You have all heard, over and over, the remark attributed to Uncle Marty: If I knew that the world would end tomorrow, I should still plant my apple tree. Or variations thereof.
There are two questions for a preacher tempted by this remark: (1) is it genuine; and (2) does it reflect Luther's own eschatology?
The first answer appears to be no. Such is the conclusion of Martin Schloemann, in Luthers Apfelbaumchen? Ein kapital deutscher Mentalitats-geschichte seit dem zweiten Welt-krieg (1975; repr. Gottingen: Vandenhoeck & Ruprecht, 1994). We gather that Scholoemann can trace the remark only back to 1944.
The second answer is probably fodder for a graduate seminar, rather than a Sunday sermon. Frederick Gaiser, in a 2005 Word & World editorial (here, if you have ATLA access) says that Schloemann's own answer is no, if the remark is to argue for a life focused on the present reality without concern for the last days, and yes -- or at least maybe -- if it is used to talk about "creaturely service [to] neighbor and world" grounded in an eschatological context. (Gaiser doesn't mention it, but the appropriation of the remark to support environmental initiatives is opportunistic and arbitrary -- and by the way, Luther did have some truly interesting things to say about the degradation of German farmland.)
But be warned: Schloemann found the misattribution of the remark suspicious, not merely because it doesn't necessarily reflect Luther's thought, but also because it reflects "the slippery malleability of the 'Luther'" whose imprimatur people seek to stick on the cause of the moment. If there are any two words that fail to describe Luther, they are slippery and malleable.
In a somewhat confusing essay on tracing suspect quotations, Yoel Natan suggests that this one may have evolved from a Jewish source:
[Rabban Yochanan ben Zakkai] used to say: ‘If there were a plant in your hand and they should say to you, ‘Look, the Messiah is here!’ Go and plant your plant, and after that go forth to receive him’ (‘Abot R. Nat. B 31; quoted from John T. Carroll et al. The Return of Jesus in Early Christianity. Hendrickson Publishers, Peabody, Massachusetts, 2000, pp. 180-181)
So if, having considered questions (1) and (2), a preacher goes on to ask (3), "What if this remark reflects my own eschatological ideas, which I want to share in a convenient form with the faithful?" it seems to us that there are two good answers. Either attribute it to Yochanan ben Zakkai, or just say it yourself.