I'm not sure I understand your question - I haven't personally heard of using boric acid for rhododendrons or in the garden at all except to deter ants.

A few of the older leaves turning yellow (even red, orange) then dropping off is normal - 'evergreen' rhododendrons shed leaves older than about three years.

If more than those older leaves are yellowing while leaf veins remain green, chlorosis or a nutrient deficiency could be a possibility and correcting your soil PH perhaps in order....

I don't find boron often mentioned in poor performance/appearance of rhododendron.

Nutrition of Rhododendron - "In addition to the major elements (nitrogen, phosphorus, potassium), most gardeners are familiar with iron and magnesium deficiency, but all do not realize a rhododendron requires some, but often only a minute amount of boron, manganese, zinc, molybdenum, copper and possibly aluminum. Most of these elements are usually in the soil, but if not available, they could be the cause of poor rhododendron performance of some kind." Not common.

Sorry - meant Epsom Salt for yellowing of leaves? Chelated iron too?

Is it too late to apply to rhodies now?

Thank You Morz

Sorry - meant Epsom Salt for yellowing of leaves? Chelated iron too?

Is it too late to apply to rhodies now?

Thank You Morz

That one I know, I should have guessed that's what you meant.

You can do a chelated iron foliar spray or soil application if the cause is unavailable iron - do you know if your soil PH is a little high to be considered acidic?

A foliar spray of 2 T Epsom salts per gallon of water will indicate whether or not lack of magnesium uptake is causing off color leaves...If your plant greens up within a few days of your spraying, you can get a longer-lasting remedy by broadcasting the crystals onto the soil at a rate of 1 pound per 100 square feet.

Now is fine for either chelated iron or epsom salts.