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jim_wa Posted - August 25 2020 : 3:30:22 PM
Older leaves of our young rhododendrons (leaves produced last year and older) are turning yellow. We are in Vancouver, southwest Washington.

We planted them this spring, a mix of Double Besse and Wojnar's Purple. They were 1-gallon size. With great effort, we mixed the native heavy, alkaline clay soil roughly equal parts aged, fairly fine-ground bark and peat moss. The soil is mounded to facilitate drainage. Since planting I've covered the mixed soil with medium bark nuggets as mulch.

We had the soil tested some time after planting. pH is 5.85 so I think that's OK. But all nutrients were very high except nitrogen, which was extremely low (2 ppm).

This was already late July, and I've read on rhododendron.org that you have to be careful with fertilizing, and don't do it late in the season. I went ahead and put on some ammonium sulfate, but I was stingy with it because it was late in the season.

Also, we have had some unusual heat, and maybe a bit late I took to watering daily when temps were in the mid 80s or higher.

My best guess is the yellowing of the older leaves is a symptom of nitrogen deficiency.

1. Does this seem reasonable?
2. What is worse, leaving them nitrogen deficient until late winter, or adding nitrogen now and whatever problem that causes? I.e., should I fertilize now with N?
3. How long do rhododendron leaves normally last?
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JulianaMedeiros Posted - October 20 2020 : 2:00:10 PM
We've had a lot of die back on lower leaves here as well here in Ohio, people here are blaming the long drought period that we had this summer, which could cause the plant to let go of older leaves that no longer have good rates of photosynthesis.
jim_wa Posted - August 26 2020 : 9:30:10 PM
Wow, thank you for the excellent and on-point information. Very helpful!
BobW Posted - August 26 2020 : 7:53:10 PM
Nitrogen deficiency can result in yellowing of rhododendron leaves, as well as stunted plant growth. Leaf senescence can also cause leaves to turn yellow. It's common to see yellow leaves on evergreen rhododendrons in the late summer and early fall. This would occur for the oldest leaves, usually not last years’ leaves. As the green stems begin to turn woody the oldest leaves of evergreen rhododendrons experience chlorophyll degradation and leaf senescence. The process can be accelerated by high temperature and/or insufficient water absorption. Leaf retention is variable. 'Wojnar's Purple' leaves are typically retained for 2 years and 'Double Besse' leaves for 3 years. Both nitrogen deficiency and leaf senescence can contribute to the yellowing of leaves.

You asked about late summer fertilization. Once buds are formed in late summer or early fall, plant growth slows down. In the late fall/winter rhododendrons are mostly dormant. The concern with late fertilization is it can continue growth and create tender shoots that may then be damaged by cold weather. Vancouver WA has mild fall and winter weather so late fertilization may not be such a concern, as it would be true for East coast or Midwest plants.

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