Posted: 10/07/2014 at 12:58
You won't kill it - it's a willow - they are incredibly eager to grow!
The danger will be that if you cut it back too hard you will stimulate a lot of new growth and you'll be left with a tree shaped like a loo brush.
Do not cut the tree back until late autumn/early winter, this way there will not be so much 'growth hormone' in the tissues. Then I would take out one third of the branches as low down as you can.
The following autumn I would repeat the above with the another third of the branches, and the autumn after that I would cut back the final third as with the others.
Cutting back one third at a time should guard against stimulating the tree to grow lots of new shoots.
The tree will grow new branches and if you keep them at a reasonable height it shouldn't get out of hand again. However, the corkscrew willow is by its nature a large tree, often planted where there is not really enough room for it to grow to its full potential.