Fire fighters were called to the site in Holder House Way, South Shields, at 12.30am on Tuesday to tackle the blaze.
Crews from South Shields, Hebburn, Marley Park and Washington stations, took three hours to extinguish the fire and were called back to the allotment at around 11.40am when the blaze began to smoulder again.
Northumbria Police has confirmed the fire was stared deliberately.
A spokesman said: "It has now been established the fire was started deliberately and is being treated as arson.
"Enquiries are on-going. Any witnesses should ring Northumbria Police on 77 060916 or Crimestoppers anonymously on 0800 555 111."