Four fire engines and around 25 firefighters tackled a fire in a field on George V Avenue in Pinner.
Part of a seven-hectare field compromising of freshly cut hay was damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: “The warmer weather we have been having makes the ground extremely dry, which unfortunately means grassland and parks will burn quickly when exposed to even the smallest of sparks.
“If you see a grass fire, don't attempt to put it out yourself as grass fires can travel very quickly and change direction without warning. Call the Brigade and let us know where the fire is.”
The Brigade was called at 2153 and the fire was under control by 2230. Fire crews from Northolt, Stanmore, Harrow and Ruislip fire stations attended the scene.