I had a 6ft tall healthy Rhododendron that a tree decided to fall on. Now it has a big chunk taken out of the middle. Either side of the chunk is still 4ft or so but the middle goes right to the base of the plant.
I'm not sure how far back to prune the sides or how much inward growth to expect either.
How should I proceed?
Cut all broken branches back to the last healthy branchlet or crotch. Remove everything broken. Once that's done, if you still have a large hole in it where little light will penetrate, thin out the surrounding growth, and head it back. This will promote regrowth, and let light in to where the broken branches were removed, allowing stronger regrowth in that area. Expect a good covering or regrowth the first year (if your bush is healthy), but it will probably be a few years until the area has filled out again.
If you want, you can head back the rest of the bush the next season, so that it looks more symmetrical, but don't do this the first year, as you want to encourage as much regrowth to the damaged area as you can the first year.
There is another alternative. Rhododendrons are open to renovation, but this is usually carried out in midwinter, when the plant is dormant. I don't know where you are, so can't say whether your plant is still completely dormant or not, depends on air temperatures/daylight lengths, but if it is still dormant, saw it down - the equivalent of stooling. As it puts out new growth, give it a good feed and reshape and prune as growth continues, till you've got something you like the shape of. You won't get any flowers for 2 or 3 years, but you will still have the shrub and it will flower again eventually - provided lacebug doesn't get it.
