Emergency services were called to Hartlepool’s Chester Road allotments early on Saturday (July 30) morning.
Cleveland Fire Brigade said it had first been alerted at about 6.45 am and received a “high number of calls” about the incident due to the large amounts of black smoke produced by the blaze.
Several chickens were also killed in the incident, which officers are now examining to determine whether it was started deliberately.
In a statement, a spokesman for Cleveland Police said: "I can confirm that the fire on Chester Road was reported to us around 7:40am on 30 July and is being investigated as arson."
Allotment holder Malcolm Horsely, 62, told the Mail: "When I came down there was nothing left – it was like a bomb site. It’s really upsetting as I use the allotment with my granddaughter. I’ve had it for 25 years.”