Vaseline trees,

Vaseline has been used by photographers for many years! they smear it over a filter in front of their lens, the interesting thing is if you wish to have vertical smearing then the vaseline must be applied horizontally!

I have used a double exposure in camera to produce an similar effect by an alternative method.  I first exposed the first image as normal but at half the metered exposure time. I then moved the camera vertically during the second exposure, which resulted in the blurred image with some sharpness in places.