Via the Scientific Alliance:

The St George’s mushroom is so named because in days long gone it could be harvested on the saint’s day, 23rd April. However, a few decades ago, the average fruiting time for this fungus was mid-May. More recently, this has moved forward to 22nd or 23rd April, making the name apt once more. Although reported as a sign of current climate change, the other implication is, of course, that the climate in this country was indeed warmer in centuries gone by. This is further indirect evidence of the Medieval Warm Period (or Medieval Climate Optimum), which was certainly not brought about by profligate use of fossil fuel by industry.