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Rhody planting


Andy Collins
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Rhododendron nutrition and culture have been debated for years and if you have a successful formula then stay with it. The usual recommendation concerns low pH (4.5 to 5.5) and organic or ammoniacal nitrogen sources. Some authorities state that "fertilizing is better left undone than overdone." In general ericaceous plants have low nutrient requirements compared to most ornamental plants. Rhododendrons may produce optimum growth when the tissue nitrogen levels range from 1.50 to 1.75 percent of dry weight. Cotoneaster and Pyracantha might require 3 to 3.3 percent for maximum growth. In simplest terms, rhododendrons do not need as much fertilizer as other plants. Research shows that the ammonium form of nitrogen has proven extremely toxic to rhododendrons and other plants when applied in high concentrations (100 ppm NH4-N at every watering) The crux with ammonium fertilization is to keep the levels low. Many people have described symptoms of "fertilizer burn" which, in fact, was an expression of ammonium toxicity The ammonium ion can severely impair metabolic processes if it accumulates to any degree in plants. At the low pH ranges which are recommended for rhododendrons the ammonium form of nitrogen predominates. If the pH rises above 5.5, a conversion of ammonium to nitrate (NO3) occurs because of the presence of certain bacterial species which catalyze this reaction. This chemical change is influenced by pH which in turn affects microbial populations and species. Nitrate (NO3) can serve as an effective nitrogen form for rhododendrons. The pH increases if a nitrate fertilizer ((KNO3 or Ca(NO3)2)) is used. The most important aspect of using NO3-N is to provide an available iron form such as a chelate or sequestered iron. Nitrate is less toxic than NH4 and will promote good growth provided the other nutrients are supplied in an available form. It's noted that Rhodo's comparitively grown in container culture with N03 or NH4 nitrogen supplied at 75 and 150 ppm showed markedly different results. Tremendous growth resulted from the N03 fertilization while toxicity and reduced growth resulted from the NH4 -N. The crucial thing is to supply of all essential elements in an available form.

 

I'd use the ericaceous mix!

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