Rhododendron Nutrition vs Budding
Richard W. Bosley, Mentor, Ohio
A lot has been written and spoken about the relationship of the nutrition level with or without a rhododendron plant and it's inclination to set bloom buds. This past summer I had an experience at my firm, Plant Systems, which grows one red rhododendron in large volume, that should add new doubt about the relationship of nutrition to reproductive bud set.
To give a little background on the crop it is the variety R. 'Nova Zembla' and the plants, at the stage at which the bud vs. no-bud data was taken, were 24 months old from the time the cuttings were struck. They were grown the first year in pure sphagnum peat on a low feed diet and the second growing season in a bark mix and were under a constant feed irrigation system. Leaf and medium samples were taken every three to four weeks during the growing season and adjustments Were made to the electronic fertilizer units accordingly. A serious attempt was made to keep plant nutrition within optimum levels although little is known about exactly what is optimum for any specific variety in a given month.
During November, while checking the plants one day, I noticed that there were two plants which had been side by side all during the growing season. One of these plants had 25 bloom buds and the other had none. Both plants were bushy, about the same size and had good dark green color as were all the production plants. I became curious as to whether there was a nutritional difference that might have caused one to remain vegetative while the other produced heavy reproductive bud set. I would have been surprised if there had been a difference as they had both been on the same program and they were in the middle of the pack so to speak. I took twenty-five of the first mature leaves back of the terminal buds from each plant and sent them in for foliar analysis.
The results show that these two plants were very close in the mineral levels within the leaves. This leads me to conclude that under my production conditions with the cultivar R. 'Nova Zembla' that reproductive bud set is a function of something other than nutrition, providing there is an optimum balance available for normal plant growth. This is not to say that putting a plant under stress of some sort will not cause budding but these techniques are often dangerous and invite entry of disease organisms.
There is a clear need for further research to be conducted into what the mechanisms are that cause the rhododendron to produce either vegetative or reproductive buds.