March - if you're in the south you can do it mid March, in the north end of March, unless we're having freakishly cold weather. I don't know if you're sure what to do when cutting it back, but you remove any plumes by their bases, where they come out of the centre at the top, as low as possible, then cut off all leaves right back to the central core of the plant so there aren't any leaves left, rake out any debris in the centre of the plant where the plumes were removed. And that's it - but you will need to protect your skin, the leaves are unpleasant and cause grazes.

If you live in the countryside and you've got a flamethrower, you can just burn all the leaves off.

March sounds about right - or just when it wakes from dormancy.

The plant is tough enough that it won't be much bothered if you are a week or two late--the shoots will just keep on growing.

Mine was done last spring. I attached a toothed disc to a strimmer and went crazy. It had plumes in the fall and grew right back - it's like you can't kill it. And wear gloves because, like Bamboo says, the sides of the leaves are serrated and will give you paper cuts.

Nitrile or goatskin gloves work best--I've had the leaves cut right through cloth gloves!