Four fire engines and 21 firefighters and officers were called to a fire at a farm in The Ridgeway, Enfield.
Around 500 tonnes of mulch was alight in an open field.
The Brigade was called at 0647 and the fire was under control at 0855. Fire crews from Enfield, Southgate and Edmonton fire stations attended the scene along with colleagues from Hertfordshire Fire and Rescue Service.
The cause of the fire is under investigation.