One of the Rhododendron plants seems to be doing badly

Asked July 13, 2020, 1:54 PM EDT

Hello - We have these 2 Rhododendron plants, since over 8 years. They were very small when they were plotted in 2 separate large rectangular bins. They have become rather large now; they get enough flowers every spring, and grow shoots every year, getting bigger and taller. This spring one of the Rhododendrons had about 10 flowers - i will call this one as "A". The other had about 50! Let me call this plant as "B". Now, even though A had fewer flowers, it definitely threw out more shoots and has grown taller, with many, many healthy new leaves. Plant B instead seems to have had very little new growth. The leaves have become a pale navy green, with cigaring, much like in the dead of winter, and appear to be narrower than the healthy, wider-leaved plant next to it. It is odd as this plant had some much bloom this year in May. Is this plant dying? Can we do something to help it get healthy. Please help if you can. Thank you so much, Bindu p.s. Attached is a pic of the two Rhododendrons side-by-side, and then 2 pics, 1 of the leaves of the not-doing-well plant B, and the other of the healthy plant A.

Philadelphia County Pennsylvania

3 Responses

Thanks for your question.

When rhododendron leaves start turning yellow, it's their way of telling us "Hey I'm stressed." Unfortunately trying to tell just why they're stressed can be difficult.

First, look for insects. Spider mites often feed off the buds and leaves causing the leaves to turn yellow. They are very tiny and difficult to see, but with close observation you can generally see fine webbing on the stems or leaves. While there are insecticides that will take care of them, control is often as simple as spraying them with a strong stream of water.

Other causes of yellowing are over-watering, under-watering, lack of fertilization and weather that is just too darn hot. Is it possible that one of your plants is getting significantly more sunshine during this hot weather?

Rhododendrons like water but they don't like wet roots. Sample the soil from both plants. If one is significantly wetter or drier than the other, adjust your watering to compensate.

Finally, it's easy to forget to fertilize plants in containers. It might be worthwhile to give them a good dose of fertilizer. Maybe even an iron supplement.

I hope this helps get you pointed in the right direction. Please think of us again if you have any other questions.

Thank you for your earlier reply. I added some compost to the Rhododendron plants to give some boost, and also paid special attention to their watering needs. We water the plants very well, and they have live prospered and grown so much these last 8 years, but since the last couple of months, something has gone seriously wrong.

But both the plants are getting worse, with some more visual symptoms... Curling / cigarring of leaves has been happening, but now I see various leaves showing yellowing. Also there are burnt / brown spots on some leaves. Furthermore, I noticed that the back of the curled leaves have this brown veins (See attached pic 3)..

I have looked at pictures of Rhododendron with problems. Sometimes I think it is a fungus infection, or may be root rot, or it is something else... I just don't know. Please help!
Thank you so much for your help.

Hello again. We're sorry to hear that your rhododendrons are still struggling.

While you're correct to assume your problems might be caused by some sort of fungal disease, these usually only set in when your plant is under some sort of stress. Here's some ideas to check.

We've had a much hotter than normal summer so far. Try moving your plants temporarily to a shadier, cooler location. They're in containers so that should be fairly easy to accomplish, although it will probably take two people. I know it does at my house.

While you have that second person available, try turning the pots on their side and examine the roots. If they are firm and tan or white, they are healthy. If they are dark and mushy, that's indicative of root rot and your plants are in trouble.

Rhododendrons tend to have very shallow root systems so they are probably not root bound. Still, they've been in the same containers for eight years and growing well, so it's worth checking while you have them out of their pots.

You said you've added some compost which is great for moisture control, but doesn't really help if we're dealing with a mineral deficiency. Yellowing leaves can be caused by either a magnesium or iron deficiency. If older leaves are yellowing, magnesium deficiency. Younger leaves indicate iron. For a magnesium deficiency, sprinkle epsom salts or spray the foliage with a solution of 2 tbsp epsom salts to one gallon water. If it's an iron deficiency, try blood meal.

If all else fails, you can try spraying your rhododendrons with a copper fungicide. You'll probably have to repeat the application every couple weeks until your plants recover.

This should give you some ideas to check and hopefully gets your plants back to good health. Again, if you have any further questions please don't hesitate to give us a shout.