Best time to remove stems from celeriac

I'd say with an inch or so of shoulder showing, then I keep on removing them as and when they appear.

Just the outer leaves growing down over the shoulders. Leave the more central ones which should be in a more upright position.

Yes. I leave 3 to 4 leaves on my celeriac and it makes quite a difference. Mind, they keep trying to put out more so I keep picking to keep that same number.

It's the variety of turnip that you grow. Some give a small root but loads of leaves to pick for turnip greens, other standard varieties produce fewer leaves for a bigger root ball so basically they self-regulate their top growth. But not celeriac - it will send loads of shoots up at the expense of the root which is why we must prune them.