Winter Propagation of Dwarf Rhododendrons
Berry Botanic Garden
Adapted from Summer 1985 Berry Botanic Garden newsletter
Last winter at the Berry Botanic Garden we decided to try a fairly new and inexpensive method of propagating dwarf rhododendrons frequently used in Holland. The only equipment needed was a greenhouse bench with heating cable set at 70 degrees F, a sharp knife, and some thin gauge plastic - a dry cleaning bag did nicely.
In contrast to the usual summer cuttings in June or July, these were taken in mid-February. Cuttings taken were both single shoot nodal, cut just below a leaf node, and basal, sliced through the swelling at the base of last year's growth. The flower buds were removed and two light wounds made on both sides of the stem at its base.
The propagation material was completely immersed in a Captan solution (1 tablespoon/1 gallon water), allowed to air dry a bit, the bottoms dipped in Rootone powder then gently but securely inserted into the heated propagation bench filled with a firmed and well watered 60:40 peat: perlite mix. The dry cleaning bag was draped directly on the cuttings, weighted on the sides with laths, the ends tucked in to make an enclosed tent which immediately fogged. The leaves are thought to breathe through the thin gauge plastic.
Once a day the plastic was gently lifted to release the droplets without allowing air to enter. Each week the cover was removed for an hour or two and replaced with the dry side over the plants. When the compost showed signs of drying, it was well watered at the same time and decaying leaves were removed.
Within 8-10 weeks most of the cuttings had rooted. The cuttings were taken out when their root balls were about the same size as their leaf area and gently potted, for although the root systems were large, they could easily have been broken off. The potted plants were left in the greenhouse a week before moving to the frames.
The chief problem with this method was an unexpected sunny period in February: some of the tender new shoots were burned before shading could be placed over the bench.
A number of species were tried including Rhododendron williamsianum, R. yakushimanum, R. floccigerum, R. ciliatum, R. hanceanum, R. chrysanthum, R. forrestii var. repens, and R. leucaspis.
These cuttings seemed to develop better root systems more quickly than semi-ripe cuttings and the absence of a growth check early in their life seems to have allowed the plants from hardwood cuttings to pull ahead of the ones begun a few months earlier.
For more information consult Nursery Stock Manual, by Lamb, Kelly and Bowbrick, Grower Books, 49 Doughty Street, London, 1975.
Jan Palmer is a horticulturist at the Berry Botanic Garden, Portland, Oregon, involved primarily in propagation work.