow to drain until all excess water has drained.

N.B. This method usually requires the seedlings to be transplanted into the mix in method #2 when they have 2 true leaves.

Method #2

1. Mix 60% very good quality long-fibre peat moss (and this is hard to find) with approximately 40% high grade fine bark mulch (southern pine is best, as opposed to our local bulk bark mulch).
2. Mix these thoroughly with a pinch of gypsum or lime and 1/2 teaspoon ground superphosphate (0-20-0, NOT treble superphosphate which is too strong) for every cubic foot of mix.
3. Sterlize the dampened mix in a zip-lock bag in the microwave (no metal twist-ties in the microwave!) for 10 minutes on high until the damped mix reaches 180ºF.
4. Allow to cool.
5. Fill pots almost to the brim.
6. Topdress the pots with 1/4 inch plus of straight peat moss similarly sterilized. This medium is extremely difficult to wet and extremely thirsty. Dry mix can only be wetted with very hot water or with water containing a surfactant; the society sells surfactant.
7. Set pots in a tray and water. If hot water has been used, allow the medium to cool to room temperature for at least 3 hours or cool it off with another liberal watering of cold water. Drain thoroughly.
8. I do not recommend the use of ‘No Damp.’

Sowing – Transplanting – Hardening Off – Moving Out

A. Whichever method you use, the seeds are now ready to be thinly surface sown.

B. Once sown, the seedlings should never dry out. They should also not be over-watered! Sprinkle your seeds thinly and evenly on the surface. Do not cover the seed.

C. Using a fine mist, moisten the seeds so they make good contact with the moss. Be careful not to blow the seeds away when misting!

D. Put the pots in a plastic bag and place them under fluorescent lights (preferably one ‘cool white’ and one ‘warm white’) – approximately 8-10″ below the tubes.

E. Mist occasionally if the surface appears at all dry as this is fatal. The seeds require light to germinate and will usually appear at 21 days. Temperature should be around 18-21C. Cooler temperatures will slow germination. Too much heat can be detrimental. If you have no fluorescent lights, a bright window with absolutely no direct sunlight would be a decidedly poor alternative. Direct sunlight on the enclosed containers will quickly cook the youngster.

F. As seedlings sprout you will first notice two opposite seedling leaves (cotyledons) followed shortly thereafter by single consecutive true leaves. Seedlings may stay in the mix of Method #1 for a very long time but it is better to move them at the second true leaf stage.

G. To accelerate growth, they can be pricked out just after the second true leaf has finished developing as a very short dormant period follows this stage. (If you have sown your seed in the peat and bark medium you need not transplant them at this time.) Delicately tease the seedlings apart and pot into soil mix similar to that used in Method #2 omitting the peat top-dressing. Similar sterilization procedures are required. Seedlings can go into single pots or flats.

H. Move (a hatpin, dental probe or lobster fork will come in handy) without letting the roots dry. Keep the seedlings misted until they go back under domes or into plastic bags. The roots can be quite seriously torn apart and reduced without much harm at this point. Moving into a mist unit, as described in Bruce Clyburn’s article (RSCAR Newsletter, May 1995), at this point can accelerate growth and rehabilitate damaged and rootless transplants within a few weeks.

I. As the seedlings resume growth they must be hardened off very gradually by cutting a small hole initially and increasingly larger holes in the plastic bags until the seedlings are growing without cover and with no sign of wilting. Do not be alarmed if the seedlings lean to one side and touch the soil as they grow; they will right themselves and seem all the more stable by doing so. In time, the bend will become indiscernible.

J. Fertilize every 3-4 weeks with an appropriate water soluble fertilizer at 1/4 strength. I use Peters Rhodo Fertilizer, 3/4 tsp. to 1 Imperial Gallon of water.

K. Shift outside to a protected cold frame shaded with lathe or 50% shade cloth after all danger of frost is past (tomato time). Initially more shade will be required. If planted directly in prepared soil, mulch lightly with pine bark mulch. Protect from wind, birds, cats, rabbits and slugs. If the seedlings were not directly planted in a frame, sink the pots in wood chips to keep the roots cool.

L. Keep evenly moist throughout the first growing season A little drying between waterings later in the season helps develop a better root system.

M. Tightly cover the frame in late November with white (not clear) polyethylene. Slit the ends in mid- to late- March to allow excess heat to escape. Remove the polyethylene during a foggy period. This whole procedure can be skipped if you want to weed out tender ones.

N. Move on to wider spacings as required.