Sounds like classic water damage.
If they were own root they should recover
if they are grafted and the rot reached the graft
it is bye bye.

Who gave you that advice?
Covering with mulch does not help
cos the cold/warm air gets in there
any way. All recomendations typically
ask for covering w/ compost.

And the the plastic pots holds the moisture in as you
are learning.


--
Theo in Zone 5
Kansas City



"SBHCOM" wrote in message
...
I think I messed up bad, this was the first year I winterized roses (I
live in
Michigan). I cut them down to about 6 inches from the ground and then
placed
old plastic pots on top of them. I then cut slits in the top and filled
them
with black pine mulch. When I removed the pots yesterday I found the
roses in
terrible shape, the stems seem to all have rotted. I think I really
screwed up

A pipe gives a wise man time to think and a dumb man something to stick in
his
mouth.