Firefighters from across south London battled a blaze at a stables in Coulsdon throughout the night.
And crews are expected to remain there throughout the day to make sure the pile of trees does not reignite.
By the time crews were called to the scene at the old Cane Hill Hospital the fire had been burning for several hours but it was not detected until flames were seen leaping into the sky at dusk.
A pile of conifer trees, which was at least 5m high and 10m deep and 40m wide, was well alight and crews surrounded the pile and soaked it with water so it did not spread to the nearby stables.
They were replaced by crews from Erith, Southwark and Lewisham at 4am.
Crews will be at the site for much of the day damping down and a digger may be brought in to turn over the pile and make sure the fire is completely out.
There were four horses inside the stables but these did not need to be evacuated.
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