That depends a lot on your winter. In Montana probably not much of any decomposition, in South Florida winter decomposition can happen as fast or faster than in the summer in Montana.

Temperature and humidity/rainfall are the big variables. Where I live woodchips break down as quickly in the wet fall as the dry summer even though temperature is 60+ deg cooler. Mid winter is usually pretty dry for us so I imagine that plus te cold all but stop significant decomposition.

It depends on more than your winter. I have seen piles of mulch material with no snow on them and a good bit of steam rising from the pile. I have also seen piles of mulch material burning in mid January telling us they can generate significant amounts of heat even when air temperatures are near to or below zero.

So, yes bacterial activity in a mulched bed may be active enough to digest that material

Heh Kimmq. Truth be told, I have not looked at it during the winter to spot any steam ;-). Its more of a dreary bleak for a auto fanatic like me, to see the cars are all salty and just waiting for the dreary bleak to end ;-)

I actually like the crispness in their air and don't mind getting out the snowblower to just ~blow~ some snow, but it get's real tired after the 2nd/3rd go around..

I'll be paying more attention this year just to see if I can catch a whisp of steam ...

Bark mulch is going to decompose slowly, winter or summer.

Off topic or on, but how much are you willing to pay for mulch......I used to be the memorial day kickoff - big box store ... type of guy. It was cheap and easy to get..

Then with kids in my life, in which they touch everything, literally, I've switched now to a pb mulch that I know will not contain old wood, wood from unknown sources with unknown chems. Local homers does carry cedar and pb mulch, but the quality is bleh. The next closest is Lowes, and their bark mulch is more like 30 bark, 70 percent sapwood.

Local nursery has them priced at $7 per bag, $6 when you buy 5 for more and $5 at 10 bags plus...At $1 a bag compared to $5, I can't complain....

Macroscopic members of the soil food web don't all overwinter as corpses and eggs; some of them just hide out in a sheltered area and wait. A good thick layer of mulch serving as a blanket on the soil can slow or prevent it freezing, keep them active, and they'll be eating on it and breaking it down that way. The isopods around here sure don't seem to stop chewing cardboard and wood just because it's cold out.

There are some fungi and other decomposers that work in very, very cold temperatures, and under an insulating blanket of snow ground temperatures may be warmer. Even in places where the ground freezes, it may warm up and thaw before snow cover disappears - and this is in addition to the physical action of freezing and thawing and worms and other bugs that may stay active under there. That said, there's no question that decomposition is slower in colder temperatures, so your mileage may vary. Will also depend on the type of mulch, of course.