PWD's move comes a few days after DJB requested it to promote use of recycled water for non-drinking purpose.
A senior government official said groundwater is presently used by PWD for watering plants on the central verge and in parks and lawns, but from next month the use of recycled water will be mandatory to meet horticultural needs.
According to government, the new move will prevent extraction of ground water in a big way and 1.75 million litres of groundwater will be saved daily.
"All agencies providing water to various sites of PWD for irrigation or horticultural use through agreements shall switch to supplying only recycled water," the official said.
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He added that all such agencies have been asked to make this switch by October 5 at the latest.
"PWD has also agreed that in future, agreements and tenders would clearly specify that the source of water for horticultural purposes is from WWTPs," he said.
Delhi Water Minister Kapil Mishra, also DJB's chairperson, said, "Recycled water can satisfy most water demands as long as it is adequately treated and is appropriate for the use. To reduce the burden on potable water, it is important to use recycled water for non-potable purposes."
DJB has also taken up the matter with DDA and all three municipal corporations and asked them to use only recycled water for irrigation in parks and other green spaces.