Pleaching or plashing is a technique of interweaving living and dead branches through a hedge for stock control. Trees are planted in lines, the branches are woven together to strengthen and fill any weak spots until the hedge thickens. Branches in close contact may grow together, due to a natural phenomenon called inosculation, a natural graft. Pleach also means weaving of thin, whippy stems of trees to form a basketry effect.The photo, via the WoahDude subreddit, was taken at the Schönbrunn Palace gardens.
For me the image immediately conjured up memories of the scene in The Third Man where Anna walks out of Holly's life, but that one was taken at a Viennese cemetery.
This photo, and the watermarked one here, show the equipment used at the palace to accomplish the effect:
Although the top photo was described in the discussion thread as an example of pleaching, a review of Google Images retrieved by keyword pleaching suggests that the process at the palace is just elaborate pruning, without the interweaving indicated by the term "pleaching."
Still an interesting effect, though.