Propagation and After Care of Rhododendron Cuttings
Oregon State University
Coarse peat and perlite 50:50 by volume is the propagating media used at the North Willamette Experiment Station for Rhododendron cuttings which are taken in July with a second batch in October or November when the first lot is well rooted. Most of the cuttings are stuck in 2¼" square pots 3½" deep which fit 49 to a square plastic mesh flat. A Mist-O-Matic control and 72°F bottom heat are used. The pots are lifted periodically while on the rooting bench to see whether roots are visible in the drain holes. The pots showing roots are removed from the propagation bench but are held in the same 45°F minimum temperature house for 2-4 weeks. The next move is to a holding house with 35°F minimum temperature. The propagation house has 50% saran shade cloth over the polyethylene from May 1 to October 15th. No shade is used on the holding house.
Occasionally cuttings are rooted in 3" deep screen bottom flats filled with the same media. This is done if we wish to examine the roots to observe the effects of hormones, fertilizers or other treatments. Cuttings which have been rooted in flats often have root balls wider than 2¼" so are potted in 4" square pots. The space around the roots is filled with peat perlite or bark without fertilizer.
Once or twice during the winter dilute liquid 30-10-10 fertilizer is applied to the rooted cuttings. We have found that slow release fertilizer in the potting media with newly rooted cuttings held under cool conditions usually results in dead cuttings. At the beginning of February the heat in the holding house is turned up to 45°F and potting into 6" pots is started. Bark that has been run through a ½" screen is used as a potting media. To this is added sufficient slow release fertilizer to supply 2 pounds of actual nitrogen per cubic yard of bark. We have used a number of different nitrogen sources and most of them will work. Two fertilizer mixtures and the quantities of nutrients per cubic yard that we use routinely are:
Osmocote 18-6-12 11.0 lbs 0-18-0 Superphosphate 2.5 lbs Gypsum (Granular) 1.5 lbs Ground Limestone 1.5 lbs Dolomite #10 1.5 lbs Micro Max 1.5 lbs North Willamette Container Mix Ammonium Nitrate 1.0 lb 0-18-0 Superphosphate 4.0 lbs Gypsum (Granular) 2.5 lbs Ground Limestone 2.5 lbs Dolomite #10 2.5lbs Sulfate of Potash 1.0 lb Nitro-Form 38-0-0 5.0 lbs F.T.E.503 4 oz.
Both of these will supply the major and minor elements needed to get rhododendrons off to a good start. The Osmocote base mix is longer lasting, but costs more, while the North Willamette Container Mix is available premixed. Twenty pounds of North Willamette Container Mix is normally used per cubic yard or 1 cup per cubic foot of potting media.
Even with the minor element package in the potting mix, often the first growth on the cuttings is off color and we apply Peter's S.T.E.M. according to directions. Every two weeks 30-10-10 liquid is applied after the minimum temperature is raised the first of March to 55°F and lights are turned on from 10 p.m. to 2 a.m. In the interest of economy, the higher temperature and lights were delayed to March 20th in 1981.
Around the end of May, the pots are top dressed with a slow release fertilizer containing at least twice as much nitrogen as potassium. Another application is made around the end of July if we want to get maximum growth.
The lights are turned off in mid May. The time of removing the plastic cover and spacing the plants varies with the weather from mid May to early June.
One caution, if you attempt to use this amount of fertilizer, be prepared to water heavily or you will have salt injury. Water at the experiment station is controlled by a time clock whose cycle is varied with weather conditions. In the hottest part of the summer, the sprinklers operate 30 minutes twice a day.
On this program we get three flushes of growth from the rooted cutting in one growing season and with the right cultivars such as 'Molly Ann', 'Purple Splendour', or 'Vulcan' we average 5 or more flower buds. The same program can be used with seedlings started in late fall. Some crosses will set flower buds by the end of the second growing season. No heat or supplemental light is used on the seedlings after the first growing season but in colder areas some heat might be needed in the polyethylene winter protection houses.