Like many artists and intellectuals of the time, Edna St. Vincent Millay was profoundly disturbed by the execution of Sacco and Vanzetti. She thought their trumped-up convictions were patently unfair, based on nothing more than political and xenophobic prejudice.
But more than this, Millay sees this miscarriage of justice as arising from a general attitude of complacency in contemporary society, a convenient collective amnesia over the importance of liberty in American public life. Instead of rousing themselves to a pique of righteous anger, the American people, especially the educated middle-classes, have chosen to cultivate their gardens. Millay doesn't mean this literally, of course; she's using a well-known saying, derived from Voltaire's Candide, which has come to mean the same as minding one's own business.
In due course, such a selfish retreat from the world and its manifest injustices will leave behind a worthless patrimony to our children and grandchildren, who will inherit a "blighted earth," a land in which the kind of injustice meted out to Sacco and Vanzetti will become the norm.