RHODODENDRONS FROM SEED

By 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. They can be expected to germinate close to one hundred percent in most cases. Choose hybrid seed or open-pollinated seed of hybrids or species with great care. Many years can be wasted growing worthless seedlings whose worth could have been determined before sowing. Do not hesitate to ask any of our more experienced members which reasonable choices to make from our local Seed Exchange.

        The methods following 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 #I or #2 can be used for any seeds. Method #2, I sometimes use as a back-up method for the more difficult species and hybrid crosses.

        Mix a very good quality long-fibered peat moss (and this is hard to find) with approximately 1/4 'Lecadan' or coarse Perlite cleaned of dust and 1/4 high grade fine bark mulch (southern pine is best as opposed to our local bulk bark mulch). Mix these thoroughly with a small pinch of gypsum or lime and fill pots almost to the brim. You can put three or four pellets of slow release fertilizer deep down in each pot. This medium is extremely difficult to wet and extremely thirsty. Dry peat can only be wetted with very hot water or with water containing a surfactant. (Do not use hot water if you have added slow release fertilizer.) A trial 4" inch pot should be watered then checked a half way down, a third of the way down, and so on to determine how much water it is going to take to soak the medium thoroughly. After doing this start on the pots filled with medium that you are going to use. If hot water has been used, allow the medium to cool to room temperature for at least two hours or cool it off with cold water. Cover the surface of the medium with about a ¼ to a ½ inch of finely chopped green live sphagnum moss making sure that this is also thoroughly wet. Do not use 'No Damp.'

        Whichever the above two mediums you use, the seeds are now ready to be surface sown. Once sown, they should never dry out. Sprinkle your seeds evenly on to the sphagnum. Using a fine mist, moisten them so they make good contact with the moss. Place the pots in tied plastic bag and place them under fluorescent lights (preferably one 'cool white' and one 'warm white') - approximately 8-10" below the tubes.

        Mist occasionally if the surface appears at all dry as drying is fatal. The seeds require light to germinate and will usually appear within 21 days. Temperature should be around 18-21°C. 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 an alternative. Direct sunlight on the enclosed containers will cook the youngsters immediately.

        As seedlings sprout you will first see two seedling leaves followed shortly thereafter by one true leaf at time. Seedlings may stay in their containers for a very long time. 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. Wrapping the roots in a small quantity of green sphagnum moss before potting seems to act as an inhibitor to damping off. I put a plug of living sphagnum moss in a hole in the potting mixture and place the seedling there. Move without letting the roots to dry and keep the seedlings misted until they go back under domes or into plastic bags. The roots can be quite seriously torn and reduced without much harm at this point. Moving into a mist unit as described in Bruce Clyburn's article (Rhododendron Society of Canada, Atlantic Region, Newsletter, May 1995) at this point can accelerate growth and get even damaged and rootless transplants growing again within a few weeks.

        As the seedlings grow, they must be hardened off very gradually by cutting larger and larger holes in the plastic bags. Shift outside in a protected frame shaded with lathe or 50% shade cloth after all danger of frost is past. Protect from birds, cats, rabbits and slugs. If the seedlings were not directly planted in the frame, sink the pots in wood chips. Tightly cover the frame in late November with white (not clear) polyethylene. Slit the ends in late March to allow excess heat to escape. Remove the polyethylene during a foggy period.

Rhododendron and Azalea Planting Instructions

        Rhododendrons and azaleas require light, porous, well-drained soil having a pH close to 5.5 (acid). If you're fortunate enough to be planting in woods or in good upland loam, you need little soil preparation, but if your soil is clay like mine, you've got a little work to do.

        In clay soils, it is beneficial to incorporate relatively large amounts of organic matter and coarse sand to improve the texture and porosity. If drainage is a problem creating a raised bed by increasing the soil height by 8 - 14 inches works well. pH can be laboratory tested by using a soil test sampling kit from the Penn State laboratory, available from your county agricultural extension agent. You can also purchase a test kit at any of your local nurseries / plant centers. I bought a "Sudbury Laboratory" lime / acid test kit years ago and it has proven to be totally adequate for my pH testing needs. Various chemical amendments are readily available to change soil pH to make it suitable for rhododendron / azalea growing. I don't intend to elaborate here on changing soil pH, but I am available to you to discuss any problems you may have with soil pH and to recommend solutions.

        The most often heard question regarding planting is "How big a hole should I dig?" I have found through my own experience and the experience of other growers that for container grown plants, the hole should be no deeper than to the depth of the root ball in the container and the diameter of the hole should be 3-4 times the diameter of the container. Most root growth will be lateral as opposed to downward so it is better to have a well prepared wide planting than a well prepared deep planting. Remember also that the surface of the root ball is to be level or slightly above the level of the planting soil, never, never, never lower than the level of the surrounding soil.

        Now that you have dug the hole and have mixed the equal amount of coarse sand, peat moss and soil, you should back fill the hole and firm in your plant. First, however, it is necessary with container grown plants to assure that the roots are in a condition that will enable them to initiate growth into the new soil. If, upon removing the plant from the container, you find that it is pot-bound, you must tear open the root ball so that you can work the new soil mix into it. The amount of cutting and tearing is directly proportional to the degree of being pot-bound. It is very difficult at times to tear these pot bound roots - you just have to cut them. I use a sharp pocket knife and cut maybe 4 or 5 one-inch deep cuts equally spaced around the circumference of the root ball from its top to its bottom. I try not to cut through large woody roots if any are near the outside surface. You may find that once you have cut into the root ball you can get your fingers into the cut and tear it more easily so that you can work the new soil mix into the roots. [Editor Tom Schuetz’ note: here in the eastern US, the soil temperature can rise significantly in the summer and moisture and humidity can be plentiful. This set of conditions can make plants with wounded roots susceptible to a fatal condition known as root rot. I avoid this potential problem by not moving plants when the soil temperature rises above 75øF. This means that typically I do not move plants in the period from mid to late June to mid to late September, depending on the weather.]

        Once you have the plant's root ball in the condition described above you are ready to plant it. Set it in the hole and backfill with your new soil mix. I find it better to do this in a layering method so that the soil mix is mixed into and on to the exposed roots of the plant. Firm the soil into and about the roots until you have reached a height level with the top of the root ball. Do not heap soil higher than the root ball or on top of the root ball. It must be level with the surface of the root ball. You should not have any problem with settling if in fact you dug your hole to the exact depth of the plant root ball. If the bottom of the root ball is resting on undisturbed subsoil, there will be no settling.

        If the soil mix is relatively damp I wait a day or two before watering, recognizing that the plant was well watered before planting. I water with a spray can and make sure that the entire planting hole is well watered without flooding.

        Next, mulch the plant. Use pine needles, oak leaves or pine bark. Do not use the "tan bark" stuff which is nothing more than ground up wood. Do not mulch too deeply. A heavy deep mulch prevents the aeration of surface soil / roots and death quickly follows. Keep the mulch about 2 inches away from the trunk of the plant.