The regional court in the northern city of Hildesheim found 66-year-old Wilfried Reinecke guilty of beating to death the 33-year-old son of his neighbours on their parcel of land with a wooden club last September.
The white-haired Reinecke then turned the weapon on the man's parents, aged 59 and 64, when they tried to intervene, the court found.
The explosion of violence marked the culmination of a long-running dispute with his neighbours in the town of Gifhorn.
Witnesses told the court that Reinecke had been locked in a feud with nearly all the other tenants for eight years because of trespassing on a path, half of which belonged to the defendant.
The dispute escalated to the point that the two sides began dumping garbage on each others' allotments.
Witnesses testified that Reinecke had one day shouted: "Someday I'm going to lose it and kill you all!"
Presiding judge Ulrich Pohl dismissed defence attorneys' arguments that Reinecke was psychologically disturbed, saying he was like "hundreds of thousands of other older men over 60 who pick fights for no good reason."
Pohl said Reinecke had shown "not the slightest sense of remorse" for his actions and had actively lain "in wait" for his 33-year-old victim, gravely wounding him with the club and then killing him with four blows to the back of his head.
Reinecke had told the court he acted in self-defence.
He will not be eligible for parole in 15 years as is customary under German law, a sentence that matched the demands of the prosecution and two other sons of the dead couple.
Reinecke's lawyers said they would appeal the sentence.