Vertical antenna on a turf roof

A vertical wire antenna based on a MFJ-1904H 6.7 m (22 feet) telescopic fiberglass pole as shown here is easily tuneable for all bands from 40 m to 10 m. Here it is placed on top of a turf (sod) roof with quarter wave radials for all the 7 bands going to each of the two sides sloping down as they follow the contour of the roof. The antenna seems to work satisfactorily at least on the 30, 20, and 17 m bands which I have been able to test so far.

A turf roof is a traditional Scandinavian type of green roof covered with grass. It dates back to the Viking age and before. In modern times it has seen a renaissance in e.g. mountain cabins.

But how does the turf affect the antenna or to pose the question more precisely: Can it be used to improve the ground plane and the antenna's performance?