The flowering of blackthorn (Prunus spinosa) is said to coincide with a spell of cold weather.  At present blackthorn hedges throughout much of southern England are in full bloom, but the weather is unusually hot, with record high temperatures being recorded in some areas on 19 April.  Thus 2018 appears to be exceptional.

Addendum:  Perhaps a letter from Corston, Somerset, published in The Times of 13 April 2000 is pertinent here:  ‘My grandfather always referred to an useasonably warm spell in March as the “blackthorn summer”, as we experienced only a few weeks ago.  If this dictum is combined with the one that says if March comes in like a lamb it will go out as a lion (and vice-versa), our forebears’ folklore proves they knew well that at this time of year “unseasonable” weather, either hot or cold, is to be expected.’

Image:  Crewkerne, Somerset, 20 April 2018, when approximately 50% of the buds had opened, so there’s still a chance that the weather will become cooler as the others do so.

Updated 23 January 2020.