North Lincolnshire residents are being asked to be on the alert for anyone acting suspiciously following the theft of more than 900 freshly-planted perennials from Kingsway Gardens.
Thieves have half-stripped newly planted beds in the Scunthorpe gardens.
It is thought the plants have been taken to sell, possibly at a market or car boot sale or via a local selling site.
Displays in Kingsway Gardens have previously led to the park receiving the coveted Green Flag award, the international benchmark for parks and green spaces.
The council will be installing surveillance devices in the area.