A friend in Illinois has sent a photo this spring of a very healthy looking rhododendron – leaf buds fully elongated and beginning to unfurl, while the green, blunt flower buds remain unopened. The flower buds don’t look brown, diseased, frozen or injured, but they remain tightly closed, foliage bud growth preceding blooming. He says he has 6 plants doing the same this month. Possible reason?
Though we can’t diagnose plant problems by phone/email, early autumn frosts can inhibit flowering and not all buds are equally affected.
“Autumn frosts: These can lead to damage…if they either occur in early autumn or immediately after a late season warm spell. Continental climates with extremes of heat and cold are more likely to suffer sudden temperature changes than those with maritime climates…A sudden temperature drop will catch a plant before it has had a chance to reach maximum hardiness and it may suffer accordingly, even if normally perfectly able to withstand such a temperature in mid-winter…Speed of ripening varies considerably…There is also a variation in the hardiness of flower buds compared to foliage and growth buds. Commonly, flower buds may be as much as 10 F. less hardy than foliage…”
(Source: The Cultivation of Rhododendrons, by P. Cox, 1993, p. 119-120)