Six crews of firefighters are tackling a blaze spanning two hectares across a field near junction 21a of the M25.
The fire service was called at 12.37on to the 20-hectare field of stubble near Noke Farm cottages, off the A405.
People in the adjacent property have been evacuated.
Charlotte Gomez said: "The farmer from across the road called us and informed us of the small fire that had started in the field. As we left our home we could see the extent at which the fire was spreading. It was terrifying.
"The wind was pushing it closer to an area where there was lots of straw bales which were next to the houses.
"If the wind had decided to blow the fire over a few more meters the houses would have been up in flames.
"I would advise people living near crop fields to be aware that natural fires are happening and to keep an eye out."
Firefighters from Garston and St Albans used a hose reel and beaters to extinguish the blaze.
Herts Fire Control said: "Expect traffic delays if you are using this route this afternoon."