Dōngtiān tā kào dòngzi lǐ zhòng de qīngcài zhèngqián.
In winter he makes money from the greens he grows in the greenhouse.
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The 'greenhouse' meaning is a northern colloquial use. For a typical glass/plastic greenhouse, prefer 大棚 or 温室; 洞子 specifically suggests a sunken/sheltered winter plot.
