Emergency sets were called to garden plots near Arnold Road, Darlington, just before 6am on Friday.
Investigators have confirmed they now believe the blaze was started deliberately.
A spokesman for County Durham and Darlington Fire and Rescue Service said: “Red Watch attended a fire at the allotments off Arnold Road, Darlington.
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“The cause of this fire is unknown but is thought to have been started deliberately.
“If you have any information relating to this incident please call FireStoppers anonymously on 0800 169 5558.”
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