Blackthorn / prunus spinosa / seiyosumomo. The plant was introduced from China around 500. Sloes are the fruit of blackthorn and are actually a wild type of plums. The flavor of the fruit is bitter, so the small plums are not suitable for eating. However, the effect of frost makes them milder.
Sloe gin is traditionally made in Ireland and Britain. Sloe liqueur is also made in Scandinavia, Germany, France and Spain. The bitter flavor is lost when making liqueurs. This delicious liqueur has a flavor similar to plum liqueur and the color is dark red. スピノサスモモ (（ヨーロッパスモモ、セイヨウスモモ）For more on Prunus spinosa or Blackthorn see here and for flowers in March see here.