1lit. 'the prisons are overgrown with grass' - a metaphor for ideal governance: so few crimes are committed that the jails go unused and weeds take over
Under his administration, the region was peaceful - the jails were so empty they grew weeds.
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From 《隋书·刘旷传》: when official Liu Kuang governed for seven years, 'jails had no inmates, lawsuits ceased, and the prison cells were entirely overgrown with grass - birds could nest in the courtyard'.
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Highly literary praise for an administrator. You'll see it in classical-flavored editorials and historical writing, never in casual speech.
