A well known and widespread saying suggests that if oak (Quercus) trees produce their leaves before those of ash (Fraxinus excelsior) trees  emerge  a dry summer is foretold:

If the oak is out before the ash, We shall surely have a splash,
If the ash is out before the oak, We shall surely have a soak [1].

Similarly, in Northern Ireland:

If the oak before the ash, Then we’ll only have a splash.
If the ash before the oak, Then we’ll surely have a soak [2].

According to a letter from Marnhull, Dorset, in the Daily Telegraph of 27 June 1987:

If oak and ash leaves show together
Us may fear some awful weather.
This be a sight but seldom seen
That could remind we what has been.

It is generally accepted that oak usually comes into leaf earlier than ash, which is said to be in leaf for a shorter period of time than any other native British tree, but the problem seems to be deciding what ‘out’ means; is it sufficient for the leaves to be emerging from their buds, or do the leaves have to be fully developed?

1.  Roy Vickery, West Dorset folklore notes, Folklore 89: 154-9, 1978.                     2.  Ballycastle, Co. Antrim,  January 1991.

Images:  main, Tooting Common, London Borough of Wandsworth, 19 April 2018; the ash in the foreground is obviously more advanced than the oak behind it; inset, same two trees, oak in front, 1 May 2018.

Note:  A woman attending a plant walk on Tooting Common on 2 May 2018 reported that ash was out before oak in Norfolk this year; we shall see …