caring for my asparagus plants

In articles on what to do when the foliage dies back the advice given is to cut down the stems in autumn, leaving 5cm stumps above the ground. Does anyone know why you should leave stumps?

I cut my foilage down to stumps (as Trillium says), and then cover the whole lot with manure - this protects them in winter, and also feeds them when they start growing

didnt get round to putting manure and compost on my 1st yr asps bed before the frosts hit, wonder if they'll be OK?

I rarely remember to mulch mine before the winter and they're at least 6 years old now (although I have lost the odd one or two in that time.)

They should be OK as long as they haven't got to put up with waterlogging as well as the cold.
(Mine are now mulched with 6 inches or more of snow - and counting!!!) 

My asparagus beds (2) are raised, and they were perfectly ok after last year's winter (we don't usually get a lot of snow around Manchester) and we had a good foot of snow .  In fact I got a really nice crop this year.

So don't worry about them, they'll survive.