I do it when they look spent out whenever that time comes and do the same for my other water plants.

Any tips on how to do it please? They aren't always easy to reach...

A manual long handled (36inch)swivel grass shears does an excellent job at cutting, trimming and removing the clippings from the pond from the surface or below. The only company that sells this that I know of is Fiskars and you can find them on Amazon. This implement has a multiplicity of other uses around the garden too, especially for those of us with back problems. I liked it so much that I purchased a second one.

That looks a very nifty gadget Loosetrife - very ingenious.
Thanks.

I wait till they go black and then they rot off and pull away easily.

They've mainly all started going yellow so I might start moving them but not sure!!