My 30+-year-old rhododendrons each get several yellow leaves each summer, including this season. The affected leaves are solid yellow (no green veins, which I’m told would indicate chlorosis). Eventually the leaved fall off and turn brown. There are no signs of insects The soil is very good, not too deep over their shallow roots, top-dressed with a layer of arborist chips (not touching the trunk). The yellow leaves seem randomly distributed throughout the tree, maybe more in the front (south-facing). The two trees that get the most sun have the most yellow, but even the ones in more shade have a few affected leaves. I water via above-ground sprinklers for 10-20 minutes (depending on the day’s high temperature), 3x/week. Finally, the rhody with the most yellow has had a tight undergrowth of lily-of-the-valley that I am valiantly trying to eradicate; these rhizomes are entwined in the rhody ‘s roots & may be stealing water & nutrients. What is going on – heat/drought stress, root competition, other? What should I do about it? (I can add photos if you think they’ll be helpful.) Thanks!
Based on the pattern you're describing and what we can see in the photos, this appears most consistent with normal shedding of older rhododendron foliage that is being accelerated somewhat by summer heat and moisture stress.
Rhododendrons retain their leaves for several years, but eventually older leaves yellow uniformly and drop. The fact that the yellow leaves are scattered among otherwise healthy green foliage, occur each summer, and are more prevalent on the plants receiving the most sun all support this explanation. We don't see a pattern in the photos that strongly suggests an insect or foliar disease problem, and the uniform yellowing isn't typical of iron chlorosis.
The irrigation regime could be contributing. Rather than relying on 10–20 minute sprinkler cycles, we generally recommend less frequent but deeper watering. Longer, thorough irrigation allows moisture to penetrate throughout the rhododendron’s active root zone and encourages more consistent soil moisture between waterings. Short, frequent cycles can wet the mulch and soil surface without adequately replenishing moisture several inches below. During dry weather, check the soil beneath the mulch at approximately 4–6 inches deep; if it remains moist, additional watering is unnecessary. The goal is to allow the soil to begin drying between waterings without allowing the root zone to become excessively dry.
The lily-of-the-valley may also be contributing to stress. Continuing to remove it is reasonable, but we'd avoid aggressive digging that could damage the rhododendron's shallow root system.
At this point, we would not recommend treating the plants for disease or automatically adding fertilizer. Maintaining adequate soil moisture, continuing the arborist-chip mulch, reducing competing vegetation, and monitoring the plants would be appropriate. If yellowing begins affecting newer foliage, significant branch dieback develops, or the overall canopy starts thinning, that would warrant a closer on-site evaluation and potentially soil testing.