That is the question....
Ornamental grasses tend to hold on to their old dead growth , and this can become confusing for gardeners with busy lives, who often mistake evergreen for deciduous and vice-versa. There is no sure-fire way of dealing with this problem except to take a little time to identify your grasses.
If new green growth is appearing from the base of the plant then it’s probably deciduous, and this is a good time to get rid of all the dead bits. I always used to take enormous care not to cut any of the new growth, but then I saw Carol Klein merrily massacre one specimen on the telly. “It’s a grass” she said, “just like your lawn”. After that, I became altogether less tentative. Do wait though, until you can at least see some green spikes emerging from the base. Some grasses, like miscanthus and pennisetum get going rather later in the spring, and the dead growth acts as protection.
Evergreens, like carex and festuca need a little comb-through. The new growth will be just as long as the old, and all you need to do is to dispense with this by pulling your hands through the plant (wearing gloves of course, because some of these grasses have sharp edges) and the old growth should come away easily. Although this is rather more time-consuming than chopping, it has the kind of therapeutic effects that make this extra time more than worth it.