Rhododendron: Greenhouse Plants
C – cool, minimum of 7°C (45°F) / W – warm, minimum of 13°C (55°F)
I think it is sad that almost the only type of indoorcultivated are the , correctly known as, simmsii (R. indica), and the Kurume Azalea, R. obtusum. There are many splendid species suitable for small or medium sized greenhouses. Some of these are from the Himalayas such as R. ciliatum whose flowers turn from rosy-pink to pale pink then white as they open. Others are from China such as R. ciliicalyx and others yet from Java and Malaya which are the species I find the most fascinating. Many of these are or semi epiphytic which may come as some surprise. They also like their growing conditions a little warmer than the other species. I would say that clear strong flower colour is one of their main attributes which is certainly true of R. brookianum ‘Mandarin’ which is a wonderful clear pinky-orange and comes from Mount Kinabalu in Borneo.
The ‘’ are very easy to grow providing they are kept cool and airy. The surface of their should never be allowed to dry right out but they should never be waterlogged. All the Rhododenrons are lime haters and ideally should only be watered using rain water or soft water. Potting compost should be peaty and lime-free. After an Azalea has finished flowering it is better off if placed outside in the garden when all danger of frost is passed. It should be in dappled shade and fed with iron sequestrine in spring, watered well during summer and brought back into the greenhouse at the end of summer. Potting, should it be necessary, is best done after flowering. Tip 5-8 cm (2-3 in) long of new growth can be taken in spring and will take two to three months to root in moist shady conditions. The epiphytic Rhododendron species will need a very open compost of peat, sand and some charcoal. I have also found the addition of orchid grade bark useful in keeping the compost open. These plants will do better if the root run is restricted to a comparatively shallow container. Obviously it is of great importance that they are kept moist and it is critical to pot them no higher or lower than they were in their previous container. Ordinary liquid feed can be applied weekly during the growing season, which can be reduced to a monthly high potash feed during the late autumn and winter months. Should plants show signs of yellowing then an annual dose of iron sequestrine should improve them.