Rhododendrons

(Question)

not sure if my earlier question got to you because the network may have had a brief interruption. I am in Toronto and just installed three rhododendrons in a corner under an oak tree. the area is in the shade. One of the three had buds and bloomed but died very soon after. Another had buds but they dried up before they bloomed and the bud and leaves surrounding them turned brown while the new leaves are light green instead of dark. now the third one has a few buds but they too look as though they are dried up, while the leaves appear to be curled up. Is it too much water, too little water or too alkaline? how do I go about doing an acidity test?

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(Answer)

These plants seem to have a variety of concerns: buds whose blooms were not long standing,  buds and leaves that have dried/curled and turned brown and new leaf growth with a pale colour.

Your first concern was that the blooms that were produced were short lived.  Research shows that rhododendrons bloom for one to three weeks depending on variety.  Were your blooms within this time frame?

On that same plant, did it bloom fairly early this spring, soon after planting?  Could this have been after one of our Toronto late spring frosts that we experienced.  Cold injury would mean that some buds would bloom but others would just turn brown.  As well, this type of temperature injury would partially kill or affect new growth causing leaves to be distorted.

If your leaves are turning brown from the edge into the mid-rib it may be experiencing “marginal leaf necrosis” This condition has several causes that may be pertinent in your case. Planting a rhododendron too deep can cause the leaf edges to turn brown and reduce the number of blooms.  Rhododendrons require air at the root level and have fragile surface feeding roots.  They should be planted at the same level in your garden as they were in their nursery pots, maybe a bit higher.  Do not bury the root ball under soil or heavy mulch.

Are the plants well drained.  Rhododendrons like to be moist, but not wet.  Brown leaf edges could be a symptom of a drainage problem.  Coincidentally  poor drainage, “wet feet”, can lead to iron deficiency that exhibits in some yellowing (chlorosis) of leaves (possibly your light coloured leaves on your second plant).  This last symptom can also be from a soil that is overly alkaline.  Rhododendrons like a pH of 4.5 to 6.

This leads to your last concern about soil testing.  Our Toronto Master Gardener website shows:

In conclusion: Are your rhododendrons of a variety that is hardy enough for your region?  Is their planting depth suitable?  Are they drained sufficiently?  Is the soil pH acidic enough?

Hopefully this helps and your plants will begin to thrive.