Frost - ice - will form from water vapor when solid surfaces - in this case pine needles - are cooled to below the dew point of the adjacent air. When that happens, water vapor turns into a solid, too. The size of a frost crystal is determined by the amount water vapor available as well as the amount of time available for formation. And it appears gravity has a part to play, too, dictating the direction the crystals will form.

In this photograph, there was both ample moisture in the air and a long, cold night that allowed ample growth of the frost.