Two crews from Preston were called out to a fire in an outbuilding at the allotments near to the junction of Blackpool and Garstang Road just before 2am on Monday, April 9.
Crews arrived to find a "well established" fire with flames leaping 15ft into the air.
Firefighters set about stopping the fire from spreading to a neighbouring building using hose reel jets.
Crew Manager Neil Parkinson said: "We attended a fire at a workshop on Serpentine Allotments after a member of the public called 999.
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"We arrived to find the fire was well established and was in danger of spreading to a nearby building.
"Firefighters set about extinguishing the fire using two hose reels and a jet.
"We had to tackle it quickly and crews did very well to stop it."
Fire services have said that they believe the fire was started deliberately.
The building was severely damaged by fire and crews believe the structure has been damaged.
A joint police and fire service investigation has been launched to establish the cause of the fire.
Fire crews spent three hours at the scene.