When it’s a “fedge” apparently.
“Living fences (also called “fedges”, as they are a cross between a hedge and a fence)” (http://www.westwaleswillows.co.uk/fedgeplanting.html)
I’ve seen this neologism used in various places, but I’m not sure it’s a necessary invention. The word “hedge” can already be used to refer to “dead hedges” as well as ones that contain live plants. The earliest hedges were created in a variety of ways – sometimes saplings were taken from woodland and replanted. The art of hedgelaying relies on the flexible new growth created by coppicing or plashing a row of live trees – this new growth is then woven into a more solid barrier. And it also seems likely that dead hedges were sometimes built from willow which would then reroot and become a living barrier, much like a willow fedge. So a line of interwoven willow saplings falls easily within the boundaries of the concept of a “hedge.”
To me, a “fedge” seems simply to be a relatively formal hedge, wherein the saplings are interwoven in a fence-like pattern. This is not dissimilar to a newly laid hedge, which can have a very precise pattern to it.
Over time this pattern will presumably become more ragged as the saplings grow, and the fedge will eventually be seen for what it really is – a hedge.