Often weather related. Sometimes thought to happen with a dry summer followed by a warm wet Fall, and that while some varieties may be more susceptible than others, it can also happen with individual plants within a variety.

The only suggestion I've seen for overcoming it is to water more regularly summer through late summer, especially if the rhody is sited where it's getting a fair amount of sun (where more water would be required than for a plant in shade).

I notice quite a number of red rhododendrons blooming in Fall here (we have little to no rain Aug and Sept) - but that could be too because the red stands out more and is most noticeable while driving, other plants with more subtle colors could be doing the same.

If I remember correctly, 'Elizabeth' and 'Elizabeth Hobbie' bloom in the spring and fall. Both are red.
Mike

Best guess: that it's genetic, confined to particular rhodies that have a particular gene or genes that make them more prone to reblooming. The great majority of our rhododendrons bloom only in the spring, even though all are in approximately the same growing situation.

Our Loder's White started reblooming in the fall after it was maybe 8 years old, and has never missed a single fall blooming period since then. That started about 20 years ago, and we've had consistent fall blooming, even when it has had summer and fall droughts to endure with less than ideal watering. We've had a Mrs. Charles Pearson that bloomed a little in the fall, but didn't water it well enough through its last drought to still have it to know whether it would have kept going with the fall blooming thing or not.

Anyway, there's no reason to want to avoid the fall blooming with Loder's White in our yard. The fall bloom - very scanty most years but any blooms then are most welcome! - doesn't subtract from its generous spring display at all, at least not here, in this zone 7 yard. Our Loder's White gets a reasonable amount of sun from the west and overhead, maybe as much as 5 or 6 hours in June/July, and approximately full sun (deciduous trees overhead to the south) after the fall bloom. That's when it sets new bloom buds in a jiffy for the spring bloom fest to com.

Your large white is either a clearly different variety or in a clearly different situation than our Loder's White. I don't really know why your fall display occurs at such a price to the spring bloom, but I wonder whether as it ages it might handle the fall bloom period with slightly more grace than you're currently seeing? Perhaps less watering in the fall than might be ideal would discourage so much fall blooming? Then maybe laying on plenty of water in the winter and early spring so the spring season goes well with lots of buds opening then...

If your white rhododendron's site, like ours, has deciduous trees overhead, then your rhododendron is getting much more sunlight in the fall than it did in the summer. In a way, that mimic's the spring situation where more sunlight calls for buds to open. An evergreen tree nearby to the south might take care of the problem?

I just noticed that you are gardening in zone 5. That probably is vastly different than our zone 7 situation, where winters tend on average to be fairly mild, with plenty of winter days where new buds might be still forming on a rhododendron that bloomed some in the fall. So if you have deciduous trees, the extra fall sunlight encourages buds to open, but your very cold zone 5 winter doesn't provide enough warmth for new bloom buds to form for a spring display? That, of course, applies only to a particular variety prone toward fall blooming anyway.

Well, maybe you don't have deciduous trees at all, and maybe this figuring is all wrong. I'm just guessing!

Best wishes,
Mary