Well yes! It is after all fantasy, and it does have many odder things than coffee that grows in strange climates.
The Shire could indeed (going right back to the need for scientific explanation of course) have micro-climates. Would anyone here believe that palm trees not only grow but bear fruit on Scotland's west coast? Well that's due to the gulf stream. Even odder, I live at high altitude in a city that's already at high altitude, on the lee of the Pennines in Northern England and I can grow tropical plants in my garden.
There is also the possibility of a changed climate. Did Mount Doom and Sauron's industry contribute to global warming? It would certainly explain the extreme weather Middle-earth could get, the harsh winters they'd had (which brought down Wolves from the North), and maybe uncommonly hot summers, seasonal shifts which might vary wildly from region to region.